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Title: General Science

Author: Bertha May Clark

Release date: August 25, 2005 [eBook #16593]
Most recently updated: December 12, 2020

Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK GENERAL
SCIENCE ***

GENERAL SCIENCE

BY

BERTHA M. CLARK, PH.D.
HEAD OF THE SCIENCE DEPARTMENT
WILLIAM PENN HIGH SCHOOL FOR GIRLS,
PHILADELPHIA

NEW YORK - CINCINNATI - CHICAGO
AMERICAN BOOK COMPANY

1912

PREFACE

This book is not intended to prepare for college entrance examinations; it
will not, in fact, prepare for any of the present-day stock examinations in
physics, chemistry, or hygiene, but it should prepare the thoughtful reader
to meet wisely and actively some of life's important problems, and should
enable him to pass muster on the principles and theories underlying
scientific, and therefore economic, management, whether in the shop or in
the home.
We hear a great deal about the conservation of our natural resources, such
as forests and waterways; it is hoped that this book will show the vital
importance of the conservation of human strength and health, and the
irreparable loss to society of energy uselessly dissipated, either in idle
worry or in aimless activity. Most of us would reproach ourselves for lack
of shrewdness if we spent for any article more than it was worth, yet few of
us consider that we daily expend on domestic and business tasks an amount
of energy far in excess of that actually required. The farmer who flails his
grain instead of threshing it wastes time and energy; the housewife who
washes with her hands alone and does not aid herself by the use of washing
machine and proper bleaching agents dissipates energy sadly needed for
other duties.
The Chapter on machines is intended not only as a stimulus to the invention
of further labor-saving devices, but also as an eye opener to those who, in
the future struggle for existence, must perforce go to the wall unless they
understand how to make use of contrivances whereby man's limited
physical strength is made effective for larger tasks.
The Chapter on musical instruments is more detailed than seems warranted
at first sight; but interest in orchestral instruments is real and general, and
there is a persistent desire for intelligent information relative to musical
instruments. The child of the laborer as well as the child of the merchant
finds it possible to attend some of the weekly orchestral concerts, with their
tiers of cheap seats, and nothing adds more to the enjoyment and instruction
of such hours than an intimate acquaintance with the leading instruments.
Unless this is given in the public schools, a large percentage of mankind is

deprived of it, and it is for this reason that so large a share of the treatment
of sound has been devoted to musical instruments.
The treatment of electricity is more theoretical than that used in preceding
Chapters, but the subject does not lend itself readily to popular presentation;
and, moreover, it is assumed that the information and training acquired in
the previous work will give the pupil power to understand the more
advanced thought and method.
The real value of a book depends not so much upon the information given
as upon the permanent interest stimulated and the initiative aroused. The
youthful mind, and indeed the average adult mind as well, is singularly non-
logical and incapable of continued concentration, and loses interest under
too consecutive thought and sustained style. For this reason the author has
sacrificed at times detail to general effect, logical development to present-
day interest and facts, and has made use of a popular, light style of writing
as well as of the more formal and logical style common to books of science.
No claim is made to originality in subject matter. The actual facts, theories,
and principles used are such as have been presented in previous textbooks
of science, but the manner and sequence of presentation are new and, so far
as I know, untried elsewhere. These are such as in my experience have
aroused the greatest interest and initiative, and such as have at the same
time given the maximum benefit from the informational standpoint. In no
case, however, is mental training sacrificed to information; but mental
development is sought through the student's willing and interested
participation in the actual daily happenings of the home and the shop and
the field, rather than through formal recitations and laboratory experiments.
Practical laboratory work in connection with the study of this book is
provided for in my Laboratory Manual in General Science, which contains
directions for a series of experiments designed to make the pupil familiar
with the facts and theories discussed in the textbook.
I have sought and have gained help from many of the standard textbooks,
new and old. The following firms have kindly placed cuts at my disposal,
and have thus materially aided in the preparation of the illustrations:
American Radiator Company; Commercial Museum, Philadelphia; General
Electric Company; Hershey Chocolate Company; Scientific American; The

Goulds Manufacturing Company; Victor Talking Machine Company.
Acknowledgment is also due to Professor Alvin Davison for figures 19, 23,
29, 142, and 161.
Mr. W.D. Lewis, Principal of the William Penn High School, has read the
manuscript and has given me the benefit of his experience and interest.
Miss. Helen Hill, librarian of the same school, has been of invaluable
service as regards suggestions and proof reading. Miss. Droege, of the
Baldwin School, Bryn Mawr, has also been of very great service.
Practically all of my assistants have given of their time and skill to the
preparation of the work, but the list is too long for individual mention.
BERTHA M. CLARK.

William Penn High School.

CONTENTS

CHAPTER PAGE
I. Heat 9
II. Temperature and Heat 27
III. Other Facts about Heat 31
IV. Burning or Oxidation 45
V. Food 60
VI. Water 70
VII. Air 81
VIII. General Properties of Gases 93
IX. Invisible Objects 100
X. Light 104
XI. Refraction 113
XII. Photography 126
XIII. Color 134
XIV. Heat and Light as Companions 142
XV. Artificial Lighting 148
XVI. Man's Way of Helping Himself 154
XVII. The Power behind the Engine 176
XVIII. Pumps and their Value to Man 187
XIX. The Water Problem of a Large City 206
XX. Man's Conquest of Substances 218
XXI. Fermentation 232
XXII. Bleaching 237
XXIII. Dyeing 244
XXIV. Chemicals as Disinfectants and
Preservatives 250

XXV. Drugs and Patent Medicines 255
XXVI. Nitrogen and its Relation to Plants 261
XXVII. Sound 266
XXVIII. Musical Instruments 284
XXIX. Speaking and Hearing 300
XXX. Electricity 306
XXXI. Some Uses of Electricity 312
XXXII. Modern Electrical Inventions 318
XXXIII. Magnets and Currents 328
XXXIV. How Electricity may be Measured 339
XXXV. How Electricity is obtained on a
Large Scale 346
Index 353

GENERAL SCIENCE

CHAPTER I
HEAT

I. Value of Fire. Every day, uncontrolled fire wipes out human lives and
destroys vast amounts of property; every day, fire, controlled and regulated
in stove and furnace, cooks our food and warms our houses. Fire melts ore
and allows of the forging of iron, as in the blacksmith's shop, and of the
fashioning of innumerable objects serviceable to man. Heated boilers
change water into the steam which drives our engines on land and sea. Heat
causes rain and wind, fog and cloud; heat enables vegetation to grow and
thus indirectly provides our food. Whether heat comes directly from the sun
or from artificial sources such as coal, wood, oil, or electricity, it is vitally
connected with our daily life, and for this reason the facts and theories
relative to it are among the most important that can be studied. Heat, if
properly regulated and controlled, would never be injurious to man; hence
in the following paragraphs heat will be considered merely in its helpful
capacity.
2. General Effect of Heat. Expansion and Contraction.
One of the best-known effects of heat is the change
which it causes in the size of a substance. Every
housewife knows that if a kettle is filled with cold water
to begin with, there will be an overflow as soon as the
water becomes heated. Heat causes not only water, but
all other liquids, to occupy more space, or to expand, and
in some cases the expansion, or increase in size, is
surprisingly large. For example, if 100 pints of ice water
is heated in a kettle, the 100 pints will steadily expand
until, at the boiling point, it will occupy as much space
as 104 pints of ice water.

FIG. 1.--As the water

becomes warmer it The expansion of water can be
expands and rise in easily shown by heating a flask
the narrow tube. (Fig. I) filled with water and closed
by a cork through which a narrow
tube passes. As the water is heated, it expands and
forces its way up the narrow tube. If the heat is
removed, the liquid cools, contracts, and slowly falls
in the tube, resuming in time its original size or
volume. A similar observation can be made with FIG. 2.—When the ball
alcohol, mercury, or any other convenient liquid. is heated, it become too

Not only liquids are affected by heat and cold, but large to slip through the
ring.
solids also are subject to similar changes. A metal ball
which when cool will just slip through a ring (Fig. 2)
will, when heated, be too large to slip through the ring. Telegraph and
telephone wires which in winter are stretched taut from pole to pole, sag in
hot weather and are much too long. In summer they are exposed to the
fierce rays of the sun, become strongly heated, and expand sufficiently to
sag. If the wires were stretched taut in the summer, there would not be
sufficient leeway for the contraction which accompanies cold weather, and
in winter they would snap.
Air expands greatly when heated (Fig. 3), but
since air is practically invisible, we are not
ordinarily conscious of any change in it. The
expansion of air can be readily shown by putting
a drop of ink in a thin glass tube, inserting the
tube in the cork of a flask, and applying heat to
the flask (Fig. 4). The ink is forced up the tube
by the expanding air. Even the warmth of the
hand is generally sufficient to cause the drop to
rise steadily in the tube. The rise of the drop of
ink shows that the air in the flask occupies more
FIG. 3—As the air in A is
space than formerly, and since the quantity of air
heated, it expands and escapes in
has not changed, each cubic inch of space must
the form of bubbles.
hold less warm air than| it held of cold air; that
is, one cubic inch of warm air weighs less than
one cubic inch of cold air, or warm air is less dense than cold air. All gases,

if not confined, expand when heated and contract as they cool. Heat, in
general, causes substances to expand or become less dense.
3. Amount of Expansion and Contraction. While most substances expand
when heated and contract when cooled, they are not all affected equally by
the same changes in temperature. Alcohol expands more than water, and
water more than mercury. Steel wire which measures 1/4 mile on a snowy
day will gain 25 inches in length on a warm summer day, and an aluminum
wire under the same conditions would gain 50 inches in length.
4. Advantages and Disadvantages of Expansion and Contraction. We
owe the snug fit of metal tires and bands to the expansion and contraction
resulting from heating and cooling. The tire of a wagon wheel is made
slightly smaller than the wheel which it is to protect; it is then put into a
very hot fire and heated until it has expanded sufficiently to slip on the
wheel. As the tire cools it contracts and fits the wheel closely.
In a railroad, spaces are usually left between consecutive
rails in order to allow for expansion during the summer.
The unsightly cracks and humps in cement floors are
sometimes due to the expansion resulting from heat (Fig.
5). Cracking from this cause can frequently be avoided by
cutting the soft cement into squares, the spaces between
them giving opportunity for expansion just as do the
spaces between the rails of railroads.
In the construction of
long wire fences
provision must be
made for tightening
the wire in summer,
FIG. 5: A cement walk broken by expansion
FIG. 4.—As the air otherwise great
due to sun heat.
in A is heated, it sagging would occur.
expands and forces
the drop of ink up Heat plays an important part in the splitting of rocks and
the tube. in the formation of débris. Rocks in exposed places are
greatly affected by changes in temperature, and in regions
where the changes in temperature are sudden, severe, and

frequent, the rocks are not able to withstand the strain of expansion and
contraction, and as a result crack and split. In the Sahara Desert much
crumbling of the rock into sand has been caused by the intense heat of the
day followed by the sharp frost of night. The heat of the day causes the
rocks to expand, and the cold of night causes them to contract, and these
two forces constantly at work loosen the grains of the rock and force them
out of place, thus producing crumbling.
The surface of the rock is the most exposed part,
and during the day the surface, heated by the
sun's rays, expands and becomes too large for
the interior, and crumbling and splitting result
from the strain. With the sudden fall of
temperature in the late afternoon and night, the
surface of the rock becomes greatly chilled and
colder than the rock beneath; the surface rock
therefore contracts and shrinks more than the
underlying rock, and again crumbling results
(Fig. 6).
On bare mountains,
FIG. 6.—Splitting and
the heating and
crumbling of rock caused by
cooling effects of the
alternating heat and cold.
sun are very
striking(Fig. 7); the
surface of many a mountain peak is covered
with cracked rock so insecure that a touch or
step will dislodge the fragments and start them
down the mountain slope. The lower levels of
mountains are frequently buried several feet
under débris which has been formed in this way FIG. 7.—Debris formed from
from higher peaks, and which has slowly crumbled rock.
accumulated at the lower levels.
5. Temperature. When an object feels hot to the touch, we say that it has a
high temperature; when it feels cold to the touch, that it has a low
temperature; but we are not accurate judges of heat. Ice water seems
comparatively warm after eating ice cream, and yet we know that ice water

is by no means warm. A room may seem warm to a person who has been
walking in the cold air, while it may feel decidedly cold to some one who
has come from a warmer room. If the hand is cold, lukewarm water feels
hot, but if the hand has been in very hot water and is then transferred to
lukewarm water, the latter will seem cold. We see that the sensation or
feeling of warmth is not an accurate guide to the temperature of a
substance; and yet until 1592, one hundred years after the discovery of
America, people relied solely upon their sensations for the measurement of
temperature. Very hot substances cannot be touched without injury, and
hence inconvenience as well as the necessity for accuracy led to the
invention of the thermometer, an instrument whose operation depends upon
the fact that most substances expand when heated and contract when
cooled.
6. The Thermometer. The modern
thermometer consists of a glass tube at the
lower end of which is a bulb filled with
mercury or colored alcohol (Fig. 8). After
the bulb has been filled with the mercury, it
is placed in a beaker of water and the water
is heated by a Bunsen burner. As the water
becomes warmer and warmer the level of
the mercury in the tube steadily rises until
the water boils, when the level remains
stationary (Fig. 9). A scratch is made on the
tube to indicate the point to which the
mercury rises when the bulb is placed in
boiling water, and this point is marked
212°. The tube is then removed from the FIG. 9.—Determining
boiling water, and after cooling for a few one of the fixed points
minutes, it is placed in a vessel containing of a thermometer.

finely chopped ice (Fig. 10). The mercury
column falls rapidly, but finally remains stationary, and at this
FIG. 8.— level another scratch is made on the tube and the point is
Making a marked 32°. The space between these two points, which
thermometer. represent the temperatures of boiling water and of melting ice,
is divided into 180 equal parts called degrees. The thermometer

in use in the United States is marked in this way and is called the Fahrenheit
thermometer after its designer. Before the degrees are etched on the
thermometer the open end of the tube is sealed.
The Centigrade thermometer, in use in foreign countries and in all scientific
work, is similar to the Fahrenheit except that the fixed points are marked
100° and 0°, and the interval between the points is divided into 100 equal
parts instead of into 180.
The boiling point of water is 212° F. or 100° C.
The melting point of ice is 32° F. or 0° C.
Glass thermometers of the above type are the ones most generally used, but
there are many different types for special purposes.
7. Some Uses of a Thermometer. One of the chief
values of a thermometer is the service it has rendered
to medicine. If a thermometer is held for a few
minutes under the tongue of a normal, healthy person,
the mercury will rise to about 98.4° F. If the
temperature of the body registers several degrees
above or below this point, a physician should be
consulted immediately. The temperature of the body is
a trustworthy indicator of general physical condition;
hence in all hospitals the temperature of patients is
carefully taken at stated intervals.
Commercially, temperature readings are extremely
important. In sugar refineries the temperature of the
heated liquids is observed most carefully, since a
FIG. 10.—Determining
difference in temperature, however slight, affects not
the lower fixed point of
only the general appearance of sugars and sirups, but
a thermometer.
the quality as well. The many varieties of steel
likewise show the influence which heat may have on
the nature of a substance. By observation and tedious experimentation it has
been found that if hardened steel is heated to about 450° F. and quickly
cooled, it gives the fine cutting edge of razors; if it is heated to about 500°
F. and then cooled, the metal is much coarser and is suitable for shears and

farm implements; while if it is heated but 50° F. higher, that is, to 550° F., it
gives the fine elastic steel of watch springs.
A thermometer could be put to good use in every kitchen; the
inexperienced housekeeper who cannot judge of the "heat" of
the oven would be saved bad bread, etc., if the thermometer
were a part of her equipment. The thermometer can also be used
in detecting adulterants. Butter should melt at 94° F.; if it does
not, you may be sure that it is adulterated with suet or other
cheap fat. Olive oil should be a clear liquid above 75° F.; if,
above this temperature, it looks cloudy, you may be sure that it
too is adulterated with fat.
8. Methods of Heating Buildings. Open Fireplaces and Stoves.
Before the time of stoves and furnaces, man heated his modest
dwelling by open fires alone. The burning logs gave warmth to
the cabin and served as a primitive cooking agent; and the
smoke which usually accompanies burning bodies was carried
away by means of the chimney. But in an open fireplace much
heat escapes with the smoke and is lost, and only a small
portion streams into the room and gives warmth.
When fuel is placed in an open fireplace (Fig. 12) and lighted,
the air immediately surrounding the fire becomes warmer and,
because of expansion, becomes lighter than the cold air above.
The cold air, being heavier, falls and forces the warmer air
upward, and along with the warm air goes the disagreeable
FIG. 11.—
smoke. The fall of the colder and heavier air, and the rise of the
A well-
warmer and hence lighter air, is similar to the exchange which
made
takes place when water is poured on oil; the water, being
commercial
heavier than oil, sinks to the bottom and forces the oil to the
thermomete
surface. The warmer air which escapes up the chimney carries
r.
with it the disagreeable smoke, and when all the smoke is got
rid of in this way, the chimney is said to draw well.
As the air is heated by the fire it expands, and is pushed up the chimney by
the cold air which is constantly entering through loose windows and doors.
Open fireplaces are very healthful because the air which is driven out is

impure, while the air which rushes in is fresh and brings oxygen to the
human being.
But open fireplaces, while pleasant
to look at, are not efficient for either
heating or cooking. The
possibilities for the latter are
especially limited, and the invention
of stoves was a great advance in
efficiency, economy, and comfort.
A stove is a receptacle for fire,
provided with a definite inlet for air
and a definite outlet for smoke, and
able to radiate into the room most
of the heat produced from the fire
which burns within. The inlet, or
draft, admits enough air to cause the
fire to burn brightly or slowly as the
case may be. If we wish a hot fire,
the draft is opened wide and enough FIG. 12.—The open fireplace as an early method
air enters to produce a strong glow. of heating.
If we wish a low fire, the inlet is
only partially opened, and just enough air enters to keep the fuel
smoldering.
When the fire is started, the damper should be opened wide in order to
allow the escape of smoke; but after the fire is well started there is less
smoke, and the damper may be partly closed. If the damper is kept open,
coal is rapidly consumed, and the additional heat passes out through the
chimney, and is lost to use.
9. Furnaces. Hot Air. The labor involved in the care of numerous stoves is
considerable, and hence the advent of a central heating stove, or furnace,
was a great saving in strength and fuel. A furnace is a stove arranged as in
Figure 13. The stove S, like all other stoves, has an inlet for air and an outlet
C for smoke; but in addition, it has built around it a chamber in which air
circulates and is warmed. The air warmed by the stove is forced upward by
cold air which enters from outside. For example, cold air constantly

entering at E drives the air heated
by S through pipes and ducts to the
rooms to be heated.
The metal pipes which convey the
heated air from the furnace to the
ducts are sometimes covered with
felt, asbestos, or other non-
conducting material in order that
heat may not be lost during
transmission. The ducts which
receive the heated air from the
pipes are built in the non-
conducting walls of the house, and
FIG. 13.—A furnace. Pipes conduct hot air to the hence lose practically no heat. The
rooms. air which reaches halls and rooms is
therefore warm, in spite of its long
journey from the cellar.
Not only houses are warmed by a central heating stove, but whole
communities sometimes depend upon a central heating plant. In the latter
case, pipes closely wrapped with a non-conducting material carry steam
long distances underground to heat remote buildings. Overbrook and
Radnor, Pa., are towns in which such a system is used.

FIG. 14.—Hot-water heating.

10. Hot-water Heating. The heated air which rises from furnaces is seldom
hot enough to warm large buildings well; hence furnace heating is being
largely supplanted by hot-water heating.
The principle of hot-water heating is shown by the following simple
experiment. Two flasks and two tubes are arranged as in Figure 15, the
upper flask containing a colored liquid and the lower flask clear water. If
heat is applied to B, one can see at the end of a few seconds the downward
circulation of the colored liquid and the upward circulation of the clear
water. If we represent a boiler by B, a radiator by the coiled tube, and a
safety tank by C, we shall have a very fair illustration of the principle of a
hot-water heating system. The hot water in the radiators cools and, in
cooling, gives up its heat to the rooms and thus warms them.

In hot-water heating systems, fresh air is not
brought to the rooms, for the radiators are closed
pipes containing hot water. It is largely for this
reason that thoughtful people are careful to raise
windows at intervals. Some systems of hot-water
heating secure ventilation by confining the
radiators to the basement, to which cold air from
outside is constantly admitted in such a way that it
circulates over the radiators and becomes strongly
heated. This warm fresh air then passes through
ordinary flues to the rooms above.
In Figure 16, a radiator is shown in a boxlike
FIG. 15.—The principle of
structure in the cellar. Fresh air from outside enters
hot-water heating.
a flue at the right, passes the radiator, where it is
warmed, and then makes its way to the room
through a flue at the left. The warm air which thus enters the room is
thoroughly fresh. The actual labor involved in furnace heating and in hot-
water heating is practically the same, since coal must be fed to the fire, and
ashes must be removed; but the hot-water system has the advantage of
economy and cleanliness.

FIG. 16.—Fresh air from outside circulates over the radiators and then rises into the rooms to be
heated.

11. Fresh Air. Fresh air is essential to normal healthy living, and 2000
cubic feet of air per hour is desirable for each individual. If a gentle breeze
is blowing, a barely perceptible opening of a window will give the needed

amount, even if there are no additional drafts of fresh air into the room
through cracks. Most houses are so loosely constructed that fresh air enters
imperceptibly in many ways, and whether we will or no, we receive some
fresh air. The supply is, however, never sufficient in itself and should not be
depended upon alone. At night, or at any other time when gas lights are
required, the need for ventilation increases, because every gas light in a
room uses up the same amount of air as four people.

FIG. 17.—The air which goes to the schoolrooms is warmed by passage over the radiators.

In the preceding Section, we learned that many houses heated by hot water
are supplied with fresh-air pipes which admit fresh air into separate rooms
or into suites of rooms. In some cases the amount which enters is so great
that the air in a room is changed three or four times an hour. The constant
inflow of cold air and exit of warm air necessitates larger radiators and
more hot water and hence more coal to heat the larger quantity of water, but
the additional expense is more than compensated by the gain in health.
12. Winds and Currents. The gentlest summer breezes and the fiercest
blasts of winter are produced by the unequal heating of air. We have seen
that the air nearest to a stove or hot object becomes hotter than the adjacent
air, that it tends to expand and is replaced and pushed upward and outward
by colder, heavier air falling downward. We have learned also that the
moving liquid or gas carries with it heat which it gradually gives out to
surrounding bodies.
When a liquid or a gas moves away from a hot object, carrying heat with it,
the process is called convection.

Convection is responsible for winds and ocean currents, for land and sea
breezes, and other daily phenomena.
The Gulf Stream illustrates the transference of heat by convection. A large
body of water is strongly heated at the equator, and then moves away,
carrying heat with it to distant regions, such as England and Norway.
Owing to the shape of the earth and its position with respect to the sun,
different portions of the earth are unequally heated. In those portions where
the earth is greatly heated, the air likewise will be heated; there will be a
tendency for the air to rise, and for the cold air from surrounding regions to
rush in to fill its place. In this way winds are produced. There are many
circumstances which modify winds and currents, and it is not always easy
to explain their direction and velocity, but one very definite cause is the
unequal heating of the surface of the earth.
13. Conduction. A poker used in stirring a fire becomes hot and heats the
hand grasping the poker, although only the opposite end of the poker has
actually been in the fire. Heat from the fire passed into the poker, traveled
along it, and warmed it. When heat flows in this way from a warm part of a
body to a colder part, the process is called conduction. A flatiron is heated
by conduction, the heat from the warm stove passing into the cold flatiron
and gradually heating it.
In convection, air and water circulate freely, carrying heat with them; in
conduction, heat flows from a warm region toward a cold region, but there
is no apparent motion of any kind.
Heat travels more readily through some substances than through others. All
metals conduct heat well; irons placed on the fire become heated throughout
and cannot be grasped with the bare hand; iron utensils are frequently made
with wooden handles, because wood is a poor conductor and does not allow
heat from the iron to pass through it to the hand. For the same reason a
burning match may be held without discomfort until the flame almost
reaches the hand.
Stoves and radiators are made of metal, because metals conduct heat
readily, and as fast as heat is generated within the stove by the burning of

fuel, or introduced into the radiator by the hot water, the heat is conducted
through the metal and escapes into the room.
Hot-water pipes and steam pipes are usually wrapped with a non-
conducting substance, or insulator, such as asbestos, in order that the heat
may not escape, but shall be retained within the pipes until it reaches the
radiators within the rooms.
The invention of the "Fireless Cooker" depended in part upon the principle
of non-conduction. Two vessels, one inside the other, are separated by
sawdust, asbestos, or other poor conducting material (Fig. 18). Foods are
heated in the usual way to the boiling point or to a high temperature, and
are then placed in the inner vessel. The heat of the food cannot escape
through the non-conducting material which surrounds it, and hence remains
in the food and slowly cooks it.
A very interesting experiment for the
testing of the efficacy of non-conductors
may be easily performed. Place hot water
in a metal vessel, and note by means of a
thermometer the rapidity with which the
water cools; then place water of the same
temperature in a second metal vessel
similar to the first, but surrounded by
asbestos or other non-conducting material,
FIG. 18.—A fireless cooker. and note the slowness with which the
temperature falls.
Chemical Change, an Effect of Heat. This effect of heat has a vital
influence on our lives, because the changes which take place when food is
cooked are due to it. The doughy mass which goes into the oven, comes out
a light spongy loaf; the small indigestible rice grain comes out the swollen,
fluffy, digestible grain. Were it not for the chemical changes brought about
by heat, many of our present foods would be useless to man. Hundreds of
common materials like glass, rubber, iron, aluminum, etc., are manufactured
by processes which involve chemical action caused by heat.

CHAPTER II
TEMPERATURE AND HEAT

14. Temperature not a Measure of the Amount of Heat Present. If two
similar basins containing unequal quantities of water are placed in the
sunshine on a summer day, the smaller quantity of water will become quite
warm in a short period of time, while the larger quantity will become only
lukewarm. Both vessels receive the same amount of heat from the sun, but
in one case the heat is utilized in heating to a high temperature a small
quantity of water, while in the second case the heat is utilized in warming to
a lower degree a larger quantity of water. Equal amounts of heat do not
necessarily produce equivalent temperatures, and equal temperatures do not
necessarily indicate equal amounts of heat. It takes more heat to raise a
gallon of water to the boiling point than it does to raise a pint of water to the
boiling point, but a thermometer would register the same temperature in the
two cases. The temperature of boiling water is 100° C. whether there is a
pint of it or a gallon. Temperature is independent of the quantity of matter
present; but the amount of heat contained in a substance at any temperature
is not independent of quantity, being greater in the larger quantity.
15. The Unit of Heat. It is necessary to have a unit of heat just as we have a
unit of length, or a unit of mass, or a unit of time. One unit of heat is called
a calorie, and is the amount of heat which will change the temperature of 1
gram of water 1° C. It is the amount of heat given out by 1 gram of water
when its temperature falls 1° C., or the amount of heat absorbed by 1 gram
of water when its temperature rises 1° C. If 400 grams of water are heated
from 0° to 5° C., the amount of heat which has entered the water is
equivalent to 5 × 400 or 2000 calories; if 200 grams of water cool from 25°
to 20° C., the heat given out by the water is equivalent to 5 × 200 or 1000
calories.

16. Some Substances Heat more readily than Others. If two equal
quantities of water at the same temperature are exposed to the sun for the
same length of time, their final temperatures will be the same. If, however,
equal quantities of different substances are exposed, the temperatures
resulting from the heating will not necessarily be the same. If a basin
containing 1 lb. of mercury is put on the fire, side by side with a basin
containing an equal quantity of water, the temperatures of the two
substances will vary greatly at the end of a short time. The mercury will
have a far higher temperature than the water, in spite of the fact that the
amount of mercury is as great as the amount of water and that the heat
received from the fire has been the same in each case. Mercury is not so
difficult to heat as water; less heat being required to raise its temperature 1°
than is required to raise the temperature of an equal quantity of water 1°. In
fact, mercury is 30 times as easy to heat as water, and it requires only one
thirtieth as much fire to heat a given quantity of mercury 1° as to heat the
same quantity of water 1°.
17. Specific Heat. We know that different substances are differently
affected by heat. Some substances, like water, change their temperature
slowly when heated; others, like mercury, change their temperature very
rapidly when heated. The number of calories needed by 1 gram of a
substance in order that its temperature may be increased 1° C. is called the
specific heat of a substance; or, specific heat is the number of calories given
out by 1 gram of a substance when its temperature falls 1° C. For
experiments on the determination of specific heat, see Laboratory Manual.
Water has the highest specific heat of any known substance except
hydrogen; that is, it requires more heat to raise the temperature of water a
definite number of degrees than it does to raise the temperature of an equal
amount of any other substance the same number of degrees. Practically this
same thing can be stated in another way: Water in cooling gives out more
heat than any other substance in cooling through the same number of
degrees. For this reason water is used in foot warmers and in hot-water
bags. If a copper lid were used as a foot warmer, it would give the feet
only.095 as much heat as an equal weight of water; a lead weight only.031
as much heat as water. Flatirons are made of iron because of the relatively
high specific heat of iron. The flatiron heats slowly and cools slowly, and,
because of its high specific heat, not only supplies the laundress with

considerable heat, but eliminates for her the frequent changing of the
flatiron.
18. Water and Weather. About four times as much heat is required to heat
a given quantity of water one degree as to heat an equal quantity of earth. In
summer, when the rocks and the sand along the shore are burning hot, the
ocean and lakes are pleasantly cool, although the amount of heat present in
the water is as great as that present in the earth. In winter, long after the
rocks and sand have given out their heat and have become cold, the water
continues to give out the vast store of heat accumulated during the summer.
This explains why lands situated on or near large bodies of water usually
have less variation in temperature than inland regions. In the summer the
water cools the region; in the winter, on the contrary, the water heats the
region, and hence extremes of temperature are practically unknown.
19. Sources of Heat. Most of the heat which we enjoy and use we owe to
the sun. The wood which blazes on the hearth, the coal which glows in the
furnace, and the oil which burns in the stove owe their existence to the sun.
Without the warmth of the sun seeds could not sprout and develop into the
mighty trees which yield firewood. Even coal, which lies buried thousands
of feet below the earth's surface, owes its existence in part to the sun. Coal
is simply buried vegetation,—vegetation which sprouted and grew under
the influence of the sun's warm rays. Ages ago trees and bushes grew "thick
and fast," and the ground was always covered with a deep layer of decaying
vegetable matter. In time some of this vast supply sank into the moist soil
and became covered with mud. Then rock formed, and the rock pressed
down upon the sunken vegetation. The constant pressure, the moisture in
the ground, and heat affected the underground vegetable mass, and slowly
changed it into coal.
The buried forest and thickets were not all changed into coal. Some were
changed into oil and gas. Decaying animal matter was often mixed with the
vegetable mass. When the mingled animal and vegetable matter sank into
moist earth and came under the influence of pressure, it was slowly changed
into oil and gas.
The heat of our bodies comes from the foods which we eat. Fruits, grain,
etc., could not grow without the warmth and the light of the sun. The

animals which supply our meats likewise depend upon the sun for light and
warmth.
The sun, therefore, is the great source of heat; whether it is the heat which
comes directly from the sun and warms the atmosphere, or the heat which
comes from burning coal, wood, and oil.

CHAPTER III
OTHER FACTS ABOUT HEAT

20. Boiling. Heat absorbed in Boiling. If a kettle of water is placed above a
flame, the temperature of the water gradually increases, and soon small
bubbles form at the bottom of the kettle and begin to rise through the water.
At first the bubbles do not get far in their ascent, but disappear before they
reach the surface; later, as the water gets hotter and hotter, the bubbles
become larger and more numerous, rise higher and higher, and finally reach
the surface and pass from the water into the air; steam comes from the
vessel, and the water is said to boil. The temperature at which a liquid boils
is called the boiling point.
While the water is heating, the temperature steadily rises, but as soon as the
water begins to boil the thermometer reading becomes stationary and does
not change, no matter how hard the water boils and in spite of the fact that
heat from the flame is constantly passing into the water.
If the flame is removed from the boiling water for but a second, the boiling
ceases; if the flame is replaced, the boiling begins again immediately.
Unless heat is constantly supplied, water at the boiling point cannot be
transformed into steam.
The number of calories which must be supplied to 1 gram of water at the
boiling point in order to change it into steam at the same temperature is
called the heat of vaporization; it is the heat necessary to change 1 gram of
water at the boiling point into steam of the same temperature.
21. The Amount of Heat Absorbed. The amount of heat which must be
constantly supplied to water at the boiling point in order to change it into
steam is far greater than we realize. If we put a beaker of ice water (water at
0° C.) over a steady flame, and note (1) the time which elapses before the
water begins to boil, and (2) the time which elapses before the boiling water

completely boils away, we shall see that it takes about 5-1/4 times as long to
change water into steam as it does to change its temperature from 0° C. to
100° C. Since, with a steady flame, it takes 5-1/4 times as long to change
water into steam as it does to change its temperature from 0° C. to the
boiling point, we conclude that it takes 5-1/4 times as much heat to convert
water at the boiling point into steam as it does to raise it from the
temperature of ice water to that of boiling water.
The amount of heat necessary to raise the temperature of 1 gram of water 1°
C. is equal to 1 calorie, and the amount necessary to raise the temperature
100° C. is equal to 100 calories; hence the amount of heat necessary to
convert 1 gram of water at the boiling point into steam at that same
temperature is equal to approximately 525 calories. Very careful
experiments show the exact heat of vaporization to be 536.1 calories. (See
Laboratory Manual.)
22. General Truths. Statements similar to the above hold for other liquids
and for solutions. If milk is placed upon a stove, the temperature rises
steadily until the boiling point is reached; further heating produces, not a
change in temperature, but a change of the water of the milk into steam. As
soon as the milk, or any other liquid food, comes to a boil, the gas flame
should be lowered until only an occasional bubble forms, because so long
as any bubbles form the temperature is that of the boiling point, and further
heat merely results in waste of fuel.
We find by experiment that every liquid has its own specific boiling point;
for example, alcohol boils at 78° C. and brine at 103° C. Both specific heat
and the heat of vaporization vary with the liquid used.
23. Condensation. If one holds a cold lid in the steam of boiling water,
drops of water gather on the lid; the steam is cooled by contact with the
cold lid and condenses into water. Bottles of water brought from a cold
cellar into a warm room become covered with a mist of fine drops of water,
because the moisture in the air, chilled by contact with the cold bottles,
immediately condenses into drops of water. Glasses filled with ice water
show a similar mist.
In Section 21, we saw that 536 calories are required to change 1 gram of
water into steam; if, now, the steam in turn condenses into water, it is

natural to expect a release of the heat used in transforming water into steam.
Experiment shows not only that vapor gives out heat during condensation,
but that the amount of heat thus set free is exactly equal to the amount
absorbed during vaporization. (See Laboratory Manual.)
We learn that the heat of vaporization is the same whether it is considered
as the heat absorbed by 1 gram of water in its change to steam, or as the
heat given out by 1 gram of steam during its condensation into water.
24. Practical Application. We understand now the value of steam as a
heating agent. Water is heated in a boiler in the cellar, and the steam passes
through pipes which run to the various rooms; there the steam condenses
into water in the radiators, each gram of steam setting free 536 calories of
heat. When we consider the size of the radiators and the large number of
grams of steam which they contain, and consider further that each gram in
condensing sets free 536 calories, we understand the ease with which
buildings are heated by steam.
Most of us have at times profited by the heat of condensation. In cold
weather, when there is a roaring fire in the range, the water frequently
becomes so hot that it "steams" out of open faucets. If, at such times, the hot
water is turned on in a small cold bathroom, and is allowed to run until the
tub is well filled, vapor condenses on windows, mirrors, and walls, and the
cold room becomes perceptibly warmer. The heat given out by the
condensing steam passes into the surrounding air and warms the room.
There is, however, another reason for the rise in temperature. If a large pail
of hot soup is placed in a larger pail of cold water, the soup will gradually
cool and the cold water will gradually become warmer. A red-hot iron
placed on a stand gradually cools, but warms the stand. A hot body loses
heat so long as a cooler body is near it; the cold object is heated at the
expense of the warmer object, and one loses heat and the other gains heat
until the temperature of both is the same. Now the hot water in the tub
gradually loses heat and the cold air of the room gradually gains heat by
convection, but the amount given the room by convection is relatively small
compared with the large amount set free by the condensing steam.
25. Distillation. If impure, muddy water is boiled, drops of water will
collect on a cold plate held in the path of the steam, but the drops will be

clear and pure. When impure water is boiled, the steam from it does not
contain any of the impurities because these are left behind in the vessel. If
all the water were allowed to boil away, a layer of mud or of other
impurities would be found at the bottom of the vessel. Because of this fact,
it is possible to purify water in a very simple way. Place over a fire a large
kettle closed except for a spout which is long enough to reach across the
stove and dip into a bottle. As the liquid boils, steam escapes through the
spout, and on reaching the cold bottle condenses and drops into the bottle as
pure water. The impurities remain behind in the kettle. Water freed from
impurities in this way is called distilled water, and the process is called
distillation (Fig. 19). By this method, the salt water of the ocean may be
separated into pure drinking water and salt, and many of the large ocean
liners distill from the briny deep all the drinking water used on their ocean
voyages.
Commercially, distillation is
a very important process.
Turpentine, for example, is
made by distilling the sap of
pine trees. Incisions are cut in
the bark of the long-leaf pine
trees, and these serve as
channels for the escape of
crude resin. This crude liquid
is collected in barrels and
taken to a distillery, where it
is distilled into turpentine and
rosin. The turpentine is the
product which passes off as
FIG. 19.—In order that the steam which passes through
vapor, and the rosin is the
the coiled tube may be quickly cooled and condensed,
mass left in the boiler after
cold water is made to circulate around the coil. The
the distillation of the
condensed steam escapes at w.
turpentine.
26. Evaporation. If a stopper is left off a cologne bottle, the contents of the
bottle will slowly evaporate; if a dish of water is placed out of doors on a
hot day, evaporation occurs very rapidly. The liquids which have
disappeared from the bottle and the dish have passed into the surrounding

air in the form of vapor. In Section 20, we saw that water could not pass
into vapor without the addition of heat; now the heat necessary for the
evaporation of the cologne and water was taken from the air, leaving it
slightly cooler. If wet hands are not dried with a towel, but are left to dry by
evaporation, heat is taken from the hand in the process, leaving a sensation
of coolness. Damp clothing should never be worn, because the moisture in
it tends to evaporate at the expense of the bodily heat, and this undue loss of
heat from the body produces chills. After a bath the body should be well
rubbed, otherwise evaporation occurs at the expense of heat which the body
cannot ordinarily afford to lose.
Evaporation is a slow process occurring at all times; it is hastened during
the summer, because of the large amount of heat present in the atmosphere.
Many large cities make use of the cooling effect of evaporation to lower the
temperature of the air in summer; streets are sprinkled not only to lay the
dust, but in order that the surrounding air may be cooled by the evaporation
of the water.
Some thrifty housewives economize by utilizing the cooling effects of
evaporation. Butter, cheese, and other foods sensitive to heat are placed in
porous vessels wrapped in wet cloths. Rapid evaporation of the water from
the wet cloths keeps the contents of the jars cool, and that without expense
other than the muscular energy needed for wetting the cloths frequently.
27. Rain, Snow, Frost, Dew. The heat of the sun causes constant
evaporation of the waters of oceans, rivers, streams, and marshes, and the
water vapor set free by evaporation passes into the air, which becomes
charged with vapor or is said to be humid. Constant, unceasing evaporation
of our lakes, streams, and pools would mean a steady decrease in the supply
of water available for daily use, if the escaped water were all retained by the
atmosphere and lost to the earth. But although the escaped vapor mingles
with the atmosphere, hovering near the earth's surface, or rising far above
the level of the mountains, it does not remain there permanently. When this
vapor meets a cold wind or is chilled in any way, condensation takes place,
and a mass of tiny drops of water or of small particles of snow is formed.
When these drops or particles become large enough, they fall to the earth as
rain or snow, and in this way the earth is compensated for the great loss of
moisture due to evaporation. Fog is formed when vapor condenses near the

surface of the earth, and when the drops are so small that they do not fall
but hover in the air, the fog is said "not to lift" or "not to clear."
If ice water is poured into a glass, a mist will form on the outside of the
glass. This is because the water vapor in the air becomes chilled by contact
with the glass and condenses. Often leaves and grass and sidewalks are so
cold that the water vapor in the atmosphere condenses on them, and we say
a heavy dew has formed. If the temperature of the air falls to the freezing
point while the dew is forming, the vapor is frozen and frost is seen instead
of dew.
The daily evaporation of moisture into the atmosphere keeps the
atmosphere more or less full of water vapor; but the atmosphere can hold
only a definite amount of vapor at a given temperature, and as soon as it
contains the maximum amount for that temperature, further evaporation
ceases. If clothes are hung out on a damp, murky day they do not dry,
because the air contains all the moisture it can hold, and the moisture in the
clothes has no chance to evaporate. When the air contains all the moisture it
can hold, it is said to be saturated, and if a slight fall in temperature occurs
when the air is saturated, condensation immediately begins in the form of
rain, snow, or fog. If, however, the air is not saturated, a fall in temperature
may occur without producing precipitation. The temperature at which air is
saturated and condensation begins is called the dew point.
28. How Chills are Caused. The discomfort we feel in an overcrowded
room is partly due to an excess of moisture in the air, resulting from the
breathing and perspiration of many persons. The air soon becomes saturated
with vapor and cannot take away the perspiration from our bodies, and our
clothing becomes moist and our skin tender. When we leave the crowded
"tea" or lecture and pass into the colder, drier, outside air, clothes and skin
give up their load of moisture through sudden evaporation. But evaporation
requires heat, and this heat is taken from our bodies, and a chill results.
Proper ventilation would eliminate much of the physical danger of social
events; fresh, dry air should be constantly admitted to crowded rooms in
order to replace the air saturated by the breath and perspiration of the
occupants.

29. Weather Forecasts. When the air is near the saturation point, the
weather is oppressive and is said to be very humid. For comfort and health,
the air should be about two thirds saturated. The presence of some water
vapor in the air is absolutely necessary to animal and plant life. In desert
regions where vapor is scarce the air is so dry that throat trouble
accompanied by disagreeable tickling is prevalent; fallen leaves become so
dry that they crumble to dust; plants lose their freshness and beauty.
The likelihood of rain or frost is often determined by temperature and
humidity. If the air is near saturation and the temperature is falling, it is safe
to predict bad weather, because the fall of temperature will probably cause
rapid condensation, and hence rain. If, however, the air is not near the
saturation point, a fall in temperature will not necessarily produce bad
weather.
The measurement of humidity is of far wider importance than the mere
forecasting of local weather conditions. The close relation between
humidity and health has led many institutions, such as hospitals, schools,
and factories, to regulate the humidity of the atmosphere as carefully as
they do the temperature. Too great humidity is enervating, and not
conducive to either mental or physical exertion; on the other hand, too dry
air is equally harmful. In summer the humidity conditions cannot be well
regulated, but in winter, when houses are artificially heated, the humidity of
a room can be increased by placing pans of water near the registers or on
radiators.
30. Heat Needed to Melt Substances. If a spoon is placed in a vessel of
hot water for a few seconds and then removed, it will be warmer than
before it was placed in the hot water. If a lump of melting ice is placed in
the vessel of hot water and then removed, the ice will not be warmer than
before, but there will be less of it. The heat of the water has been used in
melting the ice, not in changing its temperature.
If, on a bitter cold day, a pail of snow is brought into a warm room and a
thermometer is placed in the snow, the temperature rises gradually until 32°
F. is reached, when it becomes stationary, and the snow begins to melt. If
the pail is put on the fire, the temperature still remains 32°F., but the snow
melts more rapidly. As soon as all the snow is completely melted, however,

the temperature begins to rise and rises steadily until the water boils, when
it again becomes stationary and remains so during the passage of water into
vapor.
We see that heat must be supplied to ice at 0° C. or 32° F. in order to change
it into water, and further, that the temperature of the mixture does not rise so
long as any ice is present, no matter how much heat is supplied. The amount
of heat necessary to melt 1 gram of ice is easily calculated. (See Laboratory
Manual.)
Heat must be supplied to ice to melt it. On the other hand, water, in
freezing, loses heat, and the amount of heat lost by freezing water is exactly
equal to the amount of heat absorbed by melting ice.
The number of units of heat required to melt a unit mass of ice is called the
heat of fusion of water.
31. Climate. Water, in freezing, loses heat, even though its temperature
remains at 0° C. Because water loses heat when it freezes, the presence of
large streams of water greatly influences the climate of a region. In winter
the heat from the freezing water keeps the temperature of the surrounding
higher than it would naturally be, and consequently the cold weather is less
severe. In summer water evaporates, heat is taken from the air, and
consequently the warm weather is less intense.
32. Molding of Glass and Forging of Iron. The fire which is hot enough to
melt a lump of ice may not be hot enough to melt an iron poker; on the
other hand, it may be sufficiently hot to melt a tin spoon. Different
substances melt, or liquefy, at different temperatures; for example, ice melts
at 0° C., and tin at 233° C., while iron requires the relatively high
temperature of 1200° C. Most substances have a definite melting or
freezing point which never changes so long as the surrounding conditions
remain the same.
But while most substances have a definite melting point, some substances
do not. If a glass rod is held in a Bunsen burner, it will gradually grow
softer and softer, and finally a drop of molten glass will fall from the end of
the rod into the fire. The glass did not suddenly become a liquid at a definite
temperature; instead it softened gradually, and then melted. While glass is

in the soft, yielding, pliable state, it is molded into dishes, bottles, and other
useful objects, such as lamp shades, globes, etc. (Fig. 20). If glass melted at
a definite temperature, it could not be molded in this way. Iron acts in a
similar manner, and because of this property the blacksmith can shape his
horseshoes, and the workman can make his engines and other articles of
daily service to man.

FIG. 20.—Molten glass being rolled into a form suitable for window panes.

33. Strange Behavior of Water. One has but to remember that bottles of
water burst when they freeze, and that ice floats on water like wood, to
know that water expands on freezing or on solidifying. A quantity of water
which occupies 100 cubic feet of space will, on becoming ice, need 109
cubic feet of space. On a cold winter night the water sometimes freezes in
the water pipes, and the pipes burst. Water is very peculiar in expanding on
solidification, because most substances contract on solidifying; gelatin and
jelly, for example, contract so much that they shrink from the sides of the
dish which contains them.
If water contracted in freezing, ice would be heavier than water and would
sink in ponds and lakes as fast as it formed, and our streams and ponds
would become masses of solid ice, killing all animal and plant life. But the
ice is lighter than water and floats on top, and animals in the water beneath
are as free to live and swim as they were in the warm sunny days of
summer. The most severe winter cannot freeze a deep lake solid, and in the
coldest weather a hole made in the ice will show water beneath the surface.

Our ice boats cut and break the ice of the river, and through the water
beneath our boats daily ply their way to and fro, independent of winter and
its blighting blasts.
While most of us are familiar with the bursting of water pipes on a cold
night, few of us realize the influence which freezing water exerts on the
character of the land around us.
Water sinks into the ground and, on the approach of winter, freezes,
expanding about one tenth of its volume; the expanding ice pushes the earth
aside, the force in some cases being sufficient to dislodge even huge rocks.
In the early days in New England it was said by the farmers that "rocks
grew," because fields cleared of stones in the fall became rock covered with
the approach of spring; the rocks and stones hidden underground and
unseen in the fall were forced to the surface by the winter's expansion. We
have all seen fence posts and bricks pushed out of place because of the
heaving of the soil beneath them. Often householders must relay their
pavements and walks because of the damage done by freezing water.
The most conspicuous effect of the expansive
power of freezing water is seen in rocky or
mountainous regions (Fig. 21). Water easily
finds entrance into the cracks and crevices of the
rocks, where it lodges until frozen; then it
expands and acts like a wedge, widening cracks,
chiseling off edges, and even breaking rocks
asunder. In regions where frequent frosts occur,
the destructive action of water works constant
changes in the appearance of the land; small
cracks and crevices are enlarged, massive rocks
are pried up out of position, huge slabs are split
off, and particles large and small are forced FIG. 21.—The destruction
from the parent rock. The greater part of the caused by freezing water.
debris and rubbish brought down from the
mountain slopes by the spring rains owes its origin to the fact that water
expands when it freezes.

34. Heat Necessary to Dissolve a Substance. It requires heat to dissolve
any substance, just as it requires heat to change ice to water. If a handful of
common salt is placed in a small cup of water and stirred with a
thermometer, the temperature of the mixture falls several degrees. This is
just what one would expect, because the heat needed to liquefy the salt must
come from somewhere, and naturally it comes from the water, thereby
lowering the temperature of the water. We know very well that potatoes
cease boiling if a pinch of salt is put in the water; this is because the
temperature of the water has been lowered by the amount of heat necessary
to dissolve the salt.
Let some snow or chopped ice be placed in a vessel and mixed with one
third its weight of coarse salt; if then a small tube of cold water is placed in
this mixture, the water in the test tube will soon freeze solid. As soon as the
snow and salt are mixed they melt. The heat necessary for this comes in part
from the air and in part from the water in the test tube, and the water in the
tube becomes in consequence cold enough to freeze. But the salt mixture
does not freeze because its freezing point is far below that of pure water.
The use of salt and ice in ice-cream freezers is a practical application of this
principle. The heat necessary for melting the mixture of salt and ice is taken
from the cream which thus becomes cold enough to freeze.

CHAPTER IV
BURNING OR OXIDATION

35. Why Things Burn. The heat of our bodies comes from the food we eat;
the heat for cooking and for warming our houses comes from coal. The
production of heat through the burning of coal, or oil, or gas, or wood, is
called combustion. Combustion cannot occur without the presence of a
substance called oxygen, which exists rather abundantly in the air; that is,
one fifth of our atmosphere consists of this substance which we call oxygen.
We throw open our windows to allow fresh air to enter, and we take walks
in order to breathe the pure air into our lungs. What we need for the energy
and warmth of our bodies is the oxygen in the air. Whether we burn gas or
wood or coal, the heat which is produced comes from the power which
these various substances possess to combine with oxygen. We open the
draft of a stove that it may "draw well": that it may secure oxygen for
burning. We throw a blanket over burning material to smother the fire: to
keep oxygen away from it. Burning, or oxidation, is combining with
oxygen, and the more oxygen you add to a fire, the hotter the fire will burn,
and the faster. The effect of oxygen on combustion may be clearly seen by
thrusting a smoldering splinter into a jar containing oxygen; the smoldering
splinter will instantly flare and blaze, while if it is removed from the jar, it
loses its flame and again burns quietly. Oxygen for this experiment can be
produced in the following way.

FIG. 22.—Preparing oxygen from potassium chlorate and manganese dioxide.

36. How to Prepare Oxygen. Mix a small quantity of potassium chlorate
with an equal amount of manganese dioxide and place the mixture in a
strong test tube. Close the mouth of the tube with a one-hole rubber stopper
in which is fitted a long, narrow tube, and clamp the test tube to an iron
support, as shown in Figure 22. Fill the trough with water until the shelf is
just covered and allow the end of the delivery tube to rest just beneath the
hole in the shelf. Fill a medium-sized bottle with water, cover it with a glass
plate, invert the bottle in the trough, and then remove the glass plate. Heat
the test tube very gently, and when gas bubbles out of the tube, slip the
bottle over the opening in the shelf, so that the tube runs into the bottle. The
gas will force out the water and will finally fill the bottle. When all the
water has been forced out, slip the glass plate under the mouth of the bottle
and remove the bottle from the trough. The gas in the bottle is oxygen.
Everywhere in a large city or in a small village, smoke is seen, indicating
the presence of fire; hence there must exist a large supply of oxygen to keep
all the fires alive. The supply of oxygen needed for the fires of the world
comes largely from the atmosphere.
37. Matches. The burning material is ordinarily set on fire by matches, thin
strips of wood tipped with sulphur or phosphorus, or both. Phosphorus can
unite with oxygen at a fairly low temperature, and if phosphorus is rubbed
against a rough surface, the friction produced will raise the temperature of
the phosphorus to a point where it can combine with oxygen. The burning
phosphorus kindles the wood of the match, and from the burning match the
fire is kindled. If you want to convince yourself that friction produces heat,
rub a cent vigorously against your coat and note that the cent becomes
warm. Matches have been in use less than a hundred years. Primitive man
kindled his camp fire by rubbing pieces of dry wood together until they
took fire, and this method is said to be used among some isolated distant
tribes at the present time. A later and easier way was to strike flint and steel
together and to catch the spark thus produced on tinder or dry fungus.
Within the memory of some persons now living, the tinder box was a
valuable asset to the home, particularly in the pioneer regions of the West.
38. Safety Matches. Ordinary phosphorus, while excellent as a fire-
producing material, is dangerously poisonous, and those to whom the
dipping of wooden strips into phosphorus is a daily occupation suffer with a

terrible disease which usually attacks the teeth and bones of the jaw. The
teeth rot and fall out, abscesses form, and bones and flesh begin to decay;
the only way to prevent the spread of the disease is to remove the affected
bone, and in some instances it has been necessary to remove the entire jaw.
Then, too, matches made of yellow or white phosphorus ignite easily, and,
when rubbed against any rough surface, are apt to take fire. Many
destructive fires have been started by the accidental friction of such matches
against rough surfaces.
For these reasons the introduction of the so-called safety match was an
important event. When common phosphorus, in the dangerous and easily
ignited form, is heated in a closed vessel to about 250° C., it gradually
changes to a harmless red mass. The red phosphorus is not only harmless,
but it is difficult to ignite, and, in order to be ignited by friction, must be
rubbed on a surface rich in oxygen. The head of a safety match is coated
with a mixture of glue and oxygen-containing compounds; the surface on
which the match is to be rubbed is coated with a mixture of red phosphorus
and glue, to which finely powdered glass is sometimes added in order to
increase the friction. Unless the head of the match is rubbed on the prepared
phosphorus coating, ignition does not occur, and accidental fires are
avoided.
Various kinds of safety matches have been manufactured in the last few
years, but they are somewhat more expensive than the ordinary form, and
hence manufacturers are reluctant to substitute them for the cheaper
matches. Some foreign countries, such as Switzerland, prohibit the sale of
the dangerous type, and it is hoped that the United States will soon follow
the lead of these countries in demanding the sale of safety matches only.
39. Some Unfamiliar Forms of Burning. While most of us think of
burning as a process in which flames and smoke occur, there are in reality
many modes of burning accompanied by neither flame nor smoke. Iron, for
example, burns when it rusts, because it slowly combines with the oxygen
of the air and is transformed into new substances. When the air is dry, iron
does not unite with oxygen, but when moisture is present in the air, the iron
unites with the oxygen and turns into iron rust. The burning is slow and
unaccompanied by the fire and smoke so familiar to us, but the process is
none the less burning, or combination with oxygen. Burning which is not

accompanied by any of the appearances of ordinary burning is known as
oxidation.
The tendency of iron to rust lessens its efficiency and value, and many
devices have been introduced to prevent rusting. A coating of paint or
varnish is sometimes applied to iron in order to prevent contact with air.
The galvanizing of iron is another attempt to secure the same result; in this
process iron is dipped into molten zinc, thereby acquiring a coating of zinc,
and forming what is known as galvanized iron. Zinc does not combine with
oxygen under ordinary circumstances, and hence galvanized iron is immune
from rust.
Decay is a process of oxidation; the tree which rots slowly away is
undergoing oxidation, and the result of the slow burning is the decomposed
matter which we see and the invisible gases which pass into the atmosphere.
The log which blazes on our hearth gives out sufficient heat to warm us; the
log which decays in the forest gives out an equivalent amount of heat, but
the heat is evolved so slowly that we are not conscious of it. Burning
accompanied by a blaze and intense heat is a rapid process; burning
unaccompanied by fire and appreciable heat is a slow, gradual process,
requiring days, weeks, and even long years for its completion.
Another form of oxidation occurs daily in the human body. In Section 35
we saw that the human body is an engine whose fuel is food; the burning of
that food in the body furnishes the heat necessary for bodily warmth and the
energy required for thought and action. Oxygen is essential to burning, and
the food fires within the body are kept alive by the oxygen taken into the
body at every breath by the lungs. We see now one reason for an abundance
of fresh air in daily life.
40. How to Breathe. Air, which is essential to life and health, should enter
the body through the nose and not through the mouth. The peculiar nature
and arrangement of the membranes of the nose enable the nostrils to clean,
and warm, and moisten the air which passes through them to the lungs.
Floating around in the atmosphere are dust particles which ought not to get
into the lungs. The nose is provided with small hairs and a moist inner
membrane which serve as filters in removing solid particles from the air,
and in thus purifying it before its entrance into the lungs.

In the immediate neighborhood of three Philadelphia high schools, having
an approximate enrollment of over 8000 pupils, is a huge manufacturing
plant which day and night pours forth grimy smoke and soot into the
atmosphere which must supply oxygen to this vast group of young lives. If
the vital importance of nose breathing is impressed upon these young
people, the harmful effect of the foul air may be greatly lessened, the smoke
particles and germs being held back by the nose filters and never reaching
the lungs. If, however, this principle of hygiene is not brought to their
attention, the dangerous habit of breathing through the open, or at least
partially open, mouth will continue, and objectionable matter will pass
through the mouth and find a lodging place in the lungs.
There is another very important reason why nose breathing is preferable to
mouth breathing. The temperature of the human body is approximately 98°
F., and the air which enters the lungs should not be far below this
temperature. If air reaches the lungs through the nose, its journey is
relatively long and slow, and there is opportunity for it to be warmed before
it reaches the lungs. If, on the other hand, air passes to the lungs by way of
the mouth, the warming process is brief and insufficient, and the lungs
suffer in consequence. Naturally, the gravest danger is in winter.
41. Cause of Mouth Breathing. Some people find it difficult to breathe
through the nostrils on account of growths, called adenoids, in the nose. If
you have a tendency toward mouth breathing, let a physician examine your
nose and throat.
Adenoids not only obstruct breathing and
weaken the whole system through lack of
adequate air, but they also press upon the blood
vessels and nerves of the head and interfere with
normal brain development. Moreover, they
interfere in many cases with the hearing, and in
general hinder activity and growth. The removal
of adenoids is simple, and carries with it only
temporary pain and no danger. Some physicians
claim that the growths disappear in later years,
but even if that is true, the physical and mental FIG. 23.—Intelligent expression
development of earlier years is lost, and the

person is backward in the struggle for life and is often lacking in children with
achievement. adenoid growths.

42. How to Build a Fire. Substances differ
greatly as to the ease with which they may be made to burn or, in technical
terms, with which they may be made to unite with oxygen. For this reason,
we put light materials, like shavings, chips, and paper, on the grate, twisting
the latter and arranging it so that air (oxygen in the air) can reach a large
surface; upon this we place small sticks of wood, piling them across each
other so as to allow entrance for the oxygen; and finally upon this we place
our hard wood or coal.
The coal and the large sticks cannot be kindled with a match, but the paper
and shavings can, and these in burning will heat the large sticks until they
take fire and in turn kindle the coal.
43. Spontaneous Combustion. We often hear of fires "starting
themselves," and sometimes the statement is true. If a pile of oily rags is
allowed to stand for a time, the oily matter will begin to combine slowly
with oxygen and as a result will give off heat. The heat thus given off is at
first insufficient to kindle a fire; but as the heat is retained and accumulated,
the temperature rises, and finally the kindling point is reached and the
whole mass bursts into flames. For safety's sake, all oily cloths should be
burned or kept in metal vessels.
44. The Treatment of Burns. In spite of great caution, burns from fires,
steam, or hot water do sometimes occur, and it is well to know how to
relieve the suffering caused by them and how to treat the injury in order to
insure rapid healing.
Burns are dangerous because they destroy skin and thus open up an
entrance into the body for disease germs, and in addition because they lay
bare nerve tissue which thereby becomes irritated and causes a shock to the
entire system.
In mild burns, where the skin is not broken but is merely reddened, an
application of moist baking soda brings immediate relief. If this substance
is not available, flour paste, lard, sweet oil, or vaseline may be used.

In more severe burns, where blisters are formed, the blisters should be
punctured with a sharp, sterilized needle and allowed to discharge their
watery contents before the above remedies are applied.
In burns severe enough to destroy the skin, disinfection of the open wound
with weak carbolic acid or hydrogen peroxide is very necessary. After this
has been done, a soft cloth soaked in a solution of linseed oil and limewater
should be applied and the whole bandaged. In such a case, it is important
not to use cotton batting, since this sticks to the rough surface and causes
pain when removed.
45. Carbon Dioxide. A Product of Burning. When any fuel, such as coal,
gas, oil, or wood, burns, it sends forth gases into the surrounding
atmosphere. These gases, like air, are invisible, and were unknown to us for
a long time. The chief gas formed by a burning substance is called carbon
dioxide (CO2) because it is composed of one part of carbon and two parts of
oxygen. This gas has the distinction of being the most widely distributed
gaseous compound of the entire world; it is found in the ocean depths and
on the mountain heights, in brilliantly lighted rooms, and most abundantly
in manufacturing towns where factory chimneys constantly pour forth hot
gases and smoke.
Wood and coal, and in fact all animal and vegetable matter, contain carbon,
and when these substances burn or decay, the carbon in them unites with
oxygen and forms carbon dioxide.
The food which we eat is either animal or vegetable, and it is made ready
for bodily use by a slow process of burning within the body; carbon dioxide
accompanies this bodily burning of food just as it accompanies the fires
with which we are more familiar. The carbon dioxide thus produced within
the body escapes into the atmosphere with the breath.
We see that the source of carbon dioxide is practically inexhaustible,
coming as it does from every stove, furnace, and candle, and further with
every breath of a living organism.
46. Danger of Carbon Dioxide. When carbon dioxide occurs in large
quantities, it is dangerous to health, because it interferes with normal
breathing, lessening the escape of waste matter through the breath and

preventing the access to the lungs of the oxygen necessary for life. Carbon
dioxide is not poisonous, but it cuts off the supply of oxygen, just as water
cuts it off from a drowning man.
Since every man, woman, and child constantly breathes forth carbon
dioxide, the danger in overcrowded rooms is great, and proper ventilation is
of vital importance.
47. Ventilation. In estimating the quantity of air necessary to keep a room
well aired, we must take into account the number of lights (electric lights do
not count) to be used, and the number of people to occupy the room. The
average house should provide at the minimum 600 cubic feet of space for
each person, and in addition, arrangements for allowing at least 300 cubic
feet of fresh air per person to enter every hour.
In houses which have not a ventilating system, the air should be kept fresh
by intelligent action in the opening of doors and windows; and since
relatively few houses are equipped with a satisfactory system, the following
suggestions relative to intelligent ventilation are offered.
1. Avoid drafts in ventilation.
2. Ventilate on the sheltered side of the house. If the wind is blowing from
the north, open south windows.
48. What Becomes of the Carbon Dioxide. When we reflect that carbon
dioxide is constantly being supplied to the atmosphere and that it is
injurious to health, the question naturally arises as to how the air remains
free enough of the gas to support life. This is largely because carbon
dioxide is an essential food of plants. Through their leaves plants absorb it
from the atmosphere, and by a wonderful process break it up into its
component parts, oxygen and carbon. They reject the oxygen, which passes
back to the air, but they retain the carbon, which becomes a part of the plant
structure. Plants thus serve to keep the atmosphere free from an excess of
carbon dioxide and, in addition, furnish oxygen to the atmosphere.
49. How to Obtain Carbon Dioxide. There are several ways in which
carbon dioxide can be produced commercially, but for laboratory use the
simplest is to mix in a test tube powdered marble, or chalk, and
hydrochloric acid, and to collect the effervescing gas as shown in Figure 24.

The substance which remains in the test tube after the gas has passed off is
a solution of a salt and water. From a mixture of hydrochloric acid (HCl)
and marble are obtained a salt, water, and carbon dioxide, the desired gas.
50. A Commercial Use of Carbon
Dioxide. If a lighted splinter is thrust into
a test tube containing carbon dioxide, it is
promptly extinguished, because carbon
dioxide cannot support combustion; if a
stream of carbon dioxide and water falls
upon a fire, it acts like a blanket, covering
the flames and extinguishing them. The
value of a fire extinguisher depends upon
the amount of carbon dioxide and water FIG. 24.—Making carbon dioxide from
which it can furnish. A fire extinguisher is marble and hydrochloric acid.
a metal case containing a solution of
bicarbonate of soda, and a glass vessel full of strong sulphuric acid. As long
as the extinguisher is in an upright position, these substances are kept
separate, but when the extinguisher is inverted, the acid escapes from the
bottle, and mixes with the soda solution. The mingling liquids interact and
liberate carbon dioxide. A part of the gas thus liberated dissolves in the
water of the soda solution and escapes from the tube with the outflowing
liquid, while a portion remains undissolved and escapes as a stream of gas.
The fire extinguisher is therefore the source of a liquid containing the fire-
extinguishing substance and further the source of a stream of carbon
dioxide gas.
51. Carbon. Although carbon dioxide is very injurious to health, both of the
substances of which it is composed are necessary to life. We ourselves, our
bones and flesh in particular, are partly carbon, and every animal, no matter
how small or insignificant, contains some carbon; while the plants around
us, the trees, the grass, the flowers, contain a by no means meager quantity
of carbon.
Carbon plays an important and varied role in our life, and, in some one of
its many forms, enters into the composition of most of the substances which
are of service and value to man. The food we eat, the clothes we wear, the
wood and coal we burn, the marble we employ in building, the

indispensable soap, and the ornamental diamond, all
contain carbon in some form.
52. Charcoal. One of the most valuable forms of
carbon is charcoal; valuable not in the sense that it
costs hundreds of dollars, but in the more vital sense,
that its use adds to the cleanliness, comfort, and health
of man.
The foul, bad-smelling gases which arise from sewers
can be prevented from escaping and passing to streets
and buildings by placing charcoal filters at the sewer
exits. Charcoal is porous and absorbs foul gases, and
thus keeps the region surrounding sewers sweet and
clean and free of odor. Good housekeepers drop small
bits of charcoal into vases of flowers to prevent
discoloration of the water and the odor of decaying
FIG. 25.—Inside view stems.
of a fire extinguisher.
If impure water filters through charcoal, it emerges
pure, having left its impurities in the pores of the
charcoal. Practically all household filters of drinking water are made of
charcoal. But such a device may be a source of disease instead of a
prevention of disease, unless the filter is regularly cleaned or renewed. This
is because the pores soon become clogged with the impurities, and unless
they are cleaned, the water which flows through the filter passes through a
bed of impurities and becomes contaminated rather than purified. Frequent
cleansing or renewal of the filter removes this difficulty.
Commercially, charcoal is used on a large scale in the refining of sugars,
sirups, and oils. Sugar, whether it comes from the maple tree, or the sugar
cane, or the beet, is dark colored. It is whitened by passage through filters
of finely pulverized charcoal. Cider and vinegar are likewise cleared by
passage through charcoal.
The value of carbon, in the form of charcoal, as a purifier is very great,
whether we consider it a deodorizer, as in the case of the sewage, or a
decolorizer, as in the case of the refineries, or whether we consider the

service it has rendered man in the elimination of danger from drinking
water.
53. How Charcoal is Made. Charcoal may be made by heating wood in an
oven to which air does not have free access. The absence of air prevents
ordinary combustion, nevertheless the intense heat affects the wood and
changes it into new substances, one of which is charcoal.
The wood which smolders on the hearth and in the stove is charcoal in the
making. Formerly wood was piled in heaps, covered with sod or sand to
prevent access of oxygen, and then was set fire to; the smoldering wood, cut
off from an adequate supply of air, was slowly transformed into charcoal.
Scattered over the country one still finds isolated charcoal kilns, crude
earthen receptacles, in which wood thus deprived of air was allowed to
smolder and form charcoal. To-day charcoal is made commercially by
piling wood on steel cars and then pushing the cars into strong walled
chambers. The chambers are closed to prevent access of air, and heated to a
high temperature. The intense heat transforms the wood into charcoal in a
few hours. A student can make in the laboratory sufficient charcoal for art
lessons by heating in an earthen vessel wood buried in sand. The process
will be slow, however, because the heat furnished by a Bunsen burner is not
great, and the wood is transformed slowly.
A form of charcoal known as animal charcoal, or bone black, is obtained
from the charred remains of animals rather than plants, and may be
prepared by burning bones and animal refuse as in the case of the wood.
Destructive Distillation. When wood is burned without sufficient air, it is
changed into soft brittle charcoal, which is very different from wood. It
weighs only one fourth as much as the original wood. It is evident that
much matter must leave the wood during the process of charcoal making.
We can prove this by putting some dry shavings in a strong test tube fitted
with a delivery tube. When the wood is heated a gas passes off which we
may collect and burn. Other substances also come off in gaseous form, but
they condense in the water. Among these are wood alcohol, wood tar, and
acetic acid. In the older method of charcoal making all these products were
lost. Can you give any uses of these substances?

54. Matter and Energy. When wood is burned, a small pile of ashes is left,
and we think of the bulk of the wood as destroyed. It is true we have less
matter that is available for use or that is visible to sight, but, nevertheless,
no matter has been destroyed. The matter of which the wood is composed
has merely changed its character, some of it is in the condition of ashes, and
some in the condition of invisible gases, such as carbon dioxide, but none of
it has been destroyed. It is a principle of science that matter can neither be
destroyed nor created; it can only be changed, or transformed, and it is our
business to see that we do not heedlessly transform it into substances which
are valueless to us and our descendants; as, for example, when our
magnificent forests are recklessly wasted. The smoke, gases, and ashes left
in the path of a raging forest fire are no compensation to us for the valuable
timber destroyed. The sum total of matter has not been changed, but the
amount of matter which man can use has been greatly lessened.
The principle just stated embodies one of the fundamental laws of science,
called the law of the conservation of matter.
A similar law holds for energy as well. We can transform electric energy
into the motion of trolley cars, or we can make use of the energy of streams
to turn the wheels of our mills, but in all these cases we are transforming,
not creating, energy.
When a ball is fired from a rifle, most of the energy of the gunpowder is
utilized in motion, but some is dissipated in producing a flash and a report,
and in heat. The energy of the gunpowder has been scattered, but the sum of
the various forms of energy is equal to the energy originally stored away in
the powder. The better the gun is, the less will be the energy dissipated in
smoke and heat and noise.

CHAPTER V
FOOD

55. The Body as a Machine. Wholesome food and fresh air are necessary
for a healthy body. Many housewives, through ignorance, supply to their
hard-working husbands and their growing sons and daughters food which
satisfies the appetite, but which does not give to the body the elements
needed for daily work and growth. Some foods, such as lettuce, cucumbers,
and watermelons, make proper and satisfactory changes in diet, but are not
strength giving. Other foods, like peas and beans, not only satisfy the
appetite, but supply to the body abundant nourishment. Many immigrants
live cheaply and well with beans and bread as their main diet.
It is of vital importance that the relative value of different foods as heat
producers be known definitely; and just as the yard measures length and the
pound measures weight the calorie is used to measure the amount of heat
which a food is capable of furnishing to the body. Our bodies are human
machines, and, like all other machines, require fuel for their maintenance.
The fuel supplied to an engine is not all available for pulling the cars; a
large portion of the fuel is lost in smoke, and another portion is wasted as
ashes. So it is with the fuel that runs the body. The food we eat is not all
available for nourishment, much of it being as useless to us as are smoke
and ashes to an engine. The best foods are those which do the most for us
with the least possible waste.
56. Fuel Value. By fuel value is meant the capacity foods have for yielding
heat to the body. The fuel value of the foods we eat daily is so important a
factor in life that physicians, dietitians, nurses, and those having the care of
institutional cooking acquaint themselves with the relative fuel values of
practically all of the important food substances. The life or death of a
patient may be determined by the patient's diet, and the working and
earning capacity of a father depends largely upon his prosaic three meals.

An ounce of fat, whether it is the fat of meat
or the fat of olive oil or the fat of any other
food, produces in the body two and a quarter
times as much heat as an ounce of starch. Of
the vegetables, beans provide the greatest
nourishment at the least cost, and to a large
extent may be substituted for meat. It is not
uncommon to find an outdoor laborer
consuming one pound of beans per day, and
taking meat only on "high days and holidays."
The fuel value of a food is determined by
means of the bomb calorimeter (Fig. 26). The
food substance is put into a chamber A and FIG. 26.—The bomb calorimeter
ignited, and the heat of the burning substance from which the fuel value of food
raises the temperature of the water in the can be estimated.
surrounding vessel. If 1000 grams of water are
in the vessel, and the temperature of the water is raised 2° C., the number of
calories produced by the substance would be 2000, and the fuel value would
be 2000 calories.[A] From this the fuel value of one quart or one pound of
the substance can be determined, and the food substance will be said to
furnish the body with that number of heat units, providing all of the pound
of food were properly digested.
[Footnote A: As applied to food, the calorie is greater than that used in the
ordinary laboratory work, being the amount of heat necessary to raise the
temperature of 1000 grams of water 1° C., rather than 1 gram 1° C.]

TABLE SHOWING THE NUMBER OF CALORIES FURNISHED BY
ONE POUND OF VARIOUS FOODS

FOOD CALORIES FOOD CALORIES
Leg of lean
790 Carrots 210
mutton
Rib of beef 1150 Lettuce 90
Shad 380 Onion 225
Chicken 505 Cucumber 80

Apples 290 Almonds 3030
Bananas 460 Walnuts 3306
Prunes 370 Peanuts 2560
Watermelons 140 Oatmeal 4673
Rolled
Lima beans 570 4175
wheat
Beets 215 Macaroni 1665

57. Varied Diet. The human body is a much more varied and complex
machine than any ever devised by man; personal peculiarities, as well as
fuel values, influence very largely the diet of an individual. Strawberries are
excluded from some diets because of a rash which is produced on the skin,
pork is excluded from other diets for a like reason; cauliflower is absolutely
indigestible to some and is readily digested by others. From practically
every diet some foods must be excluded, no matter what the fuel value of
the substance may be.
Then, too, there are more uses for food than the production of heat. Teeth
and bones and nails need a constant supply of mineral matter, and mineral
matter is frequently found in greatest abundance in foods of low fuel value,
such as lettuce, watercress, etc., though practically all foods yield at least a
small mineral constituent. When fuel values alone are considered, fruits
have a low value, but because of the flavor they impart to other foods, and
because of the healthful influence they exercise in digestion, they cannot be
excluded from the diet.
Care should be constantly exercised to provide substantial foods of high
fuel value. But the nutritive foods should be wisely supplemented by such
foods as fruits, whose real value is one of indirect rather then direct service.
58. Our Bodies. Somewhat as a house is composed of a group of bricks, or
a sand heap of grains of sand, the human body is composed of small
divisions called cells. Ordinarily we cannot see these cells because of their
minuteness, but if we examine a piece of skin, or a hair of the head, or a
tiny sliver of bone under the microscope, we see that each of these is
composed of a group of different cells. A merchant, watchful about the
fineness of the wool which he is purchasing, counts with his lens the

number of threads to the inch; a physician, when he wishes, can, with the
aid of the microscope, examine the cells in a muscle, or in a piece of fat, or
in a nerve fiber. Not only is the human body composed of cells, but so also
are the bodies of all animals from the tiny gnat which annoys us, and the fly
which buzzes around us, to the mammoth creatures of the tropics. These
cells do the work of the body, the bone cells build up the skeleton, the nail
cells form the finger and toe nails, the lung cells take care of breathing, the
muscle cells control motion, and the brain cells are responsible for thought.
59. Why we eat so Much. The cells of the body are constantly, day by day,
minute by minute, breaking down and needing repair, are constantly
requiring replacement by new cells, and, in the case of the child, are
continually increasing in number. The repair of an ordinary machine, an
engine, for example, is made at the expense of money, but the repair and
replacement of our human cell machinery are accomplished at the expense
of food. More than one third of all the food we eat goes to maintain the
body cells, and to keep them in good order. It is for this reason that we
consume a large quantity of food. If all the food we eat were utilized for
energy, the housewife could cook less, and the housefather could save
money on grocer's and butcher's bills. If you put a ton of coal in an engine,
its available energy is used to run the engine, but if the engine were like the
human body, one third of the ton would be used up by the engine in keeping
walls, shafts, wheels, belts, etc., in order, and only two thirds would go
towards running the engine. When an engine is not working, fuel is not
consumed, but the body requires food for mere existence, regardless of
whether it does active work or not. When we work, the cells break down
more quickly, and the repair is greater than when we are at rest, and hence
there is need of a larger amount of food; but whether we work or not, food
is necessary.
60. The Different Foods. The body is very exacting in its demands,
requiring certain definite foods for the formation and maintenance of its
cells, and other foods, equally definite, but of different character, for heat;
our diet therefore must contain foods of high fuel value, and likewise foods
of cell-forming power.
Although the foods which we eat are of widely different character, such as
fruits, vegetables, cereals, oils, meats, eggs, milk, cheese, etc., they can be

put into three great classes: the carbohydrates, the fats, and the proteids.
61. The Carbohydrates. Corn, wheat, rye, in fact all cereals and grains,
potatoes, and most vegetables are rich in carbohydrates; as are also sugar,
molasses, honey, and maple sirup. The foods of the first group are valuable
because of the starch they contain; for example, corn starch, wheat starch,
potato starch. The substances of the second group are valuable because of
the sugar they contain; sugar contains the maximum amount of
carbohydrate. In the sirups there is a considerable quantity of sugar, while
in some fruits it is present in more or less dilute form. Sweet peaches,
apples, grapes, contain a moderate amount of sugar; watermelons, pears,
etc., contain less. Most of our carbohydrates are of plant origin, being found
in vegetables, fruits, cereals, and sirups.
Carbohydrates, whether of the starch group or the sugar group, are
composed chiefly of three elements: carbon, hydrogen, and oxygen; they
are therefore combustible, and are great energy producers. On the other
hand, they are worthless for cell growth and repair, and if we limited our
diet to carbohydrates, we should be like a man who had fuel but no engine
capable of using it.
62. The Fats. The best-known fats are butter, lard, olive oil, and the fats of
meats, cheese, and chocolate. When we test fats for fuel values by means of
a calorimeter (Fig. 26), we find that they yield twice as much heat as the
carbohydrates, but that they burn out more quickly. Dwellers in cold
climates must constantly eat large quantities of fatty foods if they are to
keep their bodies warm and survive the extreme cold. Cod liver oil is an
excellent food medicine, and if taken in winter serves to warm the body and
to protect it against the rigors of cold weather. The average person avoids
fatty foods in summer, knowing from experience that rich foods make him
warm and uncomfortable. The harder we work and the colder the weather,
the more food of that kind do we require; it is said that a lumberman doing
heavy out-of-door work in cold climates needs three times as much food as
a city clerk. Most of our fats, like lard and butter, are of animal origin; some
of them, however, like olive oil, peanut butter, and coconut oil, are of plant
origin.

63. The Proteids. The proteids are the
building foods, furnishing muscle, bone,
skin cells, etc., and supplying blood and
other bodily fluids. The best-known
proteids are white of egg, curd of milk,
and lean of fish and meat; peas and beans
FIG. 27.—a is the amount of fat
have an abundant supply of this
necessary to make one calorie; b is the
substance, and nuts are rich in it. Most of
amount of sugar or proteid necessary to
our proteids are of animal origin, but
make one calorie.
some protein material is also found in the
vegetable world. This class of foods
contains carbon, oxygen, and hydrogen,
and in addition, two substances not found in carbohydrates or fats—namely,
sulphur and nitrogen. Proteids always contain nitrogen, and hence they are
frequently spoken of as nitrogenous foods. Since the proteids contain all the
elements found in the two other classes of foods, they are able to contribute,
if necessary, to the store of bodily energy; but their main function is
upbuilding, and the diet should be chosen so that the proteids do not have a
double task.
For an average man four ounces of dry proteid matter daily will suffice to
keep the body cells in normal condition.
It has been estimated that 300,000,000 blood cells alone need daily repair or
renewal. When we consider that the blood is but one part of the body, and
that all organs and fluids have corresponding requirements, we realize how
vast is the work to be done by the food which we eat.

FIG. 28.—Table of food values.

64. Mistakes in Buying. The body demands a daily ration of the three
classes of food stuffs, but it is for us to determine from what meats,
vegetables, fruits, cereals, etc., this supply shall be obtained (Figs. 28 and
29).

FIG. 29.—Diagram showing the difference in the cost of three foods which give about the same
amount of nutrition each.

Generally speaking, meats are the most expensive foods we can purchase,
and hence should be bought seldom and in small quantities. Their place can
be taken by beans, peas, potatoes, etc., and at less than a quarter of the cost.
The average American family eats meat three times a day, while the average
family of the more conservative and older countries rarely eats meat more
than once a day. The following tables indicate the financial loss arising
from an unwise selection of foods:—
FOOD CONSUMED—ONE WEEK

FAMILY No. 1 FAMILY No. 2
15 lb. flour, bread
20 loaves of bread $ 1.00 home-made $ 0.45
(skim milk used)
10 to 12 lb. loin
Yeast, shortening,
steak or meat 2.00 0.10
and skim milk
similar cost
20 to 25 lb. rib 10 lb. steak (round,
roast or similar 4.40 Hamburger and 1.50
meat some loin)

4 lb. high-priced 10 lb. other meats,
cereal 0.80 boiling pieces, rump 1.00
breakfast food, 20¢ roast, etc.
Cake and pastry
3.00 5 lb. cheese, 16¢ 0.80
purchased
8 lb. butter, 30¢ 2.40 5 lb. oatmeal (bulk) 0.15
Tea, coffee, spices,
0.75 5 lb. beans 0.25
etc.
Home-made cake
Mushrooms 0.75 1.00
and pastry
Celery 1.00 6 lb. butter, 30¢ 1.80
3 lb. home-made
Oranges 2.00 0.25
shortening
Tea, coffee, and
Potatoes 0.25 0.40
spices
Miscellaneous
2.00 Apples 0.50
canned goods
Milk 0.50 Prunes 0.25
Miscellaneous
2.00 Potatoes 0.25
foods
3 doz. eggs 0.60 Milk 1.00
Miscellaneous foods 1.00
3 doz. eggs 0.60
$23.45 $11.30

"The tables show that one family spends over twice as much in the purchase
of foods as the other family, and yet the one whose food costs the less
actually secures the larger amount of nutritive material and is better fed than
the family where more money is expended."—From Human Foods, Snyder.
The Source of the Different Foods. All of our food comes from either the
plant world or the animal world. Broadly speaking, plants furnish the
carbohydrates, that is, starch and sugar; animals furnish the fats and
proteids. But although vegetable foods yield carbohydrates mainly, some of

them, like beans and peas, contain large quantities of protein and can be
substituted for meat without disadvantage to the body. Other plant products,
such as nuts, have fat as their most abundant food constituent. The peanut,
for example, contains 43% of fat, 30% of proteids, and only 17% of
carbohydrates; the Brazil nut has 65% of fat, 17% of proteids, and only 9%
of carbohydrates. Nuts make a good meat substitute, and since they contain
a fair amount of carbohydrates besides the fats and proteins, they supply all
of the essential food constituents and form a well-balanced food.

CHAPTER VI
WATER

65. Destructive Action of Water. The action of water in stream and sea, in
springs and wells, is evident to all; but the activity of ground water—that is,
rain water which sinks into the soil and remains there—is little known in
general. The real activity of ground water is due to its great solvent power;
every time we put sugar into tea or soap into water we are using water as a
solvent. When rain falls, it dissolves substances floating in the atmosphere,
and when it sinks into the ground and becomes ground water, it dissolves
material out of the rock which it encounters (Fig. 30). We know that water
contains some mineral matter, because kettles in which water is boiled
acquire in a short time a crust or coating on the inside. This crust is due to
the accumulation in the kettle of mineral matter which was in solution in the
water, but which was left behind when the water evaporated. (See Section
25.)
The amount of dissolved mineral
matter present in some wells and
springs is surprisingly great; the
famous springs of Bath, England,
contain so much mineral matter in
solution, that a column 9 feet in
diameter and 140 feet high could be
FIG. 30.—Showing how caves and holes are built out of the mineral matter
formed by the solvent action of water. contained in the water consumed
yearly by the townspeople.
Rocks and minerals are not all equally soluble in water; some are so little
soluble that it is years before any change becomes apparent, and the
substances are said to be insoluble, yet in reality they are slowly dissolving.
Other rocks, like limestone, are so readily soluble in water that from the

small pores and cavities eaten out by the water, there may develop in long
centuries, caves and caverns (Fig. 30). Most rock, like granite, contains
several substances, some of which are readily soluble and others of which
are not readily soluble; in such rocks a peculiar appearance is presented,
due to the rapid disappearance of the soluble substance, and the persistence
of the more resistant substance (Fig. 31).
We see that the solvent power of water is
constantly causing changes, dissolving some
mineral substances, and leaving others
practically untouched; eating out crevices of
various shapes and sizes, and by gradual
solution through unnumbered years enlarging
these crevices into wonderful caves, such as the
Mammoth Cave of Kentucky.
66. Constructive Action of Water. Water does
not always act as a destructive agent; what it
breaks down in one place it builds up in another.
It does this by means of precipitation. Water
dissolves salt, and also dissolves lead nitrate, but FIG. 31.—The work of water as
if a salt solution is mixed with a lead nitrate a solvent.
solution, a solid white substance is formed in the
water (Fig. 32). This formation of a solid
substance from the mingling of two liquids is called precipitation; such a
process occurs daily in the rocks beneath the surface of the earth. (See
Laboratory Manual.)
Suppose water from different sources enters a crack in a rock, bringing
different substances in solution; then the mingling of the waters may cause
precipitation, and the solid thus formed will be deposited in the crack and
fill it up. Hence, while ground water tends to make rock porous and weak
by dissolving out of it large quantities of mineral matter, it also tends under
other conditions to make it more compact because it deposits in cracks,
crevices, and pores the mineral matter precipitated from solution.
These two forces are constantly at work; in some places the destructive
action is more prominent, in other places the constructive action; but always

the result is to
change the
character of
the original
substance.
When the
mineral matter
precipitated
FIG. 32.—From the mingling of two
from the
liquids a solid is sometimes formed.
solutions is
deposited in
cracks, veins
are formed (Fig. 33), which may consist of the
ore of different metals, such as gold, silver, FIG. 33.—Mineral matter
copper, lead, etc. Man is almost entirely precipitated from solution is
dependent upon these veins for the supply of deposited in crevices and forms
metal needed in the various industries, because veins.
in the original condition of the rocks, the
metallic substances are so scattered that they
cannot be profitably extracted.
Naturally, the veins themselves are not composed of one substance alone,
because several different precipitates may be formed. But there is a decided
grouping of valuable metals, and these can then be readily separated by
means of electricity.
67. Streams. Streams usually carry mud and sand along with them; this is
particularly well seen after a storm when rivers and brooks are muddy. The
puddles which collect at the foot of a hill after a storm are muddy because
of the particles of soil gathered by the water as it runs down the hill. The
particles are not dissolved in the water, but are held there in suspension, as
we call it technically. The river made muddy after a storm by suspended
particles usually becomes clear and transparent after it has traveled onward
for miles, because, as it travels, the particles drop to the bottom and are
deposited there. Hence, materials suspended in the water are borne along
and deposited at various places (Fig. 34). The amount of deposition by large
rivers is so great that in some places channels fill up and must be dredged

annually, and vessels are sometimes caught in the deposit and have to be
towed away.

FIG. 34.—Deposit left by running water.

Running water in the form of streams and rivers, by carrying sand particles,
stones, and rocks from high slopes and depositing them at lower levels,
wears away land at one place and builds it up at another, and never ceases
in its work of changing the nature of the earth's surface (Fig. 35).

FIG. 35.—Water by its action constantly changes the character of the land.

68. Relation of Water to Human Life. Water is one of the most essential
of food materials, and whether we drink much or little water, we
nevertheless get a great deal of it. The larger part of many of our foods is
composed of water; more than half of the weight of the meat we eat is made
up of water; and vegetables are often more than nine tenths water. (See
Laboratory Manual.) Asparagus and tomatoes have over 90 per cent. of
water, and most fruits are more than three fourths water; even bread, which
contains as little water as any of our common foods, is about one third
water (Fig. 36).

Without water, solid food material, although
present in the body, would not be in a
condition suitable for bodily use. An
abundant supply of water enables the food to
be dissolved or suspended in it, and in
solution the food material is easily distributed
to all parts of the body.
Further, water assists in the removal of the
daily bodily wastes, and thus rids the system
of foul and poisonous substances.
The human body itself consists largely of
water; indeed, about two thirds of our own
weight is water. The constant replenishing of FIG. 36.—Diagram of the
this large quantity is necessary to life, and a composition of a loaf of bread and of
considerable amount of the necessary supply a potato:
is furnished by foods, particularly the fruits 1. ash; 2, food; 3, water.
and vegetables.
But while the supply furnished by the daily food is considerable, it is by no
means sufficient, and should be supplemented by good drinking water.
69. Water and its Dangers. Our drinking water comes from far and near,
and as it moves from place to place, it carries with it in solution or
suspension anything which it can find, whether it be animal, vegetable, or
mineral matter. The power of water to gather up matter is so great that the
average drinking water contains 20 to 90 grains of solid matter per gallon;
that is, if a gallon of ordinary drinking water is left to evaporate, a residue
of 20 to 90 grains will be left. (See Laboratory Manual.) As water runs
down a hill slope (Fig. 37), it carries with it the filth gathered from acres of
land; carries with it the refuse of stable, barn, and kitchen; and too often this
impure surface water joins the streams which supply our cities. Lakes and
rivers which furnish drinking water should be carefully protected from
surface draining; that is, from water which has flowed over the land and has
thus accumulated the waste of pasture and stable and, it may be, of dumping
ground.

FIG. 37.—As water flows over the land, it gathers filth and disease germs.

It is not necessary that water should be absolutely free from all foreign
substances in order to be safe for daily use in drinking; a limited amount of
mineral matter is not injurious and may sometimes be really beneficial. It is
the presence of animal and vegetable matter that causes real danger, and it
is known that typhoid fever is due largely to such impurities present in the
drinking water.
70. Methods of Purification. Water is improved by any of the following
methods:—
(a) Boiling. The heat of boiling destroys animal and vegetable germs.
Hence water that has been boiled a few minutes is safe to use. This is the
most practical method of purification in the home, and is very efficient. The
boiled water should be kept in clean, corked bottles; otherwise foreign
substances from the atmosphere reënter the water, and the advantage gained
from boiling is lost.
(b) Distillation. By this method pure water is obtained, but this method of
purification cannot be used conveniently in the home (Section 25).
(c) Filtration. In filtration, the water is forced through porcelain or other
porous substances which allow the passage of water, but which hold back
the minute foreign particles suspended in the water. (See Laboratory
Manual.) The filters used in ordinary dwellings are of stone, asbestos, or

charcoal. They are often valueless, because they soon become choked and
cannot be properly cleaned.
The filtration plants owned and operated by large cities are usually safe;
there is careful supervision of the filters, and frequent and effective
cleanings are made. In many cities the filtration system is so good that
private care of the water supply is unnecessary.
71. The Source of Water. In the beginning, the earth was stored with water
just as it was with metal, rock, etc. Some of the water gradually took the
form of rivers, lakes, streams, and wells, as now, and it is this original
supply of water which furnishes us all that we have to-day. We quarry to
obtain stone and marble for building, and we fashion the earth's treasures
into forms of our own, but we cannot create these things. We bore into the
ground and drill wells in order to obtain water from hidden sources; we
utilize rapidly flowing streams to drive the wheels of commerce, but the
total amount of water remains practically unchanged.
The water which flows on the earth is constantly changing its form; the heat
of the sun causes it to evaporate, or to become vapor, and to mingle with the
atmosphere. In time, the vapor cools, condenses, and falls as snow or rain;
the water which is thus returned to the earth feeds our rivers, lakes, springs,
and wells, and these in turn supply water to man. When water falls upon a
field, it soaks into the ground, or collects in puddles which slowly
evaporate, or it runs off and drains into small streams or into rivers. That
which soaks into the ground is the most valuable because it remains on the
earth longest and is the purest.
Water which soaks into the ground moves slowly downward and after a
longer or shorter journey, meets with a non-porous layer of rock through
which it cannot pass, and which effectually hinders its downward passage.
In such regions, there is an accumulation of water, and a well dug there
would have an abundant supply of water. The non-porous layer is rarely
level, and hence the water whose vertical path is obstructed does not "back
up" on the soil, but flows down hill parallel with the obstructing non-porous
layer, and in some distant region makes an outlet for itself, forming a spring
(Fig. 38). The streams originating in the springs flow through the land and
eventually join larger streams or rivers; from the surface of streams and

rivers evaporation occurs, the water
once more becomes vapor and passes
into the atmosphere, where it is
condensed and again falls to the earth.
Water which has filtered through
many feet of earth is far purer and
safer than that which fell directly into
the rivers, or which ran off from the
land and joined the surface streams
without passing through the soil.
FIG. 38.—How springs are formed. A, porous
72. The Composition of Water.
layer; B, non-porous layer; C, spring.
Water was long thought to be a simple
substance, but toward the end of the eighteenth century it was found to
consist of two quite different substances, oxygen (O) and hydrogen (H.)
If we send an electric current
through water (acidulated to make it
a good conductor), as shown in
Figure 39, we see bubbles of gas
rising from the end of the wire by
which the current enters the water,
and other bubbles of gas rising from
the end of the wire by which the
current leaves the water. These
gases have evidently come from the
water and are the substances of
FIG. 39.—The decomposition of water.
which it is composed, because the
water begins to disappear as the
gases are formed. If we place over each end of the wire an inverted jar filled
with water, the gases are easily collected. The first thing we notice is that
there is always twice as much of one gas as of the other; that is, water is
composed of two substances, one of which is always present in twice as
large quantities as the other.
73. The Composition of Water. On testing the gases into which water is
broken up by an electric current, we find them to be quite different. One

proves to be oxygen, a substance with which we are already familiar. The
other gas, hydrogen, is new to us and is interesting as being the lightest
known substance, being even "lighter than a feather."
An important fact about hydrogen is that in burning it gives as much heat as
five times its weight of coal. Its flame is blue and almost invisible by
daylight, but intensely hot. If fine platinum wire is placed in an ordinary gas
flame, it does not melt, but if placed in a flame of burning hydrogen, it
melts very quickly.
74. How to prepare Hydrogen. There are many different methods of
preparing hydrogen, but the easiest laboratory method is to pour sulphuric
acid, or hydrochloric acid, on zinc shavings and to collect in a bottle the gas
which is given off. This gas proves to be colorless, tasteless, and odorless.
(See Laboratory Manual.)

CHAPTER VII
AIR

75. The Instability of the Air. We are usually not conscious of the air
around us, but sometimes we realize that the air is heavy, while at other
times we feel the bracing effect of the atmosphere. We live in an ocean of
air as truly as fish inhabit an ocean of water. If you have ever been at the
seashore you know that the ocean is never still for a second; sometimes the
waves surge back and forth in angry fury, at other times the waves glide
gently in to the shore and the surface is as smooth as glass; but we know
that there is perpetual motion of the water even when the ocean is in its
gentlest moods. Generally our atmosphere is quiet, and we are utterly
unconscious of it; at other times we are painfully aware of it, because of its
furious winds. Then again we are oppressed by it because of the vast
quantity of vapor which it holds in the form of fog, or mist. The atmosphere
around us is as restless and varying as is the water of the sea. The air at the
top of a high tower is very different from the air at the base of the tower.
Not only does the atmosphere vary greatly at different altitudes, but it varies
at the same place from time to time, at one period being heavy and raw, at
another being fresh and invigorating.
Winds, temperature, and humidity all have a share in determining
atmospheric conditions, and no one of these plays a small part.
76. The Character of the Air. The atmosphere which envelops us at all
times extends more than fifty miles above us, its height being far greater
than the greatest depths of the sea. This atmosphere varies from place to
place; at the sea level it is heavy, on the mountain top less heavy, and far
above the earth it is so light that it does not contain enough oxygen to
permit man to live. Figure 40 illustrates by a pile of pillows how the
pressure of the air varies from level to level.

Sea level is a low portion of the earth's
surface, hence at sea level there is a high
column of air, and a heavy air pressure.
As one passes from sea level to mountain
top a gradual but steady decrease in the
height of the air column occurs, and hence
a gradual but definite lessening of the air
pressure.
77. Air
Pressure. If
an empty tube
(Fig. 41) is
placed upright
FIG. 40.—To illustrate the decrease in
in water, the
pressure with height.
water will not
rise in the
tube, but if the tube is put in water and the air is
then drawn out of the tube by the mouth, the
water will rise in the tube (Fig. 42). This is what
happens when we take lemonade through a FIG. 41.—The water in the tube
straw. When the air is withdrawn from the straw is at the same level as that in the
by the mouth, the pressure within the straw is glass.
reduced, and the liquid is forced up the straw by
the air pressure on the surface of the liquid in the glass. Even the ancient
Greeks and Romans knew that water would rise in a tube when the pressure
within the tube was reduced, and hence they tried to obtain water from
wells in this fashion, but the water could never be raised higher than 34
feet. Let us see why water could rise 34 feet and no more. If an empty pipe
is placed in a cistern of water, the water in the pipe does not rise above the
level of the water in the cistern. If, however, the pressure in the tube is
removed, the water in the tube will rise to a height of 34 feet approximately.
If now the air pressure in the tube is restored, the water in the tube sinks
again to the level of that in the cistern. The air pressing on the liquid in the
cistern tends to push some liquid up the tube, but the air pressing on the
water in the tube pushes downwards, and tends to keep the liquid from
rising, and these two pressures balance each other. When, however, the

pressure within the tube is reduced, the liquid rises because of the
unbalanced pressure which acts on the water in the cistern.
The column of water which can be raised this way
is approximately 34 feet, sometimes a trifle more,
sometimes a trifle less. If water were twice as
heavy, just half as high a column could be
supported by the atmosphere. Mercury is about
thirteen times as heavy as water and, therefore, the
column of mercury supported by the atmosphere is
about one thirteenth as high as the column of water
supported by the atmosphere. This can easily be
demonstrated. Fill a glass tube about a yard long
with mercury, close the open end with a finger, and
quickly insert the end of the inverted tube in a dish
of mercury (Fig. 43). When the finger is removed,
the mercury falls somewhat, leaving an empty space FIG. 42.—Water rises in the
in the top of the tube. If we measure the column in tube when the air is
the tube, we find its height is about one thirteenth of withdrawn.
34 feet or 30 inches, exactly what we should expect.
Since there is no air pressure within the tube, the
atmospheric pressure on the mercury in the dish is balanced solely by the
mercury within the tube, that is, by a column of mercury 30 inches high.
The shortness of the mercury column as compared with that of water makes
the mercury more convenient for both experimental and practical purposes.
(See Laboratory Manual.)
78. The Barometer. Since the pressure of the air changes from time to time,
the height of the mercury will change from day to day, and hour to hour.
When the air pressure is heavy, the mercury will tend to be high; when the
air pressure is low, the mercury will show a shorter column; and by reading
the level of the mercury one can learn the pressure of the atmosphere. If a
glass tube and dish of mercury are attached to a board and the dish of
mercury is inclosed in a case for protection from moisture and dirt, and
further if a scale of inches or centimeters is made on the upper portion of
the board, we have a mercurial barometer (Fig. 44).

If the barometer is taken to the
mountain top, the column of
mercury falls gradually during
the ascent, showing that as one
ascends, the pressure decreases in
agreement with the statement in
Section 76. Observations similar
to these were made by Torricelli
as early as the sixteenth century.
Taking a barometric reading
consists in measuring the height
FIG. 43.—The air supports a
of the mercury column.
column of mercury 30 inches
high. 79. A Portable Barometer. The
mercury barometer is large and FIG. 44.—A
inconvenient to carry from place to place, and a more simple
portable form has been devised, known as the aneroid barometer.
barometer (Fig. 45). This form of barometer is extremely
sensitive; indeed, it is so delicate that it shows the slight difference between
the pressure at the table top and the pressure at the floor level, whereas the
mercury barometer would indicate only a much greater variation in
atmospheric pressure. The aneroid barometers are frequently made no larger
than a watch and can be carried conveniently in the pocket, but they get out
of order easily and must be frequently readjusted. The aneroid barometer is
an air-tight box whose top is made of a thin metallic disk which bends
inward or outward according to the pressure of the atmosphere. If the
atmospheric pressure increases, the thin disk is pushed slightly inward; if,
on the other hand, the atmospheric pressure decreases, the pressure on the
metallic disk decreases and the disk is not pressed so far inward. The
motion of the disk is small, and it would be impossible to calculate changes
in atmospheric pressure from the motion of the disk, without some
mechanical device to make the slight changes in motion perceptible.
In order to magnify the slight changes in the position of the disk, the thin
face is connected with a system of levers, or wheels, which multiplies the
changes in motion and communicates them to a pointer which moves
around a graduated circular face. In Figure 45 the real barometer is scarcely

visible, being securely inclosed in a metal case
for protection; the principle, however, can be
understood by reference to Figure 46.
80. The Weight of
the Air. We have seen
that the pressure of
the atmosphere at any
point is due to the
weight of the air
column which
stretches from that FIG. 45.—Aneroid barometer.
point far up into the
sky above. This
weight varies slightly from time to time and from
place to place, but it is equal to about 15 pounds
FIG. 46.—Principle of the to the square inch as shown by actual
aneroid barometer. measurement. It comes to us as a surprise
sometimes that air actually has weight; for
example, a mass of 12 cubic feet of air at average pressure weighs 1 pound,
and the air in a large assembly hall weighs more than 1 ton.
We are practically never conscious of this really enormous pressure of the
atmosphere, which is exerted over every inch of our bodies, because the
pressure is exerted equally over the outside and the inside of our bodies; the
cells and tissues of our bodies containing gases under atmospheric pressure.
If, however, the finger is placed over the open end of a tube and the air is
sucked out of the tube by the mouth, the flesh of the finger bulges into the
tube because the pressure within the finger is no longer equalized by the
usual atmospheric pressure (Fig. 47).
Aëronauts have never ascended much higher than 7 miles; at that height the
barometer stands at 7 inches instead of at 30 inches, and the internal
pressure in cells and tissues is not balanced by an equal external pressure.
The unequalized internal pressure forces the blood to the surface of the
body and causes rupture of blood vessels and other physical difficulties.

81. Use of the Barometer. Changes in air
pressure are very closely connected with changes
in the weather. The barometer does not directly
foretell the weather, but a low or falling pressure,
accompanied by a simultaneous fall of the
mercury, usually precedes foul weather, while a
rising pressure, accompanied by a simultaneous
rise in the mercury, usually precedes fair weather.
The barometer is not an infallible prophet, but it
is of great assistance in predicting the general FIG. 47.—The flesh bulges
trend of the weather. There are certain changes in out.
the barometer which follow no known laws, and
which allow of no safe predictions, but on the
other hand, general future conditions for a few days ahead can be fairly
accurately determined. Figure 48 shows a barograph or self-registering
barometer which automatically registers air pressure.

FIG. 48.—Barograph.

Seaport towns in particular, but all cities, large or small, and villages too,
are on request notified by the United States Weather Bureau ten hours or
more in advance, of probable weather conditions, and in this way
precautions are taken which annually save millions of dollars and hundreds
of lives.
I recollect a summer spent on a New Hampshire farm, and know that an old
farmer started his farm hands haying by moonlight at two o'clock in the

morning, because the Special Farmer's Weather Forecast of the preceding
evening had predicted rain for the following day. His reliance on the
weather report was not misplaced, since the storm came with full force at
noon. Sailing vessels, yachts, and fishing dories remain within reach of port
if the barometer foretells storms.

FIG. 49.—Isotherms.

82. Isobaric and Isothermal Lines. If a line were drawn through all points
on the surface of the earth having an equal barometric pressure at the same
time, such a line would be called an isobar. For example, if the height of
barometers in different localities is observed at exactly the same time, and if
all the cities and towns which have the same pressure are connected by a
line, the curved lines will be called isobars. By the aid of these lines the
barometric conditions over a large area can be studied. The Weather Bureau
at Washington relies greatly on these isobars for statements concerning
local and distant weather forecasts, any shift in isobaric lines showing
change in atmospheric pressure.
If a line is drawn through all points on the surface of the earth having the
same temperature at the same instant, such a line is called an isotherm (Fig.
49).
83. Weather Maps. Scattered over the United States are about 125
Government Weather Stations, at each of which three times a day, at the
same instant, accurate observations of the weather are made. These

observations, which consist of the reading of barometer and thermometer,
the determination of the velocity and direction of the wind, the
determination of the humidity and of the amount of rain or snow, are
telegraphed to the chief weather official at Washington. From the reports of
wind storms, excessive rainfall, hot waves, clearing weather, etc., and their
rate of travel, the chief officials predict where the storms, etc., will be at a
definite future time. In the United States, the general movement of weather
conditions, as indicated by the barometer, is from west to east, and if a
certain weather condition prevails in the west, it is probable that it will
advance eastward, although with decided modifications. So many
influences modify atmospheric conditions that unfailing predictions are
impossible, but the Weather Bureau predictions prove true in about eight
cases out of ten.
The reports made out at Washington are telegraphed on request to cities in
this country, and are frequently published in the daily papers, along with the
forecast of the local office. A careful study of these reports enables one to
forecast to some extent the probable weather conditions of the day.
The first impression of a weather map (Fig. 50) with its various lines and
signals is apt to be one of confusion, and the temptation comes to abandon
the task of finding an underlying plan of the weather. If one will bear in
mind a few simple rules, the complexity of the weather map will disappear
and a glance at the map will give one information concerning general
weather conditions just as a glance at the thermometer in the morning will
give some indication of the probable temperature of the day. (See
Laboratory Manual.)

FIG. 50. Weather Map

On the weather map solid lines represent isobars and dotted lines represent
isotherms. The direction of the wind at any point is indicated by an arrow
which flies with the wind; and the state of the weather—clear, partly
cloudy, cloudy, rain, snow, etc.—is indicated by symbols.
84. Components of the Air. The best known constituent of the air is
oxygen, already familiar to us as the feeder of the fire without and within
the body. Almost one fifth of the air which envelops us is made up of the
life-giving oxygen. This supply of oxygen in the air is constantly being used
up by breathing animals and glowing fires, and unless there were some
constant source of additional supply, the quantity of oxygen in the air would
soon become insufficient to support animal life. The unfailing constant
source of atmospheric oxygen is plant life (Section 48). The leaves of plants
absorb carbon dioxide from the air, and break it up into oxygen and carbon.
The plant makes use of the carbon but it rejects the oxygen, which passes
back into the atmosphere through the pores of the leaves.
Although oxygen constitutes only one fifth of the atmosphere, it is one of
the most abundant and widely scattered of all substances. Almost the whole

earth, whether it be rich loam, barren clay, or granite boulder, contains
oxygen in some form or other; that is, in combination with other substances.
But nowhere, except in the air around us, do we find oxygen free and
uncombined with other substances.
A less familiar but more abundant constituent of the atmosphere is the
nitrogen. Almost four fifths of the air around us is made up of nitrogen. If
the atmosphere were composed of oxygen alone, the merest flicker of a
match would set the whole world ablaze. The fact that the oxygen of the air
is diluted as it were with so large a proportion of nitrogen, prevents fires
from sweeping over the world and destroying everything in their path.
Nitrogen does not support combustion, and a burning match placed in a
corked bottle goes out as soon as it has used up the oxygen in the bottle.
The nitrogen in the bottle, not only does not assist the burning of the match,
but it acts as a damper to the burning.
Free nitrogen, like oxygen, is a colorless, odorless gas. It is not poisonous;
but one would die if surrounded by nitrogen alone, just as one would die if
surrounded by water. The vast supply of nitrogen in the atmosphere would
be useless if the smaller amount of oxygen were not present to keep the
body alive. Nitrogen is so important a factor in daily life that an entire
chapter will be devoted to it later.
Another constituent of the air with which we are familiar is carbon dioxide.
In pure air, carbon dioxide is present in very small proportion, being
continually taken from the air by plants in the manufacture of their food.
Various other substances are present in the air in very minute proportions,
but of all the substances in the air, oxygen, nitrogen, and carbon dioxide are
the most important.

CHAPTER VIII
GENERAL PROPERTIES OF GASES

85. Bicycle Tires. We know very well that we cannot put more than a
certain amount of water in a tube, but we know equally well that the amount
of air which can be pumped into a bicycle or automobile tire depends
largely upon our muscular energy. A gallon of water remains a gallon of
water and requires a perfectly definite amount of space, but air can be
compressed and compressed, and made to occupy less and less space. While
it is true that air is easily compressed, it is also true that air is elastic and
capable of very rapid and easy expansion. If a puncture occurs in a tire, the
compressed air escapes very quickly; that is, the compressed air within the
tube has taken the first opportunity offered for expansion.
The fact that air is elastic has added
materially to the comfort of the world.
Transportation by bicycles and automobiles
has been greatly facilitated by the use of air
tires. In many hospitals, air mattresses are
used in place of hair, feather, or cotton
mattresses, and in this way the bed is kept
fresher and cleaner, and can be moved with
less danger of discomfort to the patient.
Every time we squeeze the bulb of an FIG. 51.—By squeezing the bulb, air
is forced out of the nozzle.
atomizer, we force compressed or condensed
air through the atomizer, and the condensed
air pushes the liquid out of the nozzle (Fig. 51). Thus we see that in the
necessities and conveniences of life compressed air plays an important part.
86. The Danger of Compression. Air under ordinary atmospheric
conditions exerts a pressure of 15 pounds to the square inch. If, now, large
quantities of air are compressed into a small space, the pressure exerted

becomes correspondingly greater. If too much air is blown into a toy
balloon, the balloon bursts because it cannot support the great pressure
exerted by the compressed air within. What is true of air is true of all gases.
Dangerous boiler explosions have occurred because the boiler walls were
not strong enough to withstand the pressure of the steam (which is water in
the form of gas). The pressure within the boilers of engines is frequently
several hundred pounds to the square inch, and such a pressure needs a
strong boiler.
87. How Pressure is Measured in Buildings. In the preceding Section we
saw that undue pressure of a gas may cause explosion. It is important,
therefore, that authorities keep strict watch on gases confined within pipes
and reservoirs, never allowing the pressure to exceed that which the walls
of the reservoir will safely bear.
Pressure in a gas pipe may be measured by a
simple instrument called the pressure gauge: The
gauge consists of a bent glass tube containing
mercury, and so made that one end can be fitted
to a gas jet (Fig. 52). When the gas cock is
closed, the mercury stands at the same level in
both arms, but when the cock is opened, the gas
whose pressure is being measured forces the
mercury up the opposite arm. If the pressure of
the gas is small, the mercury changes its level but
very little. It is clear that the height of a column
of mercury is a measure of the gas pressure. Now
it is known that one cubic inch of mercury weighs
about half a pound. Hence a column of mercury
one inch high indicates a pressure of about one
FIG. 52.—A pressure gauge. half pound to the square inch; a column two
inches high indicates a pressure of about one
pound to the square inch, and so on.
This is a very convenient way to measure the pressure of the illuminating
gas in our homes and offices. The gauge is attached to the gas burner and
the pressure is read by means of a scale attached to the gauge. (See
Laboratory Manual.)

In order to have satisfactory illumination, the pressure must be strong
enough to give a steady, broad flame. If the flame from any gas jet is
flickering and weak, it is usually an indication of insufficient pressure and
the gas company should investigate conditions and see to it that the
consumer receives his proper value.
87. The Gas Meter. Most householders are deeply interested in the actual
amount of gas which they consume (gas is charged for according to the
number of cubic feet used), and therefore they should be able to read the
gas meter which indicates their consumption of gas. Such gas meters are
furnished by the companies, and can be read easily.
The instrument itself is somewhat
complex. It will suffice to say that
within the meter box are thin disks
which are moved by the stream of
gas that passes them. This
movement of the disks is recorded
by clockwork devices on a dial
face. In this way, the number of
cubic feet of gas which pass
through the meter is automatically
registered.
FIG. 53.—The gas meter indicates the number of
89. The Relation between cubic feet of gas consumed.
Pressure and Volume. It was long
known that as the pressure of a gas
increases, that is, as it becomes compressed, its volume decreases, but
Robert Boyle was the first to determine the exact relation between the
volume and the pressure of a gas. He did this in a very simple manner.
Pour mercury into a U-shaped tube until the level of the mercury in the
closed end of the tube is the same as the level in the open end. The air in the
long arm is pressing upon the mercury in that arm, and is tending to force it
up the short arm. The air in the short closed arm is pressing down upon the
mercury in that arm and tending to send it up the long arm. Since the
mercury is at the same level in the two arms, the pressure in the long arm
must be equal to the pressure in the short arm. But the long arm is open, and

the pressure in that arm is the pressure of the atmosphere. Therefore the
pressure in the short arm must be one atmosphere. Measure the distance bc
between the top of the mercury and the closed end of the tube.
Pour more mercury into the open end of the
tube, and as the mercury rises higher and higher
in the long arm, note carefully the decrease in
the volume of the air in the short arm. Pour
mercury into the tube until the difference in
level bd is just equal to the barometric height,
approximately 32 inches. The pressure of the air
in the closed end now supports the pressure of
one atmosphere, and in addition, a column of
mercury equal to another atmosphere. If now the
air column in the closed end is measured, its
volume will be only one half of its former
volume. By doubling the pressure we have
reduced the volume one half. Similarly, if the
FIGS. 54, 55.—As the pressure pressure is increased threefold, the volume will
on the gas increases, its volume be reduced to one third of the original volume.
decreases.
90. Heat due to Compression. We saw in
Section 89 that whenever the pressure exerted
upon a gas is increased, the volume of the gas is decreased; and that
whenever the pressure upon a gas is decreased, the volume of the gas is
increased. If the pressure is changed very slowly, the change in the
temperature of the gas is imperceptible; if, however, the pressure is
removed suddenly, the temperature falls rapidly, or if the pressure is applied
suddenly, the temperature rises rapidly. When bicycle tires are being
inflated, the pump becomes hot because of the compression of the air.
The amount of heat resulting from compression is surprisingly large; for
example, if a mass of gas at 0° C. is suddenly compressed to one half its
original volume, its temperature rises 87° C.
91. Cooling by Expansion. If a gas expands suddenly, its temperature falls;
for example, if a mass of gas at 87° C. is allowed to expand rapidly to twice
its original volume, its temperature falls to 0° C. If the compressed air of a

bicycle tire is allowed to expand and a sensitive thermometer is held in the
path of the escaping air, the thermometer will show a decided drop in
temperature.
The low temperature obtained by the expansion of air or other gases is
utilized commercially on a large scale. By means of powerful pistons air is
compressed to one third or one fourth its original volume, is passed through
a coil of pipe surrounded with cold water, and is then allowed to escape into
large refrigerating vaults, which thereby have their temperatures noticeably
lowered, and can be used for the permanent storage of meats, fruits, and
other perishable material. In summer, when the atmospheric temperature is
high, the storage and preservation of foods is of vital importance to
factories and cold storage houses, and but for the low temperature
obtainable by the expansion of compressed gases, much of our food supply
would be lost to use.
92. Unexpected Transformations. If the pressure on a gas is greatly
increased, a sudden transformation sometimes occurs and the gas becomes a
liquid. Then, if the pressure is reduced, a second transformation occurs, and
the liquid evaporates or returns to its original form as a gas.
In Section 23 we saw that a fall of temperature caused water vapor to
condense or liquefy. If temperature alone were considered, most gases could
not be liquefied, because the temperature at which the average gas liquefies
is so low as to be out of the range of possibility; it has been calculated, for
example, that a temperature of 252° C. below zero would have to be
obtained in order to liquefy hydrogen.
Some gases can be easily transformed into liquids by pressure alone, some
gases can be easily transformed into liquids by cooling alone; on the other
hand, many gases are so difficult to liquefy that both pressure and low
temperature are needed to produce the desired result. If a gas is cooled and
compressed at the same time, liquefaction occurs much more surely and
easily than though either factor alone were depended upon. The air which
surrounds us, and of whose existence we are scarcely aware, can be reduced
to the form of a liquid, but the pressure exerted upon the portion to be
liquefied must be thirty-nine times as great as the atmospheric pressure, and
the temperature must have been reduced to a very low point.

93. Artificial Ice. Ammonia gas is liquefied by strong pressure and low
temperature and is then allowed to flow into pipes which run through tanks
containing salt water. The reduction of pressure causes the liquid to
evaporate or turn to a gas, and the fall of temperature which always
accompanies evaporation means a lowering of the temperature of the salt
water to 16° or 18° below zero. But immersed in the salt water are molds
containing pure water, and since the freezing point of water is 0° C, the
water in the molds freezes and can be drawn from the mold as solid cakes
of ice.

FIG. 56.—Apparatus for making artificial ice.

Ammonia gas is driven by the pump C into the coil D (Fig. 56) under a
pressure strong enough to liquefy it, the heat generated by this compression
being carried off by cold water which constantly circulates through B. The
liquid ammonia flows through the regulating valve V into the coil E, in
which the pressure is kept low by the pump C. The accompanying
expansion reduces the temperature to a very low degree, and the brine
which circulates around the coil E acquires a temperature below the
freezing point of pure water. The cold brine passes from A to a tank in
which are immersed cans filled with water, and within a short time the
water in the cans is frozen into solid cakes of ice.

CHAPTER IX
INVISIBLE OBJECTS

94. Very Small Objects. We saw in Section 84 that gases have a tendency
to expand, but that they can be compressed by the application of force. This
observation has led scientists to suppose that substances are composed of
very minute particles called molecules, separated by small spaces called
pores; and that when a gas is condensed, the pores become smaller, and that
when a gas expands, the pores become larger.
The fact that certain substances are soluble, like sugar in water, shows that
the molecules of sugar find a lodging place in the spaces or pores between
the molecules of water, in much the same way that pebbles find lodgment in
the chinks of the coal in a coal scuttle. An indefinite quantity of sugar
cannot be dissolved in a given quantity of liquid, because after a certain
amount of sugar has been dissolved all the pores become filled, and there is
no available molecular space. The remainder of the sugar settles at the
bottom of the vessel, and cannot be dissolved by any amount of stirring.
If a piece of potassium permanganate about the size of a grain of sand is put
into a quart of water, the solid disappears and the water becomes a deep rich
red. The solid evidently has dissolved and has broken up into minute
particles which are too small to be seen, but which have scattered
themselves and lodged in the pores of the water, thus giving the water its
rich color.
There is no visible proof of the existence of molecules and molecular
spaces, because not only are our eyes unable to see them directly, but even
the most powerful microscope cannot make them visible to us. They are so
small that if one thousand of them were laid side by side, they would make
a speck too small to be seen by the eye and too small to be visible under the
most powerful microscope.

We cannot see molecules or molecular pores, but the phenomena of
compression and expansion, solubility and other equally convincing facts,
have led us to conclude that all substances are composed of very minute
particles or molecules separated by spaces called pores.
95. Journeys Made by Molecules. If a gas jet is turned on and not lighted,
an odor of gas soon becomes perceptible, not only throughout the room, but
in adjacent halls and even in distant rooms. An uncorked bottle of cologne
scents an entire room, the odor of a rose or violet permeates the atmosphere
near and far. These simple everyday occurrences seem to show that the
molecules of a gas must be in a state of continual and rapid motion. In the
case of the cologne, some molecules must have escaped from the liquid by
the process of evaporation and traveled through the air to the nose. We
know that the molecules of a liquid are in motion and are continually
passing into the air because in time the vessel becomes empty. The only
way in which this could happen would be for the molecules of the liquid to
pass from the liquid into the surrounding medium; but this is really saying
that the molecules are in motion.
From these phenomena and others it is reasonably clear that substances are
composed of molecules, and that molecules are not inert, quiet particles, but
that they are in incessant motion, moving rapidly hither and thither,
sometimes traveling far, sometimes near. Even the log of wood which lies
heavy and motionless on our woodpile is made up of countless billions of
molecules each in rapid incessant motion. The molecules of solid bodies
cannot escape so readily as those of liquids and gases, and do not travel far.
The log lies year after year in an apparently motionless condition, but if
one's eyes were keen enough, the molecules would be seen moving among
themselves, even though they cannot escape into the surrounding medium
and make long journeys as do the molecules of liquids and gases.
96. The Companions of Molecules. Common sense tells us that a molecule
of water is not the same as a molecule of vinegar; the molecules of each are
extremely small and in rapid motion, but they differ essentially, otherwise
one substance would be like every other substance. What is it that makes a
molecule of water differ from a molecule of vinegar, and each differ from
all other molecules? Strange to say, a molecule is not a simple object, but is
quite complex, being composed of one or more smaller particles, called

atoms, and the number and kind of atoms in a molecule determine the type
of the molecule, and the type of the molecule determines the substance. For
example, a glass of water is composed of untold millions of molecules, and
each molecule is a company of three still smaller particles, one of which is
called the oxygen atom and two of which are alike in every particular and
are called hydrogen atoms.
97. Simple Molecules. Generally molecules are composed of atoms which
are different in kind. For example, the molecule of water has two different
atoms, the oxygen atom and the hydrogen atoms; alcohol has three different
kinds of atoms, oxygen, hydrogen, and carbon. Sometimes, however,
molecules are composed of a group of atoms all of which are alike. Now
there are but seventy or eighty different kinds of atoms, and hence there can
be but seventy or eighty different substances whose molecules are
composed of atoms which are alike. When the atoms comprising a molecule
are all alike, the substance is called an element, and is said to be a simple
substance. Throughout the length and breadth of this vast world of ours
there are only about eighty known elements. An element is the simplest
substance conceivable, because it has not been separated into anything
simpler. Water is a compound substance. It can be separated into oxygen
and hydrogen.
Gold, silver, and lead are examples of elements, and water, alcohol, cider,
sand, and marble are complex substances, or compounds, as we are apt to
call them. Everything, no matter what its size or shape or character, is
formed from the various combinations into molecules of a few simple
atoms, of which there exist about eighty known different kinds. But few of
the eighty known elements play an important part in our everyday life. The
elements in which we are most interested are given in the following table,
and the symbols by which they are known are placed in columns to the
right:

Oxygen O Copper Cu Phosphorus P
Hydrogen H Iodine I Potassium K
Carbon C Iron Fe Silver Ag
Aluminium Al Lead Pb Sodium Na

Calcium Ca Nickel Ni Sulphur S
Chlorine Cl Nitrogen N Tin Sn

We have seen in an earlier experiment that twice as much hydrogen as
oxygen can be obtained from water. Two atoms of the element hydrogen
unite with one atom of the element oxygen to make one molecule of water.
In symbols we express this H2O. A group of symbols, such as this,
expressing a molecule of a compound is called a formula. NaCl is the
formula for sodium chloride, which is the chemical name of common salt.

CHAPTER X
LIGHT

98. What Light Does for Us. Heat keeps us warm, cooks our food, drives
our engines, and in a thousand ways makes life comfortable and pleasant,
but what should we do without light? How many of us could be happy even
though warm and well fed if we were forced to live in the dark where the
sunbeams never flickered, where the shadows never stole across the floor,
and where the soft twilight could not tell us that the day was done? Heat
and light are the two most important physical factors in life; we cannot say
which is the more necessary, because in the extreme cold or arctic regions
man cannot live, and in the dark places where the light never penetrates
man sickens and dies. Both heat and light are essential to life, and each has
its own part to play in the varied existence of man and plant and animal.
Light enables us to see the world around us, makes the beautiful colors of
the trees and flowers, enables us to read, is essential to the taking of
photographs, gives us our moving pictures and our magic lanterns, produces
the exquisite tints of stained-glass windows, and brings us the joy of the
rainbow. We do not always realize that light is beneficial, because
sometimes it fades our clothing and our carpets, and burns our skin and
makes it sore. But we shall see that even these apparently harmful effects of
light are in reality of great value in man's constant battle against disease.
99. The Candle. Natural heat and light are furnished by the sun, but the
absence of the sun during the evening makes artificial light necessary, and
even during the day artificial light is needed in buildings whose structure
excludes the natural light of the sun. Artificial light is furnished by
electricity, by gas, by oil in lamps, and in numerous other ways. Until
modern times candles were the main source of light, and indeed to-day the
intensity, or power, of any light is measured in candle power units, just as
length is measured in yards; for example, an average gas jet gives a 10

candle power light, or is ten times as bright as a candle; an ordinary
incandescent electric light gives a 16 candle power light, or furnishes
sixteen times as much light as a candle. Very strong large oil lamps can at
times yield a light of 60 candle power, while the large arc lamps which flash
out on the street corners are said to furnish 1200 times as much light as a
single candle. Naturally all candles do not give the same amount of light,
nor are all candles alike in size. The candles which decorate our tea tables
are of wax, while those which serve for general use are of paraffin and
tallow.

FIG. 57.—A photograph at a receives four times as much light as when held at b.

100. Fading Illumination. The farther we move from a light, the less
strong, or intense, is the illumination which reaches us; the light of the
street lamp on the corner fades and becomes dim before the middle of the
block is reached, so that we look eagerly for the next lamp. The light
diminishes in brightness much more rapidly than we realize, as the
following simple experiment will show. Let a single candle (Fig. 57) serve
as our light, and at a distance of one foot from the candle place a
photograph. In this position the photograph receives a definite amount of
light from the candle and has a certain brightness.
If now we place a similar photograph directly behind the first photograph
and at a distance of two feet from the candle, the second photograph
receives no light because the first one cuts off all the light. If, however, the
first photograph is removed, the light which fell on it passes outward and
spreads itself over a larger area, until at the distance of the second
photograph the light spreads itself over four times as large an area as
formerly. At this distance, then, the illumination on the second photograph

is only one fourth as strong as it was on a similar photograph held at a
distance of one foot from the candle.
The photograph or object placed at a distance of one foot from a light is
well illuminated; if it is placed at a distance of two feet, the illumination is
only one fourth as strong, and if the object is placed three feet away, the
illumination is only one ninth as strong. This fact should make us have
thought and care in the use of our eyes. We think we are sixteen times as
well off with our incandescent lights as our ancestors were with simple
candles, but we must reflect that our ancestors kept the candle near them,
"at their elbow," so to speak, while we sit at some distance from the light
and unconcernedly read and sew.
As an object recedes from a light the illumination which it receives
diminishes rapidly, for the strength of the illumination is inversely
proportional to the square of distance of the object from the light. Our
ancestors with a candle at a distance of one foot from a book were as well
off as we are with an incandescent light four feet away.
101. Money Value of Light.
Light is bought and sold almost
as readily as are the products of
farm and dairy; many factories,
churches, and apartments pay a
definite sum for electric light of
a standard strength, and
naturally full value is desired. FIG. 58.—The two shadows are equally dark.
An instrument for measuring
the strength of a light is called a
photometer, and there are many different varieties, just as there are varieties
of scales which measure household articles. One light-measuring scale
depends upon the law that the intensity of illumination decreases with the
square of the distance of the object from the light. Suppose we wish to
measure the strength of the electric light bulbs in our homes, in order to see
whether we are getting the specified illumination. In front of a screen place
a black rod (Fig. 58) which is illuminated by two different lights; namely, a
standard candle and an incandescent bulb whose strength is to be measured.
Two shadows of the rod will fall on the screen, one caused by the candle

and the other caused by the incandescent light. The shadow due to the latter
source is not so dark as that due to the candle. Now let the incandescent
light be moved away from the screen until the two shadows are of equal
darkness. If the incandescent light is four times as far away from the screen
as the candle, and the shadows are equal, we know, by Section 100, that its
strength is sixteen candle power. If the incandescent light is four times as
far away from the screen as the candle is, its power must be sixteen times as
great, and we know the company is furnishing the standard amount of light
for a sixteen candle power electric bulb. If, however, the bulb must be
moved nearer to the rod in order that the two shadows may be similar then
the light given by the bulb is less than sixteen candle power, and less than
that due the consumer.
102. How Light Travels. We never
expect to see around a corner, and if
we wish to see through pinholes in
three separate pieces of cardboard,
FIG. 59.—The candle cannot be seen unless the we place the cardboards so that the
three pinholes are in a strait line. three holes are in a straight line.
When sunlight enters a dark room
through a small opening, the dust particles dancing in the sun show a
straight ray. If a hole is made in a card, and the card is held in front of a
light, the card casts a shadow, in the center of which is a bright spot. The
light, the hole, and the bright spot are all in the same straight line. These
simple observations lead us to think that light travels in a straight line.
We can always tell the direction from which light comes, either by the
shadow cast or by the bright spot formed when an opening occurs in the
opaque object casting the shadow. If the shadow of a tree falls towards the
west, we know the sun must be in the cast; if a bright spot is on the floor,
we can easily locate the light whose rays stream through an opening and
form the bright spot. We know that light travels in a straight line, and
following the path of the beam which comes to our eyes, we are sure to
locate the light.
103. Good and Bad Mirrors. As we walk along the street, we frequently
see ourselves reflected in the shop windows, in polished metal signboards,
in the metal trimmings of wagons and automobiles; but in mirrors we get

the best image of ourselves. We resent the image given by a piece of tin,
because the reflection is distorted and does not picture us as we really are; a
rough surface does not give a fair representation; if we want a true image of
ourselves, we must use a smooth surface like a mirror as a reflector. If the
water in a pond is absolutely still, we get a clear, true image of the trees, but
if there are ripples on the surface, the reflection is blurred and distorted. A
metal roof reflects so much light that the eyes are dazzled by it, and a
whitewashed fence injures the eyes because of the glare which comes from
the reflected light. Neither of these could be called mirrors, however,
because although they reflect light, they reflect it so irregularly that not
even a suggestion of an image can be obtained.
Most of us are sufficiently familiar with mirrors to know that the image is a
duplicate of ourselves with regard to size, shape, color, and expression, but
that it appears to be back of the mirror, while we are actually in front of the
mirror. The image appears not only behind the mirror, but it is also exactly
as far back of the mirror as we are in front of it; if we approach the mirror,
the image also draws nearer; if we withdraw, it likewise recedes.
104. The Path of Light. If a mirror or any other polished surface is held in
the path of a sunbeam, some of the light is reflected, and by rotating the
mirror the reflected sunbeam may be made to take any path. School
children amuse themselves by reflecting sunbeams from a mirror into their
companions' faces. If the companion moves his head in order to avoid the
reflected beam, his tormentor moves or inclines the mirror and flashes the
beam back to his victim's face.
If a mirror is held so that a ray of light strikes it in a perpendicular direction,
the light is reflected backward along the path by which it came. If, however,
the light makes an angle with the mirror, its direction is changed, and it
leaves the mirror along a new path. By observation we learn that when a
beam strikes the mirror and makes an angle of 30° with the perpendicular,
the beam is reflected in such a way that its new path also makes an angle of
30° with the perpendicular. If the sunbeam strikes the mirror at an angle of
32° with the perpendicular, the path of the reflected ray also makes an angle
of 32° with the perpendicular. The ray (AC, Fig. 60) which falls upon the
mirror is called the incident ray, and the angle which the incident ray (AC)
makes with the perpendicular (BC) to the mirror, at the point where the ray

strikes the mirror, is called the
angle of incidence. The angle
formed by the reflected ray (CD)
and this same perpendicular is
called the angle of reflection.
Observation and experiment have
taught us that light is always
reflected in such a way that the
angle of reflection equals the angle
of incidence. Light is not the only
illustration we have of the law of
reflection. Every child who bounces
a ball makes use of this law, but he
uses it unconsciously. If an elastic
FIG. 60.—The ray AC is reflected as CD. ball is thrown perpendicularly
against the floor, it returns to the
sender; if it is thrown against the floor at an angle (Fig. 61), it rebounds in
the opposite direction, but always in such a way that the angle of reflection
equals the angle of incidence.
105. Why the Image seems to be
behind the Mirror. If a candle is
placed in front of a mirror, as in
Figure 62, one of the rays of light
which leaves the candle will fall
upon the mirror as AB and will be
reflected as BC (in such a way that
the angle of reflection equals the
angle of incidence). If an observer
stands at C, he will think that the
point A of the candle is somewhere
along the line CB extended. Such a
supposition would be justified from FIG. 61.—A bouncing ball illustrates the law of
Section 102. But the candle sends reflection.
out light in all directions; one ray
therefore will strike the mirror as AD and will be reflected as DE, and an
observer at E will think that the point A of the candle is somewhere along

the line ED. In order that both observers may be correct, that is, in order
that the light may seem to be in both these directions, the image of the point
A must seem to be at the intersection of the two lines. In a similar manner it
can be shown that every point of the image of the candle seems to be behind
the mirror.
It can be shown by experiment that
the distance of the image behind the
mirror is equal to the distance of the
object in front of the mirror.
106. Why Objects are Visible. If
the beam of light falls upon a sheet
of paper, or upon a photograph,
instead of upon a smooth polished
surface, no definite reflected ray
FIG. 62.—The image is a duplicate of the object,
will be seen, but a glare will be
but appears to be behind the mirror.
produced by the scattering of the
beam of light. The surface of the
paper or photograph is rough, and
as a result, it scatters the beam in
every direction. It is hard for us to
realize that a smooth sheet of paper
is by no means so smooth as it
looks. It is rough compared with a
polished mirror. The law of
reflection always holds, however,
no matter what the reflecting
surface is,—the angle of reflection
FIG. 63.—The surface of the paper, although
always equals the angle of
smooth in appearance, is in reality rough, and
incidence. In a smooth body the
scatters the light in every direction.
reflected beams are all parallel; in a
rough body, the reflected beams are
inclined to each other in all sorts of ways, and no two beams leave the paper
in exactly the same direction.
Hot coals, red-hot stoves, gas flames, and candles shine by their own light,
and are self-luminous. Objects like chairs, tables, carpets, have no light

within themselves and are visible only when they receive light from a
luminous source and reflect that light. We know that these objects are not
self-luminous, because they are not visible at night unless a lamp or gas is
burning. When light from any luminous object falls upon books, desks, or
dishes, it meets rough surfaces, and hence undergoes diffuse reflection, and
is scattered irregularly in all directions. No matter where the eye is, some
reflected rays enter it, and the various objects are clearly seen.

CHAPTER XI
REFRACTION

107. Bent Rays of Light. A straw in a
glass of lemonade seems to be broken at
the surface of the liquid, the handle of a
teaspoon in a cup of water appears
broken, and objects seen through a glass
of water may seem distorted and changed
in size. When light passes from air into
water, or from any transparent substance
into another of different density, its
direction is changed, and it emerges along
an entirely new path (Fig. 64). We know FIG. 64.—A straw or stick in water
that light rays pass through glass, because seems broken.
we can see through the window panes and
through our spectacles; we know that light rays pass through water, because
we can see through a glass of clear water; on the other hand, light rays
cannot pass through wood, leather, metal, etc.
Whenever light meets a transparent substance obliquely, some of it is
reflected, undergoing a change in its direction; and some of it passes
onward through the medium, but the latter portion passes onward along a
new path. The ray RO (Fig. 65) passes obliquely through the air to the
surface of the water, but, on entering the water, it is bent or refracted and
takes the new path OS. The angle AOR is called the angle of incidence. The
angle POS is called the angle of refraction.
The angle of refraction is the angle formed by the refracted ray and the
perpendicular to the surface at the point where the light strikes it.
When light passes from air into water or glass, the refracted ray is bent
toward the perpendicular, so that the angle of refraction is smaller than the

angle of incidence. When a ray of light
passes from water or glass into air, the
refracted ray is bent away from the
perpendicular so that the angle of
refraction is greater than the angle of
incidence.
The bending or deviation of light in its
passage from one substance to another is
called refraction.
FIG. 65.—When the ray RO enters the
10
water, its path changes to OS.
8.
H
ow Refraction Deceives us.
Refraction is the source of many
illusions; bent rays of light make
objects appear where they really are
FIG. 66.—A fish at A seems to be at B.
not. A fish at A (Fig. 66) seems to
be at B. The end of the stick in
Figure 64 seems to be nearer the surface of the water than it really is.
The light from the sun, moon, and stars can reach us only by passing
through the atmosphere, but in Section 76, we learned that the atmosphere
varies in density from level to level; hence all the light which travels
through the atmosphere is constantly deviated from its original path, and
before the light reaches the eye it has undergone many changes in direction.
Now we learned in Section 102, that the direction of the rays of light as
they enter the eye determines the direction in which an object is seen; hence
the sun, moon, and stars seem to be along the lines which enter the eye,
although in reality they are not.
109. Uses of Refraction. If it were not for refraction, or the deviation of
light in its passage from medium to medium, the wonders and beauties of
the magic lantern and the camera would be unknown to us; sun, moon, and
stars could not be made to yield up their distant secrets to us in
photographs; the comfort and help of spectacles would be lacking,
spectacles which have helped unfold to many the rare beauties of nature,

such as a clear view of clouds and sunset, of humming bee and flying bird.
Books with their wealth of entertainment and information would be sealed
to a large part of mankind, if glasses did not assist weak eyes.
By refraction the magnifying glass reveals objects hidden because of their
minuteness, and enlarges for our careful contemplation objects otherwise
barely visible. The watchmaker, unassisted by the magnifying glass, could
not detect the tiny grains of dust or sand which clog the delicate wheels of
our watches. The merchant, with his lens, examines the separate threads of
woolen and silk fabrics to determine the strength and value of the material.
The physician, with his invaluable microscope, counts the number of
infinitesimal corpuscles in the blood and bases his prescription on that
count; he examines the sputum of a patient to determine whether
tuberculosis wastes the system. The bacteriologist with the same instrument
scrutinizes the drinking water and learns whether the dangerous typhoid
germs are present. The future of medicine will depend somewhat upon the
additional secrets which man is able to force from nature through the use of
powerful lenses, because as lenses have, in the past, been the means of
revealing disease germs, so in the future more powerful lenses may serve to
bring to light germs yet unknown. How refraction accomplishes these
results will be explained in the following Sections.
110. The Window Pane. We have seen that light is bent when it passes
from one medium to another of different density, and that objects viewed by
refracted light do not appear in their proper positions.
When a ray of light passes through a piece of plane glass, such as a window
pane (Fig. 67), it is refracted at the point B toward the perpendicular, and
continues its course through the glass in the new direction BC. On emerging
from the glass, the light is refracted away from the perpendicular and takes
the direction CD, which is clearly parallel to its original direction. Hence,
when we view objects through the window, we see them slightly displaced
in position, but otherwise unchanged. The deviation or displacement caused
by glass as thin as window panes is too slight to be noticed, and we are not
conscious that objects are out of position.
111. Chandelier Crystals and Prisms. When a ray of light passes through
plane glass, like a window pane, it is shifted somewhat, but its direction

does not change; that is, the
emergent ray is parallel to the
incident ray. But when a beam of
light passes through a triangular
glass prism, such as a chandelier
crystal, its direction is greatly
changed, and an object viewed
through a prism is seen quite out of
its true position.
W
he
ne
FIG. 67.—Objects looked at through a window ve
pane seem to be in their natural place. r
li
ght passes through a prism, it is bent
toward the base of the prism, or toward
the thick portion of the prism, and
emerges from the prism in quite a FIG. 68.—When looked at through the
prism, A seems to be at S.
different direction from that in which it
entered (Fig. 68). Hence, when an object
is looked at through a prism, it is seen quite out of place. In Figure 68, the
candle seems to be at S, while in reality it is at A.
112. Lenses. If two prisms are arranged as
in Figure 69, and two parallel rays of light
fall upon the prisms, the beam A will be
bent downward toward the thickened
portion of the prism, and the beam B will
be bent upward toward the thick portion
FIG. 69.—Rays of light are converged
of the prism, and after passing through the
and focused at F.
prism the two rays will intersect at some
point F, called a focus.
If two prisms are arranged as in Figure 70, the ray A will be refracted
upward toward the thick end, and the ray B will be refracted downward

toward the thick end; the two rays, on
emerging, will therefore be widely
separated and will not intersect.
Lenses are very similar to prisms; indeed,
two prisms placed as in Figure 69, and
rounded off, would make a very good FIG. 70.—Rays of light are diverged and
convex lens. A lens is any transparent do not come to any real focus.
material, but usually glass, with one or
both sides curved. The various types of
lenses are shown in Figure 71.
The first three types focus parallel
rays at some common point F, as in
Figure 69. Such lenses are called
convex or converging lenses. The
last three types, called concave
lenses, scatter parallel rays so that
they do not come to a focus, but
FIG. 71.—The different types of lenses.
diverge widely after passage
through the lens.
113. The Shape and Material of a Lens. The main or principal focus of a
lens, that is, the point at which rays parallel to the base line AB meet (Fig.
71), depends upon the shape of the lens. For example, a thick lens, such as
A (Fig. 72), focuses the rays very near to the lens; B, which is not so thick,
focuses the rays at a greater distance from the lens; and C, which is a very
thin lens, focuses the rays at a considerable distance from the lens. The
distance of the principal focus from the lens is called the focal length of the
lens, and from the diagrams we see that the more convex the lens, the
shorter the focal length.
The position of the principal focus depends not only on the shape of the
lens, but also on the refractive power of the material composing the lens. A
lens made of ice would not deviate the rays of light so much as a lens of
similar shape composed of glass. The greater the refractive power of the
lens, the greater the bending, and the nearer the principal focus to the lens.

There are many different kinds of
glass, and each kind of glass
refracts the light differently. Flint
glass contains lead; the lead makes
the glass dense, and gives it great
refractive power, enabling it to bend
and separate light in all directions.
Cut glass and toilet articles are
made of flint glass because of the
brilliant effects caused by its great
refractive power, and imitation
gems are commonly nothing more
than polished flint glass.
114. How Lenses Form Images.
Suppose we place an arrow, A, in FIG. 72.—The more curved the lens, the shorter
front of a convex lens (Fig. 73). The the focal length, and the nearer the focus is to the
ray AC, parallel to the principal lens.
axis, will pass through the lens and
emerge as DE. The ray is always bent toward the thick portion of the lens,
both at its entrance into the lens and its emergence from the lens.
In Section 105, we saw that two
rays determine the position of any
point of our image; hence in order
to locate the image of the top of the
arrow, we need to consider but one
more ray from the top of the object.
The most convenient ray to choose
FIG. 73.—The image is larger than the object. By
would be one passing through O,
means of a lens, a watchmaker gets an enlarged
the optical center of the lens,
image of the dust which clogs the wheels of his
because such a ray passes through
watch.
the lens unchanged in direction, as
is clear from Figure 74. The point
where AC and AO meet after refraction will be the position of the top of the
arrow. Similarly it can be shown that the center of the arrow will be at the
point T, and we see that the image is larger than the object. This can be
easily proved experimentally. Let a convex lens be placed near a candle

(Fig. 75); move a paper screen back and forth behind the lens; for some
position of the screen a clear, enlarged image of the candle will be made.
If the candle or arrow is
placed in a new position, say
at MA (Fig. 76), the image
formed is smaller than the
object, and is nearer to the
lens than it was before. Move
the lens so that its distance
from the candle is increased,
and then find the image on a
piece of paper. The size and
FIG. 74.—Rays above O are bent downward, those below position of the image depend
O are bent upward, and rays through O emerge from the upon the distance of the
lens unchanged in direction. object from the lens (Fig. 77).
By means of a lens one can
easily get on a visiting card a picture of a distant church steeple.
115. The Value of Lenses. If it
were not for the fact that a lens can
be held at such a distance from an
object as to make the image larger
than the object, it would be
impossible for the lens to assist the
watchmaker in locating the small
particles of dust which clog the
wheels of the watch. If it were not FIG. 75.—The lens is held in such a position that
for the opposite fact—that a lens the image of the candle is larger than the object.
can be held at such a distance from
the object as to make an image
smaller than the object, it would be
impossible to have a photograph of
a tall tree or building unless the
photograph were as large as the tree
itself. When a photographer takes a
FIG. 76.—The image is smaller than the object. photograph of a person or a tree, he
moves his camera until the image

formed by the lens is of the desired size. By bringing the camera (really the
lens of the camera) near, we obtain a large-sized photograph; by increasing
the distance between the camera and the object, a smaller photograph is
obtained. The mountain top may be so far distant that in the photograph it
will not appear to be greater than a small stone.
Many familiar illustrations of
lenses, or curved refracting
surfaces, and their work, are known
to all of us. Fish globes magnify the
fish that swim within. Bottles can
be so shaped that they make the
olives, pickles, and peaches that
they contain appear larger than they
FIG. 77.—The lens is placed in such a position
really are. The fruit in bottles that the image is about the same size as the
frequently seems too large to have
object.
gone through the neck of the bottle.
The deception is due to refraction,
and the material and shape of the bottle furnish a sufficient explanation.
By using combinations of two or more lenses of various kinds, it is possible
to have an image of almost any desired size, and in practically any desired
position.
116. The Human Eye. In Section 114, we obtained on a movable screen, by
means of a simple lens, an image of a candle. The human eye possesses a
most wonderful lens and screen (Fig. 78); the lens is called the crystalline
lens, and the screen is called the retina. Rays of light pass from the object
through the pupil P, go through the crystalline lens L, where they are
refracted, and then pass onward to the retina R, where they form a distinct
image of the object.
We learned in Section 114 that a change in the position of the object
necessitated a change in the position of the screen, and that every time the
object was moved the position of the screen had to be altered before a clear
image of the object could be obtained. The retina of the eye cannot be
moved backward and forward, as the screen was, and the crystalline lens is
permanently located directly back of the iris. How, then, does it happen that

we can see clearly both near and distant
objects; that the printed page which is
held in the hand is visible at one second,
and that the church spire on the distant
horizon is visible the instant the eyes are
raised from the book? How is it possible
to obtain on an immovable screen by
means of a simple lens two distinct
images of objects at widely varying
distances?
The answer to these questions is that the
crystalline lens changes shape according
FIG. 78.—The eye.
to need. The lens is attached to the eye by
means of small muscles, m, and it is by
the action of these muscles that the lens is able to become small and thick,
or large and thin; that is, to become more or less curved. When we look at
near objects, the muscles act in such a way that the lens bulges out, and
becomes thick in the middle and of the right curvature to focus the near
object upon the screen. When we look at an object several hundred feet
away, the muscles change their pull on the lens and flatten it until it is of the
proper curvature for the new distance. The adjustment of the muscles is so
quick and unconscious that we normally do not experience any difficulty in
changing our range of view. The ability of the eye to adjust itself to varying
distances is called accommodation. The power of adjustment in general
decreases with age.
117. Farsightedness and Nearsightedness. A farsighted person is one who
cannot see near objects so distinctly as far objects, and who in many cases
cannot see near objects at all. The eyeball of a farsighted person is very
short, and the retina is too close to the crystalline lens. Near objects are
brought to a focus behind the retina instead of on it, and hence are not
visible. Even though the muscles of accommodation do their best to bulge
and thicken the lens, the rays of light are not bent sufficiently to focus
sharply on the retina. In consequence objects look blurred. Farsightedness
can be remedied by convex glasses, since they bend the light and bring it to
a closer focus. Convex glasses, by bending the rays and bringing them to a

nearer focus, overbalance a short eyeball with its tendency to focus objects
behind the retina.
A
nearsigh
ted
person
is one
who
FIG. 79.—The farsighted eye. FIG. 80.—The defect is remedied by
cannot
convex glasses.
see
objects unless they are close to the eye.
The eyeball of a nearsighted person is very wide, and the retina is too far
away from the crystalline lens. Far objects are brought to a focus in front of
the retina instead of on it, and hence are not visible. Even though the
muscles of accommodation do their best to pull out and flatten the lens, the
rays are not separated sufficiently to focus as far back as the retina. In
consequence objects look blurred. Nearsightedness can be remedied by
wearing concave glasses, since they separate the light and move the focus
farther away. Concave glasses, by separating the rays and making the focus
more distant, overbalance a wide eyeball with its tendency to focus objects
in front of the retina.

FIG. 81.—The nearsighted eye. The defect is remedied by concave glasses.

118. Headache and Eyes. Ordinarily the muscles of accommodation adjust
themselves easily and quickly; if, however, they do not, frequent and severe
headaches occur as a result of too great muscular effort toward
accommodation. Among young people headaches are frequently caused by
over-exertion of the crystalline muscles. Glasses relieve the muscles of the
extra adjustment, and hence are effective in eliminating this cause of
headache.

An exact balance is required between glasses, crystalline lens, and muscular
activity, and only those who have studied the subject carefully are
competent to treat so sensitive and necessary a part of the body as the eye.
The least mistake in the curvature of the glasses, the least flaw in the type
of glass (for example, the kind of glass used), means an improper focus,
increased duty for the muscles, and gradual weakening of the entire eye,
followed by headache and general physical discomfort.
119. Eye Strain. The extra work which is thrown upon the nervous system
through seeing, reading, writing, and sewing with defective eyes is
recognized by all physicians as an important cause of disease. The tax made
upon the nervous system by the defective eye lessens the supply of energy
available for other bodily use, and the general health suffers. The health is
improved when proper glasses are prescribed.
Possibly the greatest danger of eye strain is among school children, who are
not experienced enough to recognize defects in sight. For this reason, many
schools employ a physician who examines the pupils' eyes at regular
intervals.
The following general precautions are worth observing:—
1. Rest the eyes when they hurt, and as far as possible do close work, such
as writing, reading, sewing, wood carving, etc., by daylight.
2. Never read in a very bright or a very dim light.
3. If the light is near, have it shaded.
4. Do not rub the eyes with the fingers.
5. If eyes are weak, bathe them in lukewarm water in which a pinch of
borax has been dissolved.

CHAPTER XII
PHOTOGRAPHY

120. The Magic of the Sun. Ribbons and dresses washed and hung in the
sun fade; when washed and hung in the shade, they are not so apt to lose
their color. Clothes are laid away in drawers and hung in closets not only
for protection against dust, but also against the well-known power of light
to weaken color.
Many housewives lower the window shades that the wall paper may not
lose its brilliancy, that the beautiful hues of velvet, satin, and plush tapestry
may not be marred by loss in brilliancy and sheen. Bright carpets and rugs
are sometimes bought in preference to more delicately tinted ones, because
the purchaser knows that the latter will fade quickly if used in a sunny
room, and will soon acquire a dull mellow tone. The bright and gay colors
and the dull and somber colors are all affected by the sun, but why one
should be affected more than another we do not know. Thousands of
brilliant and dainty hues catch our eye in the shop and on the street, but not
one of them is absolutely permanent; some may last for years, but there is
always more or less fading in time.
Sunlight causes many strange, unexplained effects. If the two substances,
chlorine and hydrogen, are mixed in a dark room, nothing remarkable
occurs any more than though water and milk were mixed, but if a mixture
of these substances is exposed to sunlight, a violent explosion occurs and an
entirely new substance is formed, a compound entirely different in character
from either of its components.
By some power not understood by man, the sun is able to form new
substances. In the dark, chlorine and hydrogen are simply chlorine and
hydrogen; in the sunlight they combine as if by magic into a totally
different substance. By the same unexplained power, the sun frequently
does just the opposite work; instead of combining two substances to make

one new product, the sun may separate or break down some particular
substance into its various elements. For example, if the sun's rays fall upon
silver chloride, a chemical action immediately begins, and as a result we
have two separate substances, chlorine and silver. The sunlight separates
silver chloride into its constituents, silver and chlorine.
121. The Magic Wand in Photography. Suppose we coat one side of a
glass plate with silver chloride, just as we might put a coat of varnish on a
chair. We must be very careful to coat the plate in the dark room,[B]
otherwise the sunlight will separate the silver chloride and spoil our plan.
Then lay a horseshoe on the plate for good luck, and carry the plate out into
the light for a second. The light will separate the silver chloride into
chlorine and silver, the latter of which will remain on the plate as a thin
film. All of the plate was affected by the sun except the portion protected by
the horseshoe which, because it is opaque, would not allow light to pass
through and reach the plate. If now the plate is carried back to the dark
room and the horseshoe is removed, one would expect to see on the plate an
impression of the horseshoe, because the portion protected by the horseshoe
would be covered by silver chloride and the exposed unprotected portion
would be covered by metallic silver. But we are much disappointed because
the plate, when examined ever so carefully, shows not the slightest change
in appearance. The change is there, but the unaided eye cannot detect the
change. Some chemical, the so-called "developer," must be used to bring
out the hidden change and to reveal the image to our unseeing eyes. There
are many different developers in use, any one of which will effect the
necessary transformation. When the plate has been in the developer for a
few seconds, the silver coating gradually darkens, and slowly but surely the
image printed by the sun's rays appears. But we must not take this picture
into the light, because the silver chloride which was protected by the
horseshoe is still present, and would be strongly affected by the first
glimmer of light, and, as a result, our entire plate would become similar in
character and there would be no contrast to give an image of the horseshoe
on the plate.
[Footnote B: That is, a room from which ordinary daylight is excluded.]

But a photograph on glass, which must be carefully shielded from the light
and admired only in the dark room, would be neither pleasurable nor

practical. If there were some way by which the hitherto unaffected silver
chloride could be totally removed, it would be possible to take the plate into
any light without fear. To accomplish this, the unchanged silver chloride is
got rid of by the process technically called "fixing"; that is, by washing off
the unreduced silver chloride with a solution such as sodium thiosulphite,
commonly known as hypo. After a bath in the hypo the plate is cleansed in
clear running water and left to dry. Such a process gives a clear and
permanent picture on the plate.
122. The Camera. A camera (Fig.
82) is a light-tight box containing a
movable convex lens at one end and
a screen at the opposite end. Light
from the object to be photographed
passes through the lens, falls upon
the screen, and forms an image
there. If we substitute for the
ordinary screen a plate or film
FIG. 82.—A camera. coated with silver chloride or any
other silver salt, the light which
falls upon the sensitive plate and
forms an image there will change the silver chloride and produce a hidden
image. If the plate is then removed from the camera in the dark, and is
treated as described in the preceding Section, the image becomes visible
and permanent. In practice some gelatin is mixed with the silver salt, and
the mixture is then poured over the plate or film in such a way that a thin,
even coating is made. It is the presence of the gelatin that gives plates a
yellowish hue. The sensitive plates are left to dry in dark rooms, and when
the coating has become absolutely firm and dry, the plates are packed in
boxes and sent forth for sale.
Glass plates are heavy and inconvenient to carry, so that celluloid films
have almost entirely taken their place, at least for outdoor work.
123. Light and Shade. Let us apply the above process to a real photograph.
Suppose we wish to take the photograph of a man sitting in a chair in his
library. If the man wore a gray coat, a black tie, and a white collar, these

details must be faithfully represented in the photograph. How can the
almost innumerable lights and shades be produced on the plate?
The white collar would send through the lens the most light to the sensitive
plate; hence the silver chloride on the plate would be most changed at the
place where the lens formed an image of the collar. The gray coat would not
send to the lens so much light as the white collar, hence the silver chloride
would be less affected by the light from the coat than by that from the
collar, and at the place where the lens produced an image of the coat the
silver chloride would not be changed so much as where the collar image is.
The light from the face would produce a still different effect, since the light
from the face is stronger than the light from the gray coat, but less than that
from a white collar. The face in the image would show less changed silver
chloride than the collar, but more than the coat, because the face is lighter
than the coat, but not so light as the collar. Finally, the silver chloride would
be least affected by the dark tie. The wall paper in the background would
affect the plate according to the brightness of the light which fell directly
upon it and which reflected to the camera. When such a plate has been
developed and fixed, as described in Section 121, we have the so-called
negative (Fig. 83). The collar is very dark, the black tie and gray coat white,
and the white tidy very dark.
The lighter the object, such as tidy or collar, the
more salt is changed, or, in other words, the
greater the portion of the silver salt that is
affected, and hence the darker the stain on the
plate at that particular spot. The plate shows all
gradations of intensity—the tidy is dark, the
black tie is light. The photograph is true as far as
position, form, and expression are concerned,
but the actual intensities are just reversed. How
this plate can be transformed into a photograph
true in every detail will be seen in the following
FIG. 83.—A negative.
Section.
124. The Perfect Photograph. Bright objects, such as the sky or a white
waist, change much of the silver chloride, and hence appear dark on the
negative. Dark objects, such as furniture or a black coat, change little of the

chloride, and hence appear light on the negative. To obtain a true
photograph, the negative is placed on a piece of sensitive photographic
paper, or paper coated with a silver salt in the same manner as the plate and
films. The combination is exposed to the light. The dark portions of the
negative will act as obstructions to the passage of light, and but little light
will pass through that part of the negative to the photographic paper, and
consequently but little of the silver salt on the paper will be changed. On the
other hand, the light portion of the negative will allow free and easy
passage of the light rays, which will fall upon the photographic paper and
will change much more of the silver. Thus it is that dark places in the
negative produce light places in the positive or real photograph (Fig. 84),
and that light places in the negative produce dark places in the positive; all
intermediate grades are likewise represented with their proper gradations of
intensity.
If properly treated, a negative remains good for
years, and will serve for an indefinite number of
positives or true photographs.
125. Light and Disease. The far-reaching effect
which light has upon some inanimate objects,
such as photographic films and clothes, leads us
to inquire into the relation which exists between
light and living things. We know from daily
observation that plants must have light in order
to thrive and grow. A healthy plant brought into
a dark room soon loses its vigor and freshness, FIG. 84.—A positive or true
and becomes yellow and drooping. Plants do not photograph.
all agree as to the amount of light they require,
for some, like the violet and the arbutus, grow best in moderate light, while
others, like the willows, need the strong, full beams of the sun. But nearly
all common plants, whatever they are, sicken and die if deprived of sunlight
for a long time. This is likewise true in the animal world. During long
transportation, animals are sometimes necessarily confined in dark cars,
with the result that many deaths occur, even though the car is well aired and
ventilated and the food supply good. Light and fresh air put color into pale
cheeks, just as light and air transform sickly, yellowish plants into hardy

green ones. Plenty of fresh air, light, and pure water are the watchwords
against disease.
In addition to the plants and
animals which we see, there are
many strange unseen ones floating
in the atmosphere around us, lying
in the dust of corner and closet,
growing in the water we drink, and
thronging decayed vegetable and
animal matter. Everyone knows that
mildew and vermin do damage in
FIG. 85—Stems and leaves of oxalis growing the home and in the field, but very
toward the light. few understand that, in addition to
these visible enemies of man, there
are swarms of invisible plants and
animals some of which do far more damage, both directly and indirectly,
than the seen and familiar enemies. All such very small plants and animals
are known as microorganisms.
Not all microörganisms are harmful; some are our friends and are as helpful
to us as are cultivated plants and domesticated animals. Among the most
important of the microörganisms are bacteria, which include among their
number both friend and foe. In the household, bacteria are a fruitful source
of trouble, but some of them are distinctly friends. The delicate flavor of
butter and the sharp but pleasing taste of cheese are produced by bacteria.
On the other hand, bacteria are the cause of many of the most dangerous
diseases, such as typhoid fever, tuberculosis, influenza, and la grippe.
By careful observation and experimentation it has been shown conclusively
that sunlight rapidly kills bacteria, and that it is only in dampness and
darkness that bacteria thrive and multiply. Although sunlight is essential to
the growth of most plants and animals, it retards and prevents the growth of
bacteria. Dirt and dust exposed to the sunlight lose their living bacteria,
while in damp cellars and dark corners the bacteria thrive, increasing
steadily in number. For this reason our houses should be kept light and airy;
blinds should be raised, even if carpets do fade; it is better that carpets and
furniture should fade than that disease-producing bacteria should find a

permanent abode within our dwellings. Kitchens and pantries in particular
should be thoroughly lighted. Bedclothes, rugs, and clothing should be
exposed to the sunlight as frequently as possible; there is no better
safeguard against bacterial disease than light. In a sick room sunlight is
especially valuable, because it not only kills bacteria, but keeps the air dry,
and new bacteria cannot get a start in a dry atmosphere.

CHAPTER XIII
COLOR

126. The Rainbow. One of the most beautiful and well-known phenomena
in nature is the rainbow, and from time immemorial it has been considered
Jehovah's signal to mankind that the storm is over and that the sunshine will
remain. Practically everyone knows that a rainbow can be seen only when
the sun's rays shine upon a mist of tiny drops of water. It is these tiny drops
which by their refraction and their scattering of light produce the rainbow in
the heavens.
The exquisite tints of the rainbow can be seen if we look at an object
through a prism or chandelier crystal, and a very simple experiment enables
us to produce on the wall of a room the exact colors of the rainbow in all
their beauty.
127. How to produce
Rainbow Colors. The
Spectrum. If a beam of
sunlight is admitted into a
dark room through a narrow
opening in the shade, and is
allowed to fall upon a prism,
as shown in Figure 86, a
FIG. 86.—White light is a mixture of lights of rainbow
beautiful band of colors will
colors.
appear on the opposite wall
of the room. The ray of light
which entered the room as ordinary sunlight has not only been refracted and
bent from its straight path, but it has been spread out into a band of colors
similar to those of the rainbow.
Whenever light passes through a prism or lens, it is dispersed or separated
into all the colors which it contains, and a band of colors produced in this

way is called a spectrum. If we examine such a spectrum we find the
following colors in order, each color imperceptibly fading into the next:
violet, indigo, blue, green, yellow, orange, red.
128. Sunlight or White Light. White light or sunlight can be dispersed or
separated into the primary colors or rainbow hues, as shown in the
preceding Section. What seems even more wonderful is that these spectral
colors can be recombined so as to make white light.
If a prism B (Fig. 87) exactly similar to A in every way is placed behind A
in a reversed position, it will undo the dispersion of A, bending upward the
seven different beams in such a way that they emerge together and produce
a white spot on the screen. Thus we see, from two simple experiments, that
all the colors of the rainbow may be obtained from white light, and that
these colors may be in turn recombined to produce white light.
White light is not a simple light, but
is composed of all the colors which
appear in the rainbow.
129. Color. If a piece of red glass is
held in the path of the colored beam
of light formed as in Section 127,
all the colors on the wall will
disappear except the red, and
instead of a beautiful spectrum of FIG. 87.—Rainbow colors recombined to form
all colors there will be seen the red white light.
color alone. The red glass does not
allow the passage through it of any
light except red light; all other colors are absorbed by the red glass and do
not reach the eye. Only the red ray passes through the red glass, reaches the
eye, and produces a sensation of color.
If a piece of blue glass is substituted for the red glass, the blue band remains
on the wall, while all the other colors disappear. If both blue and red pieces
of glass are held in the path of the beam, so that the light must pass through
first one and then the other, the entire spectrum disappears and no color
remains. The blue glass absorbs the various rays with the exception of the

blue ones, and the red glass will not allow these blue rays to pass through it;
hence no light is allowed passage to the eye.
An emerald looks green because it freely transmits green, but absorbs the
other colors of which ordinary daylight is composed. A diamond appears
white because it allows the passage through it of all the various rays; this is
likewise true of water and window panes.
Stained-glass windows owe their charm and beauty to the presence in the
glass of various dyes and pigments which absorb in different amounts some
colors from white light and transmit others. These pigments or dyes are
added to the glass while it is in the molten state, and the beauty of a stained-
glass window depends largely upon the richness and the delicacy of the
pigments used.
130. Reflected Light. Opaque Objects. In Section 106 we learned that most
objects are visible to us because of the light diffusely reflected from them.
A white object, such as a sheet of paper, a whitewashed fence, or a table
cloth, absorbs little of the light which falls upon it, but reflects nearly all,
thus producing the sensation of white. A red carpet absorbs the light rays
incident upon it except the red rays, and these it reflects to the eye.
Any substance or object which reflects none of the rays which fall upon it,
but absorbs all, appears black; no rays reach the eye, and there is an absence
of any color sensation. Coal and tar and soot are good illustrations of
objects which absorb all the light which falls upon them.
131. How and Why Colors Change. Matching Colors. Most women prefer
to shop in the morning and early afternoon when the sunlight illuminates
shops and factories, and when gas and electricity do not throw their spell
over colors. Practically all people know that ribbons and ties, trimmings and
dresses, frequently look different at night from what they do in the daytime.
It is not safe to match colors by artificial light; cloth which looks red by
night may be almost purple by day. Indeed, the color of an object depends
upon the color of the light which falls upon it. Strange sights are seen on the
Fourth of July when variously colored fireworks are blazing. The child with
a white blouse appears first red, then blue, then green, according as his
powders burn red, blue, or green. The face of the child changes from its
normal healthy hue to a brilliant red and then to ghastly shades.

Suppose, for example, that a white hat is held at the red end of the spectrum
or in any red light. The characteristics of white objects is their ability to
reflect all the various rays that fall upon them. Here, however, the only light
which falls upon the white hat is red light, hence the only light which the
hat has to reflect is red light and the hat consequently appears red.
Similarly, if a white hat is placed in a blue light, it will reflect all the light
which falls upon it, namely, blue light, and will appear blue. If a red hat is
held in a red light, it is seen in its proper color. If a red hat is held in a blue
light, it appears black; it cannot reflect any of the blue light because that is
all absorbed and there is no red light to reflect.
A child wearing a green frock on Independence Day seems at night to be
wearing a black frock, if standing near powders burning with red, blue, or
violet light.
132. Pure, Simple Colors—Things as they Seem. To the eye white light
appears a simple, single color. It reveals its compound nature to us only
when passed through a prism, when it shows itself to be compounded of an
infinite number of colors which Sir Isaac Newton grouped in seven
divisions: violet, indigo, blue, green, yellow, orange, and red.
We naturally ask ourselves whether these colors which compose white light
are themselves in turn compound? To answer that question, let us very
carefully insert a second prism in the path of the rays which issue from the
first prism, carefully barring out the remaining six kinds of rays. If the red
light is compound, it will be broken up into its constituent parts and will
form a typical spectrum of its own, just as white light did after its passage
through a prism. But the red rays pass through the second prism, are
refracted, and bent from this course, and no new colors appear, no new
spectrum is formed. Evidently a ray of spectrum red is a simple color, not a
compound color.
If a similar experiment is made with the remaining spectrum rays, the result
is always the same: the individual spectrum colors remain simple, pure
colors. The individual spectrum colors are groups of simple, pure colors.
133. Colors not as they Seem—Compound Colors. If one half of a
cardboard disk (Fig. 88) is painted green, and the other half violet, and the
disk is slipped upon a toy top, and spun rapidly, the rotating disk will

appear blue; if red and green
are used in the same way
instead of green and violet,
the rotating disk will appear
yellow. A combination of red
and yellow will give orange.
The colors formed in this way
do not appear to the eye
different from the spectrum
colors, but they are actually
very different. The spectrum
FIG. 88.—Violet and green give blue. Green, blue, and colors, as we saw in the
red give white. preceding Section, are pure,
simple colors, while the
colors formed from the rotating disk are in reality compounded of several
totally different rays, although in appearance the resulting colors are pure
and simple.
If it were not that colors can be compounded, we should be limited in hue
and shade to the seven spectral colors; the wealth and beauty of color in
nature, art, and commerce would be unknown; the flowers with their
thousands of hues would have a poverty of color undreamed of; art would
lose its magenta, its lilac, its olive, its lavender, and would have to work its
wonders with the spectral colors alone. By compounding various colors in
different proportions, new colors can be formed to give freshness and
variety. If one third of the rotating disk is painted blue, and the remainder
white, the result is lavender; if fifteen parts of white, four parts of red, and
one part of blue are arranged on the disk, the result is lilac. Olive is
obtained from a combination of two parts green, one part red, and one part
black; and the soft rich shades of brown are all due to different mixtures of
black, red, orange, or yellow.
134. The Essential Colors. Strange and unexpected facts await us at every
turn in science! If the rotating cardboard disk (Fig. 88) is painted one third
red, one third green, and one third blue, the resulting color is white. While
the mixture of the spectral colors produces white, it is not necessary to have
all of the spectral colors in order to obtain white; because a mixture of the
following colors alone, red, green, and blue, will give white. Moreover, by

the mixture of these three colors in proper proportions, any color of the
spectrum, such as yellow or indigo or orange, may be obtained. The three
spectral colors, red, green, and blue, are called primary or essential hues,
because all known tints of color may be produced by the careful blending of
blue, green, and red in the proper proportions; for example, purple is
obtained by the blending of red and blue, and orange by the blending of red
and yellow.
135. Color Blindness. The nerve fibers of the eye which carry the sensation
of color to the brain are particularly sensitive to the primary colors—red,
green, blue. Indeed, all color sensations are produced by the stimulation of
three sets of nerves which are sensitive to the primary colors. If one sees
purple, it is because the optic nerves sensitive to red and blue (purple equals
red plus blue) have carried their separate messages to the brain, and the
blending of the two distinct messages in the brain has given the sensation of
purple. If a red rose is seen, it is because the optic nerves sensitive to red
have been stimulated and have carried the message to the brain.
A snowy field stimulates equally all three sets of optic nerves—the red, the
green, and the blue. Lavender, which is one part blue and three parts white,
would stimulate all three sets of nerves, but with a maximum of stimulation
for the blue. Equal stimulation of the three sets would give the impression
of white.
A color-blind person has some defect in one or more of the three sets of
nerves which carry the color message to the brain. Suppose the nerve fibers
responsible for carrying the red are totally defective. If such a person views
a yellow flower, he will see it as a green flower. Yellow contains both red
and green, and hence both the red and green nerve fibers should be
stimulated, but the red nerve fibers are defective and do not respond, the
green nerve fibers alone being stimulated, and the brain therefore interprets
green.
A well-known author gives an amusing incident of a dinner party, at which
the host offered stewed tomato for apple sauce. What color nerves were
defective in the case of the host?
In some employments color blindness in an employee would be fatal to
many lives. Engineers and pilots govern the direction and speed of trains

and boats largely by the colored signals which flash out in the night's
darkness or move in the day's bright light, and any mistake in the reading of
color signals would imperil the lives of travelers. For this reason a rigid test
in color is given to all persons seeking such employment, and the ability to
match ribbons and yarns of all ordinary hues is an unvarying requirement
for efficiency.

CHAPTER XIV
HEAT AND LIGHT AS COMPANIONS

"The night has a thousand eyes,
And the day but one;
Yet the light of the bright world dies
With the dying sun."

136. Most bodies which glow and give out light are hot; the stove which
glows with a warm red is hot and fiery; smoldering wood is black and
lifeless; glowing coals are far hotter than black ones. The stained-glass
window softens and mellows the bright light of the sun, but it also shuts out
some of the warmth of the sun's rays; the shady side of the street spares our
eyes the intense glare of the sun, but may chill us by the absence of heat.
Our illumination, whether it be oil lamp or gas jet or electric light, carries
with it heat; indeed, so much heat that we refrain from making a light on a
warm summer's night because of the heat which it unavoidably furnishes.
137. Red a Warm Color. We have seen that heat and light usually go hand
in hand. In summer we lower the shades and close the blinds in order to
keep the house cool, because the exclusion of light means the exclusion of
some heat; in winter we open the blinds and raise the shades in order that
the sun may stream into the room and flood it with light and warmth. The
heat of the sun and the light of the sun seem boon companions.
We can show that when light passes through a prism and is refracted,
forming a spectrum, as in Section 127, it is accompanied by heat. If we hold
a sensitive thermometer in the violet end of the spectrum so that the violet
rays fall upon the bulb, the reading of the mercury will be practically the
same as when the thermometer is held in any dark part of the room; if,
however, the thermometer is slowly moved toward the red end of the
spectrum, a change occurs and the mercury rises slowly but steadily,
showing that heat rays are present at the red end of the spectrum. This

agrees with the popular notion, formed independently of science, which
calls the reds the warm colors. Every one of us associates red with warmth;
in the summer red is rarely worn, it looks hot; but in winter red is one of the
most pleasing colors because of the sense of warmth and cheer it brings.
All light rays are accompanied by a small amount of heat, but the red rays
carry the most.
What seems perhaps the most unexpected thing, is that the temperature, as
indicated by a sensitive thermometer, continues to rise if the thermometer is
moved just beyond the red light of the spectrum. There actually seems to be
more heat beyond the red than in the red, but if the thermometer is moved
too far away, the temperature again falls. Later we shall see what this
means.
138. The Energy of the Sun. It is difficult to tell how much of the energy
of the sun is light and how much is heat, but it is easy to determine the
combined effect of heat and light.
Suppose we allow the sun's rays to fall
perpendicularly upon a metal cylinder coated
with lampblack and filled with a known quantity
of water (Fig. 89); at the expiration of a few
hours the temperature of the water will be
considerably higher. Lampblack is a good
absorber of heat, and it is used as a coating in
order that all the light rays which fall upon the
cylinder may be absorbed and none lost by
reflection.
Light and heat rays fall upon the lampblack,
pass through the cylinder, and heat the water.
We know that the red light rays have the largest
share toward heating the water, because if the FIG. 89.—The energy of the sun
cylinder is surrounded by blue glass which can be measured in heat units.
absorbs the red rays and prevents their passage
into the water, the temperature of the water begins to fall. That the other
light rays have a small share would have been clear from the preceding
Section.

All the energy of the sunshine which falls upon the cylinder, both as heat
and as light, is absorbed in the form of heat, and the total amount of this
energy can be calculated from the increase in the temperature of the water.
The energy which heated the water would have passed onward to the
surface of the earth if its path had not been obstructed by the cylinder of
water; and we can be sure that the energy which entered the water and
changed its temperature would ordinarily have heated an equal area of the
earth's surface; and from this, we can calculate the energy falling upon the
entire surface of the earth during any one day.
Computations based upon this experiment show that the earth receives daily
from the sun the equivalent of 341,000,000,000 horse power—an amount
inconceivable to the human mind.
Professor Young gives a striking picture of what this energy of the sun
could do. A solid column of ice 93,000,000 miles long and 2-1/4 miles in
diameter could be melted in a single second if the sun could concentrate its
entire power on the ice.
While the amount of energy received daily from the sun by the earth is
actually enormous, it is small in comparison with the whole amount given
out by the sun to the numerous heavenly bodies which make up the
universe. In fact, of the entire outflow of heat and light, the earth receives
only one part in two thousand million, and this is a very small portion
indeed.
139. How Light and Heat Travel from the Sun to Us. Astronomers tell us
that the sun—the chief source of heat and light—is 93,000,000 miles away
from us; that is, so far distant that the fastest express train would require
about 176 years to reach the sun. How do heat and light travel through this
vast abyss of space?

FIG. 90.—Waves formed by a pebble.

A quiet pool and a pebble will help to make it clear to us. If we throw a
pebble into a quiet pool (Fig. 90), waves or ripples form and spread out in
all directions, gradually dying out as they become more and more distant
from the pebble. It is a strange fact that while we see the ripple moving
farther and farther away, the particles of water are themselves not moving
outward and away, but are merely bobbing up and down, or are vibrating. If
you wish to be sure of this, throw the pebble near a spot where a chip lies
quiet on the smooth pond. After the waves form, the chip rides up and down
with the water, but does not move outward; if the water itself were moving
outward, it would carry the chip with it, but the water has no forward
motion, and hence the chip assumes the only motion possessed by the
water, that is, an up-and-down motion. Perhaps a more simple illustration is
the appearance of a wheat field or a lawn on a windy day; the wind sweeps
over the grass, producing in the grass a wave like the water waves of the
ocean, but the blades of grass do not move from their accustomed place in
the ground, held fast as they are by their roots.
If a pebble is thrown into a quiet pool, it creates ripples or waves which
spread outward in all directions, but which soon die out, leaving the pool
again placid and undisturbed. If now we could quickly withdraw the pebble
from the pool, the water would again be disturbed and waves would form. If
the pebble were attached to a string so that it could be dropped into the
water and withdrawn at regular intervals, the waves would never have a
chance to disappear, because there would always be a regularly timed
definite disturbance of the water. Learned men tell us that all hot bodies and
all luminous bodies are composed of tiny particles, called molecules, which
move unceasingly back and forth with great speed. In Section 95 we saw
that the molecules of all substances move unceasingly; their speed,
however, is not so great, nor are their motions so regularly timed as are
those of the heat-giving and the light-giving particles. As the particles of the
hot and luminous bodies vibrate with great speed and force they violently
disturb the medium around them, and produce a series of waves similar to
those produced in the water by the pebble. If, however, a pebble is thrown
into the water very gently, the disturbance is slight, sometimes too slight to
throw the water into waves; in the same way objects whose molecules are in
a state of gentle motion do not produce light.

The particles of heat-giving and light-giving bodies are in a state of rapid
vibration, and thereby disturb the surrounding medium, which transmits or
conveys the disturbance to the earth or to other objects by a train of waves.
When these waves reach their destination, the sensation of light or heat is
produced.
We see the water waves, but we can never see with the eye the heat and
light waves which roll in to us from that far-distant source, the sun. We can
be sure of them only through their effect on our bodies, and by the visible
work they do.
140. How Heat and Light Differ. If heat and light are alike due to the
regular, rapid motion of the particles of a body, and are similarly conveyed
by waves, how is it, then, that heat and light are apparently so different?
Light and heat differ as much as the short, choppy waves of the ocean and
the slow, long swell of the ocean, but not more so. The sailor handles his
boat in one way in a choppy sea and in a different way in a rolling sea, for
he knows that these two kinds of waves act dissimilarly. The long, slow
swell of the ocean would correspond with the longer, slower waves which
travel out from the sun, and which on reaching us are interpreted as heat.
The shorter, more frequent waves of the ocean would typify the short, rapid
waves which leave the sun, and which on reaching us are interpreted as
light.

CHAPTER XV
ARTIFICIAL LIGHTING

141. We seldom consider what life would be without our wonderful
methods of illumination which turn night into day, and prolong the hours of
work and pleasure. Yet it was not until the nineteenth century that the
marvelous change was made from the short-lived candle to the more
enduring oil lamp. Before the coming of the lamp, even in large cities like
Paris, the only artificial light to guide the belated traveler at night was the
candle required to be kept burning in an occasional window.
With the invention of the kerosene lamp came more efficient lighting of
home and street, and with the advent of gas and electricity came a light so
effective that the hours of business, manufacture, and pleasure could be
extended far beyond the setting of the sun.
The production of light by candle, oil, and gas will be considered in the
following paragraphs, while illumination by electricity will be reserved for
a later Chapter.
142. The Candle. Candles were originally made by dipping a wick into
melting tallow, withdrawing it, allowing the adhered tallow to harden, and
repeating the dipping until a satisfactory thickness was obtained. The more
modern method consists in pouring a fatty preparation into a mold, at the
center of which a wick has been placed.
The wick, when lighted, burns for a brief interval with a faint, uncertain
light; almost immediately, however, the intensity of the light increases and
the illumination remains good as long as the candle lasts. The heat of the
burning tallow melts more of the tallow near it, and this liquid fat is quickly
sucked up into the burning wick. The heat of the flame is sufficient to
change most of this liquid into a gas, that is, to vaporize the liquid, and

furthermore to set fire to the gas thus formed. These heated gases burn with
a bright yellow flame.
143. The Oil Lamp. The simple candle of our ancestors was now replaced
by the oil lamp, which gave a brighter, steadier, and more permanent
illumination. The principle of the lamp is similar to that of the candle,
except that the wick is saturated with kerosene or oil rather than with fat.
The heat from the burning wick is sufficient to change the oil into a gas and
then to set fire to the gas. By placing a chimney over the burning wick, a
constant and uniform draught of air is maintained around the blazing gases,
and hence a steady, unflickering light is obtained. Gases and carbon
particles are set free by the burning wick. In order that the gases may burn
and the solid particle glow, a plentiful supply of oxygen is necessary. If the
quantity of air is insufficient, the carbon particles remain unburned and
form soot. A lamp "smokes" when the air which reaches the wick is
insufficient to burn the rapidly formed carbon particles; this explains the
danger of turning a lamp wick too high and producing more carbon
particles than can be oxidized by the air admitted through the lamp
chimney.
One great disadvantage of oil lamps and oil stoves is that they cannot be
carried safely from place to place. It is almost impossible to carry a lamp
without spilling the oil. The flame soon spreads from the wick to the
overflowing oil and in consequence the lamp blazes and an explosion may
result. Candles, on the other hand, are safe from explosion; the dripping
grease is unpleasant but not dangerous.
The illumination from a shaded oil lamp is soft and agreeable, but the
trimming of the wicks, the refilling of bowls, and the cleaning of chimneys
require time and labor. For this reason, the introduction of gas met with
widespread success. The illumination from an ordinary gas jet is stronger
than that from an ordinary lamp, and the stronger illumination added to the
greater convenience has made gas a very popular source of light.
144. Gas Burners and Gas Mantles. For a long time, the only gas flame
used was that in which the luminosity resulted in heating particles of carbon
to incandescence. Recently, however, that has been widely replaced by use
of a Bunsen flame upon an incandescent mantle, such as the Welsbach. The

principle of the incandescent mantle is very simple. When certain
substances, such as thorium and cerium, are heated, they do not melt or
vaporize, but glow with an intense bright light. Welsbach made use of this
fact to secure a burner in which the illumination depends upon the glowing
of an incandescent, solid mantle, rather than upon the blazing of a burning
gas. He made a cylindrical mantle of thin fabric, and then soaked it in a
solution of thorium and cerium until it became saturated with the chemical.
The mantle thus impregnated with thorium and cerium is placed on the gas
jet, but before the gas is turned on, a lighted match is held to the mantle in
order to burn away the thin fabric. After the fabric has been burned away,
there remains a coarse gauze mantle of the desired chemicals. If now the
gas cock is opened, the escaping gas is ignited, the heat of the flame will
raise the mantle to incandescence and will produce a brilliant light. A very
small amount of burning gas is sufficient to raise the mantle to
incandescence, and hence, by the use of a mantle, intense light is secured at
little cost. The mantle saves us gas, because the cock is usually "turned on
full" whether we use a plain burner or a mantle burner. But, nevertheless,
gas is saved, because when the mantle is adjusted to the gas jet, the pressure
of the gas is lessened by a mechanical device and hence less gas escapes
and burns. By actual experiment, it has been found that an ordinary burner
consumes about five times as much gas per candle power as the best
incandescent burner, and hence is about five times as expensive. One
objection to the mantles is their tendency to break. But if the mantles are
carefully adjusted on the burner and are not roughly jarred in use, they last
many months; and since the best quality cost only twenty-five cents, the
expense of renewing the mantles is slight.
145. Gas for Cooking. If a cold object is held in the bright flame of an
ordinary gas jet, it becomes covered with soot, or particles of unburned
carbon. Although the flame is surrounded by air, the central portion of it
does not receive sufficient oxygen to burn up the numerous carbon particles
constantly thrown off by the burning gas, and hence many carbon particles
remain in the flame as glowing, incandescent masses. That some unburned
carbon is present in a flame is shown by the fact that whenever a cold object
is held in the flame, it becomes "smoked" or covered with soot. If enough
air were supplied to the flame to burn up the carbon as fast as it was set

free, there would be no deposition of soot on objects held over the flame or
in it, because the carbon would be transformed into gaseous matter.
Unburned carbon would be objectionable in cooking stoves where utensils
are constantly in contact with the flame, and for this reason cooking stoves
are provided with an arrangement by means of which additional air is
supplied to the burning gas in quantities adequate to insure complete
combustion of the rapidly formed carbon particles. An opening is made in
the tube through which gas passes to the burner, and as the gas moves past
this opening, it carries with it a draft of air. These openings are visible on all
gas stoves, and should be kept clean and free of clogging, in order to insure
complete combustion. So long as the supply of air is sufficient, the flame
burns with a dull blue color, but when the supply falls below that needed for
complete burning of the carbon, the blue color disappears, and a yellow
flame takes its place, and with the yellow flame the deposition of soot is
inevitable.
146. By-products of Coal Gas. Many important products besides
illuminating gas are obtained from the distillation of soft coal. Ammonia is
made from the liquids which collect in the condensers; anilin, the source of
exquisite dyes, is made from the thick, tarry distillate, and coke is the
residue left in the clay retorts. The coal tar yields not only anilin, but also
carbolic acid and naphthalene, both of which are commercially valuable,
the former as a widely used disinfectant, and the latter as a popular moth
preventive.
From a ton of good gas-producing coal can be obtained about 10,000 cubic
feet of illuminating gas, and as by-products 6 pounds of ammonia, 12
gallons of coal tar, and 1300 pounds of coke.
147. Natural Gas. Animal and vegetable matter buried in the depth of the
earth sometimes undergoes natural distillation, and as a result gas is formed.
The gas produced in this way is called natural gas. It is a cheap source of
illumination, but is found in relatively few localities and only in limited
quantity.
148. Acetylene. In 1892 it was discovered that lime and coal fused together
in the intense heat of the electric furnace formed a crystalline, metallic-
looking substance called calcium carbide. As a result of that discovery, this

substance was soon made on a large scale and sold at a moderate price. The
cheapness of calcium carbide has made it possible for the isolated
farmhouse to discard oil lamps and to have a private gas system. When the
hard, gray crystals of calcium carbide are put in water, they give off
acetylene, a colorless gas which burns with a brilliant white flame. If bits of
calcium carbide are dropped into a test tube containing water, bubbles of
gas will be seen to form and escape into the air, and the escaping gas may
be ignited by a burning match held near the mouth of the test tube. When
chemical action between the water and carbide has ceased, and gas bubbles
have stopped forming, slaked lime is all that is left of the dark gray crystals
which were put into the water.
When calcium carbide is used as a source of illumination, the crystals are
mechanically dropped into a tank containing water, and the gas generated is
automatically collected in a small sliding tank, whence it passes through
pipes to the various rooms. The slaked lime, formed while the gas was
generated, collects at the bottom of the tanks and is removed from time to
time.
The cost of an acetylene generator is about $50 for a small house, and the
cost of maintenance is not more than that of lamps. The generator does not
require filling oftener than once a week, and the labor is less than that
required for oil lamps. In a house in which there were twenty burners, the
tanks were filled with water and carbide but once a fortnight. Acetylene is
seldom used in large cities, but it is very widely used in small communities
and is particularly convenient in more or less remote summer residences.
Electric Lights. The most recent and the most convenient lighting is that
obtained by electricity. A fine, hairlike filament within a glass bulb is raised
to incandescence by the heat of an electric current. This form of
illumination will be considered in connection with electricity.

CHAPTER XVI
MAN'S WAY OF HELPING HIMSELF

149. Labor-saving Devices. To primitive man belonged more especially the
arduous tasks of the out-of-door life: the clearing of paths through the
wilderness; the hauling of material; the breaking up of the hard soil of
barren fields into soft loam ready to receive the seed; the harvesting of the
ripe grain, etc.

The more intelligent races among
men soon learned to help
themselves in these tasks. For
example, our ancestors in the field
soon learned to pry stones out of the
ground (Fig. 91) rather than to
undertake the almost impossible
task of lifting them out of the earth
in which they were embedded; to
swing fallen trees away from a path
by means of rope attached to one
FIG. 91.—Prying a stone out of the ground. end rather than to attempt to
remove them single-handed; to
pitch hay rather than to lift it; to clear a field with a rake rather than with the
hands; to carry heavy loads in wheelbarrows (Fig. 92) rather than on the
shoulders; to roll barrels up a plank (Fig. 93) and to raise weights by ropes.
In every case, whether in the lifting of stones, or the felling of trees, or the
transportation of heavy weights, or the digging of the ground, man used his
brain in the invention of mechanical devices which would relieve muscular
strain and lighten physical labor.
If all mankind had depended upon physical strength only, the world to-day
would be in the condition prevalent in parts of Africa, Asia, and South
America, where the natives loosen the soil with their hands or with crude
implements (Fig. 94), and transport huge weights on their shoulders and
heads.
Any mechanical device (Figs. 95 and 96), whereby man's work can be more
conveniently done, is called a machine; the machine itself never does any
work—it merely enables man to use his own efforts to better advantage.
150. When do we Work? Whenever, as a result of effort or force, an object
is moved, work is done. If you lift a knapsack from the floor to the table,
you do work because you use force and move the knapsack through a
distance equal to the height of the table. If the knapsack were twice as
heavy, you would exert twice as much force to raise it to the same height,
and hence you would do double the work. If you raised the knapsack twice

the
distance,
—say to
your
shoulder
s instead
of to the
level of
FIG. 93.—Rolling barrels up a plank. the
table,—
you would do twice the work, because while
FIG. 92.—The wheelbarrow
you would exert the same force you would
lightens labor.
continue it through double the distance.
Lifting heavy weights through great distances is
not the only way in which work is done.
Painting, chopping wood, hammering, plowing,
washing, scrubbing, sewing, are all forms of
work. In painting, the moving brush spreads
paint over a surface; in chopping wood, the
descending ax cleaves the wood asunder; in
scrubbing, the wet mop rubbed over the floor
carries dirt away; in every conceivable form of
work, force and motion occur.
FIG. 94.—Crude method of
farming. A man does work when he walks, a woman does
work when she rocks in a chair—although here
the work is less than in walking. On a windy day the work done in walking
is greater than normal. The wind resists our progress, and we must exert
more force in order to cover the same distance. Walking through a plowed
or rough field is much more tiring than to walk on a smooth road, because,
while the distance covered may be the same, the effort put forth is greater,
and hence more work is done. Always the greater the resistance
encountered, the greater the force required, and hence the greater the work
done.
The work done by a boy who raises a 5-pound knapsack to his shoulder
would be 5 × 4, or 20, providing his shoulders were 4 feet from the ground.

The amount of work done depends upon the force used and the distance
covered (sometimes called displacement), and hence we can say that
Work = force multiplied by distance,
or W = f × d.
151. Machines. A glance into our machine shops, our factories, and even
our homes shows how widespread is the use of complex machinery. But all
machines, however complicated in appearance, are in reality but
modifications and combinations of one or more of four simple machines
devised long ago by our remote ancestors. These simple devices are known
to-day, as (1) the lever, represented by a crowbar, a pitchfork; (2) the
inclined plane, represented by the plank upon which barrels are rolled into a
wagon; (3) the pulley, represented by almost any contrivance for the raising
of furniture to upper stories; (4) the wheel and axle, represented by
cogwheels and coffee grinders.
Suppose a 600-pound bowlder which is
embedded in the ground is needed for the tower
of a building. The problem of the builder is to
get the heavy bowlder out of the ground, to load
it on a wagon for transportation, and finally to
raise it to the tower. Obviously, he cannot do
this alone; the greatest amount of force of which
he is capable would not suffice to accomplish
any one of these tasks. How then does he help
himself and perform the impossible? Simply, by
the use of some of the machine types mentioned
above, illustrations of which are known in a
general way to every schoolboy. The very knife
with which a stick is whittled is a machine.
152. The Lever. Balance a foot rule, containing
a hole at its middle point F, as shown in Figure
97. If now a weight of 1 pound is suspended
from the bar at some point, say 12, the balance FIG. 95.—Primitive method of
is disturbed, and the bar swings about the point grinding corn.
F as a center. The balance can be regained by

su
sp
en
di
ng
an
eq
ui FIG. 97.—The principle of the lever.
va
le
FIG. 96.—Separating rice grains nt weight at the opposite end of the bar, or by
by flailing. applying a 2-pound weight at a point 3 inches to
the left of F. In the latter case a force of 1 pound
actually balances a force of 2 pounds, but the 1-
pound weight is twice as far from the point of suspension as is the 2-pound
weight. The small weight makes up in distance what it lacks in magnitude.
Such an arrangement of a rod or bar is called a lever. In any form of lever
there are only three things to be considered: the point where the weight
rests, the point where the force acts, and the point called the fulcrum about
which the rod rotates.
The distance from the force to the fulcrum is called the force arm. The
distance from the weight to the fulcrum is called the weight arm; and it is a
law of levers, as well as of all other machines, that the force multiplied by
the length of the force arm must equal the weight multiplied by the length
of the weight arm.
Force × force arm = weight × weight arm.
A force of 1 pound at a distance of 6, or with a force arm 6, will balance a
weight of 2 pounds with a weight arm 3; that is,
1 × 6 = 2 × 3.
Similarly a force of 10 pounds may be made to sustain a weight of 100
pounds, providing the force arm is 10 times longer than the weight arm; and
a force arm of 800 pounds, at a distance of 10 feet from the fulcrum, may

be made to sustain a weight of 8000 pounds, providing the weight is 1 foot
from the fulcrum.
153. Applications of the Lever. By means of a lever, a 600-pound bowlder
can be easily pried out of the ground. Let the lever, any strong metal bar, be
supported on a stone which serves as fulcrum; then if a man exerts his force
at the end of the rod somewhat as in Figure 91 (p. 154), the force arm will
be the distance from the stone or fulcrum to the end of the bar, and the
weight arm will be the distance from the fulcrum to the bowlder itself. The
man pushes down with a force of 100 pounds, but with that amount
succeeds in prying up the 600-pound bowlder. If, however, you look
carefully, you will see that the force arm is 6 times as long as the weight
arm, so that the smaller force is compensated for by the greater distance
through which it acts.
At first sight it seems as though the man's work were done for him by the
machine. But this is not so. The man must lower his end of the lever 3 feet
in order to raise the bowlder 6 inches out of the ground. He does not at any
time exert a large force, but he accomplishes his purpose by exerting a
small force continuously through a correspondingly greater distance. He
finds it easier to exert a force of 100 pounds continuously until his end has
moved 3 feet rather than to exert a force of 600 pounds on the bowlder and
move it 6 inches.
By the time the stone has been raised the man has done as much work as
though the stone had been raised directly, but his inability to put forth
sufficient muscular force to raise the bowlder directly would have rendered
impossible a result which was easily accomplished when through the
medium of the lever he could extend his small force through greater
distance.
154. The Wheelbarrow as a Lever. The principle of the lever is always the
same; but the relative position of the important points may vary. For
example, the fulcrum is sometimes at one end, the force at the opposite end,
and the weight to be lifted between them.
Suspend a stick with a hole at its center as in Figure 98, and hang a 4-pound
weight at a distance of 1 foot from the fulcrum, supporting the load by
means of a spring balance 2 feet from the fulcrum. The pointer on the

spring balance shows that the force
required to balance the 4-pound
load is but 2 pounds.
The force is 2 feet from the
fulcrum, and the weight (4) is 1 foot
from the fulcrum, so that
Force × distance = Weight ×
distance,
or 2 × 2 = 4 × 1.
FIG. 98.—A slightly different form of lever.
M
ov
e the 4-pound weight so that it is very
near the fulcrum, say but 6 inches from it;
then the spring balance registers a force
only one fourth as great as the weight
which it suspends. In other words a force
of 1 at a distance of 24 inches (2 feet) is
equivalent to a force of 4 at a distance of
6 inches.

FIG. 99.—The wheelbarrow lightend
labor.

FIG. 100.—A modified
wheelbarrow.

One of the most useful levers of this type is the wheelbarrow (Fig. 99). The
fulcrum is at the wheel, the force is at the handles, the weight is on the
wheelbarrow. If the load is halfway from the fulcrum to the man's hands,
the man will have to lift with a force equal to one half the load. If the load is

one fourth as far from the fulcrum as the man's hands, he will need to lift
with a force only one fourth as great as that of the load.
This shows that in loading a wheelbarrow, it is important to arrange the load
as near to the wheel as possible.
The nutcracker (Fig. 101) is an
illustration of a double lever of the
wheelbarrow kind; the nearer the
nut is to the fulcrum, the easier the
cracking.
H
a
FIG. 101.—The nutcracker is a lever. m
m
ers (Fig. 102), tack-lifters, scissors,
forceps, are important levers, and if you
will notice how many different levers (fig.
103) are used by all classes of men, you
will understand how valuable a machine
this simple device is.
FIG. 102.—The hand exerts a small force
155. The Inclined Plane. A man wishes over a long distance and draws out a nail.
to load the 600-pound bowlder on a
wagon, and proceeds to do it by means of a plank, as in Figure 93. Such an
arrangement is called an inclined plane.
The advantage of an inclined plane can be seen by the following
experiment. Select a smooth board 4 feet long and prop it so that the end A
(Fig. 104) is 1 foot above the level of the table; the length of the incline is
then 4 times as great as its height. Fasten a metal roller to a spring balance
and observe its weight. Then pull the roller uniformly upward along the
plank and notice what the pull is on the balance, being careful always to
hold the balance parallel to the incline.
When the roller is raised along the incline, the balance registers a pull only
one fourth as great as the actual weight of the roller. That is, when the roller
weighs 12, a force of 3 suffices to raise it to the height A along the incline;

but the smaller force must be applied throughout the entire length of the
incline. In many cases, it is preferable to exert a force of 30 pounds, for
example, over the distance CA than a force of 120 pounds over the shorter
distance BA.
Prop the board so that the end A is 2 feet above
the table level; that is, arrange the inclined plane
in such a way that its length is twice as great as
its height. In that case the steady pull on the
balance will be one half the weight of the roller;
or a force of 6 pounds will suffice to raise the
12-pound roller.
The
steeper
the
incline, FIG. 103.—Primitive man tried

the more to lighten his task by balancing
force his burden.

necessar
y to raise a weight; whereas if the
incline is small, the necessary
lifting force is greatly reduced. On
FIG. 104.—Less force is required to raise the an inclined plane whose length is
roller along the incline than to raise it to A ten times its height, the lifting force
directly. is reduced to one tenth the weight
of the load. The advantage of an
incline depends upon the relative length and height, or is equal to the ratio
of the length to the height.
156. Application. By the use of an inclined plank a strong man can load the
600-pound bowlder on a wagon. Suppose the floor of the wagon is 2 feet
above the ground, then if a 6-foot plank is used, 200 pounds of force will
suffice to raise the bowlder; but the man will have to push with this force
against the bowlder while it moves over the entire length of the plank.
Since work is equal to force multiplied by distance, the man has done work
represented by 200 × 6, or 1200. This is exactly the amount of work which

would have been necessary to raise the bowlder directly. A man of even
enormous strength could not lift such a weight (600 lb.) even an inch
directly, but a strong man can furnish the smaller force (200) over a distance
of 6 feet; hence, while the machine does not lessen the total amount of work
required of a man, it creates a new distribution of work and makes possible,
and even easy, results which otherwise would be impossible by human
agency.
157. Railroads and Highways. The problem of the incline is an important
one to engineers who have under their direction the construction of our
highways and the laying of our railroad tracks. It requires tremendous force
to pull a load up grade, and most of us are familiar with the struggling horse
and the puffing locomotive. For this reason engineers, wherever possible,
level down the steep places, and reduce the strain as far as possible.

FIG. 105.—A well-graded railroad bed.

The slope of the road is called its grade, and the grade itself is simply the
number of feet the hill rises per mile. A road a mile long (5280 feet) has a
grade of 132 if the crest of the hill is 132 feet above the level at which the
road started.

FIG. 106.—A long, gradual ascent is better than a shorter, steeper one.

In such an incline, the ratio of length to height is 5280 ÷ 132, or 40; and
hence in order to pull a train of cars to the summit, the engine would need
to exert a continuous pull equal to one fortieth of the combined weight of
the train.
If, on the other hand, the ascent had been gradual, so that the grade was 66
feet per mile, a pull from the engine of one eightieth of the combined
weight would have sufficed to land the train of cars at the crest of the grade.
Because of these facts, engineers spend great sums in grading down railroad
beds, making them as nearly level as possible. In mountainous regions, the
topography of the land prevents the elimination of all steep grades, but
nevertheless the attempt is always made to follow the easiest grades.
158. The Wedge. If an inclined plane is pushed underneath or within an
object, it serves as a wedge. Usually a wedge consists of two inclined
planes (Fig. 107).
A chisel and an ax are illustrations of wedges. Perhaps the most universal
form of a wedge is our common pin. Can you explain how this is a wedge?
159. The Screw. Another valuable and indispensable form of the inclined
plane is the screw. This consists of a metal rod around which passes a ridge,
and Figure 108 shows clearly that a screw is simply a rod around which (in
effect) an inclined plane has been wrapped.

The ridge encircling the
screw is called the thread,
and the distance between
two successive threads is
called the pitch. It is easy
to see that the closer the
threads and the smaller the
pitch, the greater the
advantage of the screw,
and hence the less force FIG. 108—A
needed in overcoming screw as a
FIG. 107.—By means of a wedge, the resistance. A corkscrew is a simple machine.
stump is split. familiar illustration of the
screw.
160. Pulleys. The pulley, another of the machines, is merely a grooved
wheel around which a cord passes. It is sometimes more convenient to
move a load in one direction rather than in another, and the pulley in its
simplest form enables us to do this. In order to raise a flag to the top of a
mast, it is not necessary to climb the mast, and so pull up the flag; the same
result is accomplished much more easily by attaching the flag to a movable
string, somewhat as in Figure 109, and pulling from below. As the string is
pulled down, the flag rises and ultimately reaches the desired position.
If we employ a stationary pulley, as in Figure 109, we do not change the
force, because the force required to balance the load is as large as the load
itself. The only advantage is that a force in one direction may be used to
produce motion in another direction. Such a pulley is known as a fixed
pulley.
161. Movable Pulleys. By the use of a movable pulley, we are able to
support a weight by a force equal to only one half the load. In Figure 109,
the downward pull of the weight and the downward pull of the hand are
equal; in Figure 110, the spring balance supports only one half the entire
load, the remaining half being borne by the hook to which the string is
attached. The weight is divided equally between the two parts of the string
which passes around the pulley, so that each strand bears only one half of
the burden.

We have seen in our study of the lever and the inclined
plane that an increase in force is always accompanied
by a decrease in distance, and in the case of the pulley
we naturally look for a similar result. If you raise the
balance (Fig. 110) 12 feet, you will find that the
weight rises only 6 feet; if you raise the balance 24
inches, you will find that the weight rises 12 inches.
You must exercise a force of 100 pounds over 12 feet
of space in order to raise a weight of 200 pounds a
distance of 6 feet. When we raise 100 pounds through
12 feet or 200 pounds through 6 feet the total work
done is the same; but the pulley enables those who
cannot furnish a force of 200 pounds for the space of 6
feet to accomplish the task by furnishing 100 pounds
for the space of 12 feet.
162. Combination of Pulleys. A FIG. 109.—By means of

combination of pulleys called a pulley, a force in one

block and tackle is used where direction produces

very heavy loads are to be motion in the opposite

moved. In Figure 111 the upper direction.

block of pulleys is fixed, the
lower block is movable, and one
continuous rope passes around
the various pulleys. The load is
supported by 6 strands, and each
strand bears one sixth of the load.
If the hand pulls with a force of 1
pound at P, it can raise a load of
6 pounds at W, but the hand will
have to pull downward 6 feet at P
in order to raise the load at W 1
foot. If 8 pulleys were used, a
FIG. 110.—A movable force equivalent to one eighth of
pulley lightens labor. the load would suffice to move
W, but this force would have to
FIG. 111.—An effective

be exerted over a distance 8 times as great as that arrangement of pulleys
through which W was raised. known as block and
tackle.
163. Practical Application. In our childhood many of
us saw with wonder the appearance and disappearance
of flags flying at the tops of high masts, but observation soon taught us that
the flags were raised by pulleys. In tenements, where there is no yard for
the family washing, clothes often appear flapping in mid-air. This seems
most marvelous until we learn that the lines are pulled back and forth by
pulleys at the window and at a distant support. By means of pulleys,
awnings are raised and lowered, and the use of pulleys by furniture movers,
etc., is familiar to every wide-awake observer on the streets.
164. Wheel and Axle. The wheel and axle consists of a large wheel and a
small axle so fastened that they rotate together.
When the large wheel makes one
revolution, P falls a distance equal to the
circumference of the wheel. While P
moves downward, W likewise moves, but
its motion is upward, and the distance it
moves is small, being equal only to the
circumference of the small axle. But a
small force at P will sustain a larger force
at W; if the circumference of the large
wheel is 40 inches, and that of the small
wheel 10 inches, a load of 100 at W can
be sustained by a force of 25 at P. The FIG. 112.—The wheel and axle.
increase in force of the wheel and axle
depends upon the relative size of the two parts, that is, upon the
circumference of wheel as compared with circumference of axle, and since
the ratio between circumference and radius is constant, the ratio of the
wheel and axle combination is the ratio of the long radius to the short
radius.
For example, in a wheel and axle of radii 20 and 4, respectively, a given
weight at P would balance 5 times as great a load at W.

165. Application. Windlass, Cogwheels. In the old-fashioned windlass used
in farming districts, the large wheel is replaced by a handle which, when
turned, describes a circle. Such an arrangement is equivalent to wheel and
axle (Fig. 112); the capstan used on shipboard for raising the anchor has the
same principle. The kitchen coffee grinder and the meat chopper are other
familiar illustrations.
Cogwheels are modifications of the wheel and axle. Teeth cut in A fit into
similar teeth cut in B, and hence rotation of A causes rotation of B. Several
revolutions of the smaller wheel, however, are necessary in order to turn the
larger wheel through one complete revolution; if the radius of A is one half
that of B, two revolutions of A will correspond to one of B; if the radius of
A is one third that of B, three revolutions of A will correspond to one of B.
Experiment demonstrates that a weight W
attached to a cogwheel of radius 3 can be raised
by a force P, equal to one third of W applied to a
cogwheel of radius 1. There is thus a great
increase in force. But the speed with which W is
raised is only one third the speed with which the
small wheel rotates, or increase in power has
been at the decrease of speed.
This is a very common method for raising heavy
weights by small force.
FIG. 113.—Cogwheels.
Cogwheels can be made to give speed at the
decrease of force. A heavy weight W attached to B will in its slow fall cause
rapid rotation of A, and hence rapid rise of P. It is true that P, the load
raised, will be less than W, the force exerted, but if speed is our aim, this
machine serves our purpose admirably.
An extremely important form of wheel and axle is that in which the two
wheels are connected by belts as in Figure 114. Rotation of W induces
rotation of w, and a small force at W is able to overcome a large force at w.
An advantage of the belt connection is that power at one place can be
transmitted over a considerable distance and utilized in another place.

166. Compound Machines. Out of the
few simple machines mentioned in the
preceding Sections has developed the
complex machinery of to-day. By a
combination of screw and lever, for
example, we obtain the advantage due to
each device, and some compound FIG. 114.—By means of a belt, motion
machines have been made which combine can be transferred from place to place.
all the various kinds of simple machines,
and in this way multiply their mechanical
advantage many fold.
A relatively simple complex machine called the crane (Fig. 116) maybe
seen almost any day on the street, or wherever heavy weights are being
lifted. It is clear that a force applied to turn wheel 1 causes a slower rotation
of wheel 3, and a still slower rotation of wheel 4, but as 4 rotates it winds
up a chain and slowly raises Q. A very complex machine is that seen in
Figure 117.
16
7.
M
ea
su
re
m
en
t
of
W
or
FIG. 115.—A simple derrick for raising
k.
weights.
In FIG. 116.—A traveling crane.
Se
ction 150, we learned that the
amount of work done depends upon the force exerted, and the distance
covered, or that W = force × distance. A man who raises 5 pounds a height
of 5 feet does far more work than a man who raises 5 ounces a height of 5

inches, but the product of force by distance is 25 in each case. There is
difficulty because we have not selected an arbitrary unit of work. The unit
of work chosen and in use in practical affairs is the foot pound, and is
defined as the work done when a force of 1 pound acts through a distance of
1 foot. A man who moves 8 pounds through 6 feet does 48 foot pounds of
work, while a man who moves 8 ounces (1/2 pound) through 6 inches (1/2
foot) does only one fourth of a foot pound of work.

FIG. 117.—A farm engine putting in a crop.

168. The Power or the Speed with which Work is Done. A man can load
a wagon more quickly than a growing boy. The work done by the one is
equal to the work done by the other, but the man is more powerful, because
the time required for a given task is very important. An engine which hoists
a 50-pound weight in 1 second is much more powerful than a man who
requires 50 seconds for the same task; hence in estimating the value of a
working agent, whether animal or mechanical, we must consider not only
the work done, but the speed with which it is done.
The rate at which a machine is able to accomplish a unit of work is called
power, and the unit of power customarily used is the horse power. Any
power which can do 550 foot pounds of work per second is said to be one
horse power (H.P.). This unit was chosen by James Watt, the inventor of a
steam engine, when he was in need of a unit with which to compare the new
source of power, the engine, with his old source of power, the horse.
Although called a horse power it is greater than the power of an average
horse.
An ordinary man can do one sixth of a horse power. The average
locomotive of a railroad has more than 500 H.P., while the engines of an
ocean liner may have as high as 70,000 H.P.

169. Waste Work and Efficient Work. In our study of machines we
omitted a factor which in practical cases cannot be ignored, namely,
friction. No surface can be made perfectly smooth, and when a barrel rolls
over an incline, or a rope passes over a pulley, or a cogwheel turns its
neighbor, there is rubbing and slipping and sliding. Motion is thus hindered,
and the effective value of the acting force is lessened. In order to secure the
desired result it is necessary to apply a force in excess of that calculated.
This extra force, which must be supplied if friction is to be counteracted, is
in reality waste work.
If the force required by a machine is 150 pounds, while that calculated as
necessary is 100 pounds, the loss due to friction is 50 pounds, and the
machine, instead of being thoroughly efficient, is only two thirds efficient.
Machinists make every effort to eliminate from a machine the waste due to
friction, leveling and grinding to the most perfect smoothness and
adjustment every part of the machine. When the machine is in use, friction
may be further reduced by the use of lubricating oil. Friction can never be
totally eliminated, however, and machines of even the finest construction
lose by friction some of their efficiency, while poorly constructed ones lose
by friction as much as one half of their efficiency.
170. Man's Strength not Sufficient for
Machines. A machine, an inert mass of
metal and wood, cannot of itself do any
work, but can only distribute the energy
which is brought to it. Fortunately it is not
necessary that this energy should be
contributed by man alone, because the
store of energy possessed by him is very
small in comparison with the energy
required to run locomotives, automobiles,
sawmills, etc. Perhaps the greatest value
of machines lies in the fact that they
FIG. 118.—Man's strength is not
enable man to perform work by the use of
sufficient for heavy work.
energy other than his own.

Figure 118 shows one way in which a horse's energy can be utilized in
lifting heavy loads. Even the fleeting wind has been harnessed by man, and,
as in the windmill, made to work for him (Fig. 119). One sees dotted over
the country windmills large and small, and in Holland, the country of
windmills, the landowner who does not possess a windmill is poor indeed.
For generations running water from rivers, streams, and falls has served
man by carrying his logs downstream, by turning the wheels of his mill,
etc.; and in our own day running water is used as an indirect source of
electric lights for street and house, the energy of the falling water serving to
rotate the armature of a dynamo (Section 310).
A more constant source of energy is that available from the burning of fuel,
such as coal and oil. The former is the source of energy in locomotives, the
latter in most automobiles.

FIG. 119.—The windmill pumps water into the troughs where cattle drink.

In the following Chapter will be given an account of water, wind, and fuel
as machine feeders.

CHAPTER XVII
THE POWER BEHIND THE ENGINE

171. Small boys soon learn the power of running water; swimming or
rowing downstream is easy, while swimming or rowing against the current
is difficult, and the swifter the water, the easier the one and the more
difficult the other; the river assists or opposes us as we go with it or against
it. The water of a quiet pool or of a gentle stream cannot do work, but water
which is plunging over a precipice or dam, or is flowing down steep slopes,
may be made to saw wood, grind our corn, light our streets, run our electric
cars, etc. A waterfall, or a rapid stream, is a great asset to any community,
and for this reason should be carefully guarded. Water power is as great a
source of wealth as a coal bed or a gold mine.
The most tremendous waterfall in our country is Niagara Falls, which every
minute hurls millions of gallons of water down a 163-foot precipice. The
energy possessed by such an enormous quantity of water flowing at such a
tremendous speed is almost beyond everyday comprehension, and would
suffice to run the engines of many cities far and near. Numerous attempts to
buy from the United States the right to utilize some of this apparently
wasted energy have been made by various commercial companies. It is
fortunate that these negotiations have been largely fruitless, because much
deviation of the water for commercial uses and the installation of machinery
in the vicinity of the famous falls would greatly detract from the beauty of
this world-known scene, and would rob our country of a natural beauty
unequaled elsewhere.
172. Water Wheels. In Figure 120 the water of a small but rapid mountain
stream is made to rotate a large wheel, which in turn communicates its
motion through belts to a distant sawmill or grinder. In more level regions
huge dams are built which hold back the water and keep it at a higher level
than the wheel; from the dam the water is conveyed in pipes (flumes) to the

paddle wheel which it turns.
Cogwheels or belts connect the
paddle wheel with the factory
machinery, so that motion of the
paddle wheel insures the running of
the machinery.
One of the most efficient forms of
water wheels is that shown in
Figure 121, and called the Pelton
wheel. Water issues in a narrow jet
similar to that of the ordinary
garden hose and strikes with great
FIG. 120.—A mountain stream turns the wheels
force against the lower part of the
of the mill.
wheel, thereby causing rotation of
the wheel. Belts transfer this
motion to the machinery of factory
or mill.
173. Turbines. The most efficient
form of water motor is the turbine,
a strong metal wheel shaped
somewhat like a pin wheel,
inclosed in a heavy metal case.
Water is conveyed from a reservoir
or dam through a pipe (penstock) to
the turbine case, in which is placed
FIG. 121.—The Pelton water wheel.
the heavy metal turbine wheel (Fig.
122). The force of the water causes
rotation of the turbine and of the
shaft which is rigidly fastened to it. The water which flows into the turbine
case causes rotation of the wheel, escapes from the case through openings,
and flows into the tail water.
The power which a turbine can furnish depends upon the quantity of water
and the height of the fall, and also upon the turbine wheel itself. One of the

largest turbines known has a horse power of about
20,000; that is, it is equivalent, approximately, to
20,000 horses.
174. How much is a Stream Worth? The work
which a stream can perform may be easily
calculated. Suppose, for example, that 50,000
pounds of water fall over a 22-foot dam every
second; the power of such a stream would be
1,100,000 foot pounds per second or 2000 H.P.
Naturally, a part of this power would be lost to use
by friction within the machinery and by leakage, so
that the power of a turbine run by a 2000 H.P.
stream would be less than that value.
Of course, the horse power to be obtained from a
stream determines the size of the paddle wheel or
turbine which can be run by it. It would be possible
to construct a turbine so large that the stream would
not suffice to turn the wheel; for this reason, the
power of a stream is carefully determined before
machine construction is begun, and the size of the
FIG. 122—A turbine at
machinery depends upon the estimates of the water
Niagara Falls.
power furnished by expert engineers.
A rough estimate of the volume of a stream may be
made by the method described below:—
Suppose we allow a stream of water to flow through a rectangular trough;
the speed with which the water flows through the trough can be determined
by noting the time required for a chip to float the length of the trough; if the
trough is 10 feet long and the time required is 5 seconds, the water has a
velocity of 2 feet per second.
The quantity of water which flows through the trough each second depends
upon the dimensions of the trough and the velocity of the water. Suppose
the trough is 5 feet wide and 3 feet high, or has a cross section of 15 square
feet. If the velocity of the water were 1 foot per second, then 15 cubic feet
of water would pass any given point each second, but since the velocity of

the water is 2 feet per second, 30 cubic feet will represent the amount of
water which will flow by a given point in one second.

FIG. 123.—Estimating the quantity of water which flows through the trough each second.

175. Quantity of Water Furnished by a River. Drive stakes in the river at
various places and note the time required for a chip to float from one stake
to another. If we know the distance between the stakes and the time
required for the chip to float from one stake to another, the velocity of the
water can be readily determined.
The width of the stream from bank to bank is easily measured, and the
depth is obtained in the ordinary way by sounding; it is necessary to take a
number of soundings because the bed of the river is by no means level, and
soundings taken at only one level would not give an accurate estimate. If
the soundings show the following depths: 30, 25, 20, 32, 28, the average
depth could be taken as 30 + 25 + 20 + 32 + 28 ÷ 5, or 27 feet. If, as a result
of measuring, the river at a given point in its course is found to be 27 feet
deep and 60 feet wide, the area of a cross section at that spot would be 1620
square feet, and if the velocity proved to be 6 feet per second, then the
quantity of water passing in any one second would be 1620 × 6, or 9720
cubic feet. By experiment it has been found that 1 cu. ft. of water weighs
about 62.5 lb. The weight of the water passing each second would therefore
be 62.5 × 9720, or 607,500 lb. If this quantity of water plunges over a 10-ft.
dam, it does 607,500 × 10, or 6,075,000 foot pounds of work per second, or

11,045 H.P. Such a stream would be very valuable for the running of
machinery.
176. Windmills. Those of us who have spent our vacation days in the
country know that there is no ready-made water supply there as in the cities,
but that as a rule the farmhouses obtain their drinking water from springs
and wells. In poorer houses, water is laboriously carried in buckets from the
spring or is lifted from the well by the windlass. In more prosperous houses,
pumps are installed; this is an improvement over the original methods, but
the quantity of water consumed by the average family is so great as to make
the task of pumping an arduous one.
The average amount of water used per day by one person is 25 gallons. This
includes water for drinking, cooking, dish washing, bathing, laundry. For a
family of five, therefore, the daily consumption would be 125 gallons; if to
this be added the water for a single horse, cow, and pig, the total amount
needed will be approximately 150 gallons per day. A strong man can pump
that amount from an ordinary well in about one hour, but if the well is deep,
more time and strength are required.
The invention of the windmill was a great
boon to country folks because it
eliminated from their always busy life one
task in which labor and time were
consumed.
177. The Principle of the Windmill. The
toy pin wheel is a windmill in miniature.
The wind strikes the sails, and causes
rotation; and the stronger the wind blows,
the faster will the wheel rotate. In
windmills, the sails are of wood or steel,
instead of paper, but the principle is FIG. 124.—The toy pin wheel is a
identical. miniature windmill.

As the wheel rotates, its motion is
communicated to a mechanical device which makes use of it to raise and
lower a plunger, and hence as long as the wind turns the windmill, water is
raised. The water thus raised empties into a large tank, built either in the

windmill tower or in the garret of the house, and from the tank the water
flows through pipes to the different parts of the house. On very windy days
the wheel rotates rapidly, and the tank fills quickly; in order to guard
against an overflow from the tank a mechanical device is installed which
stops rotation of the wheel when the tank is nearly full. The supply tank is
usually large enough to hold a supply of water sufficient for several days,
and hence a continuous calm of a day or two does not materially affect the
house flow. When once built, a windmill practically takes care of itself,
except for oiling, and is an efficient and cheap domestic possession.

FIG. 125.—The windmill pumps water into the tank.

178. Steam as a Working Power. If a delicate vane is held at an opening
from which steam issues, the pressure of the steam will cause rotation of the
vane (Fig. 126), and if the vane is connected with a machine, work can be
obtained from the steam.
When water is heated in an open vessel, the pressure of its steam is too low
to be of practical value, but if on the contrary water is heated in an almost
closed vessel, its steam pressure is considerable. If steam at high pressure is
directed by nozzles against the blades of a wheel, rapid rotation of the
wheel ensues just as it did in Figure 121, although in this case steam
pressure replaces water pressure. After the steam has spent itself in turning
the turbine, it condenses into water and makes its escape through openings

in an inclosing case. In Figure 127 the
protecting case is removed, in order that the
form of the turbine and the positions of the
nozzles may be visible.
A single
large
turbine
wheel
may
have as
many as
800,000
sails or
blades,
and
FIG. 126.—Steam as a source of steam
power. may
pour out
upon
these from many nozzles.
FIG. 127.—Steam turbine with many blades and
The steam turbine is very much 4 nozzles.
more efficient than its forerunner,
the steam engine. The installation of
turbines on ocean liners has been accompanied by great increase in speed,
and by an almost corresponding decrease in the cost of maintenance.
179. Steam Engines. A very simple illustration of the working of a steam
engine is given in Figure 128. Steam under pressure enters through the
opening F, passes through N, and presses upon the piston M. As a result M
moves downward, and thereby induces rotation in the large wheel L.
As M falls it drives the air in D out through O and P (the opening P is not
visible in the diagram).
As soon as this is accomplished, a mechanical device draws up the rod E,
which in turn closes the opening N, and thus prevents the steam from
passing into the part of D above M.

But when the rod E is in such a
position that N is closed, O on the
other hand is open, and steam
rushes through it into D and forces
up the piston. This up-and-down
motion of the piston causes
continuous rotation of the wheel L.
If the fire is hot, steam is formed
quickly, and the piston moves
rapidly; if the fire is low, steam is
formed slowly, and the piston
moves less rapidly.
The steam engine as seen on our
railroad trains is very complex, and
cannot be discussed here; in
principle, however, it is identical
with that just described. Figure 129
shows a steam harvester at work on
a modern farm.

FIG. 128.—The principle of the steam engine.

FIG. 129.—Steam harvester at work.

In both engine and turbine the real source of power is not the steam but the
fuel, such as coal or oil, which converts the water into steam.
180. Gas Engines. Automobiles have been largely responsible for the gas
engine. To carry coal for fuel and water for steam would be impracticable
for most motor cars. Electricity is used in some cars, but the batteries are

heavy, expensive, and short-lived, and are not always easily replaceable.
For this reason gasoline is extensively used, and in the average automobile
the source of power is the force generated by exploding gases.
It was discovered some years ago that if the vapor of gasoline or naphtha
was mixed with a definite quantity of air, and a light was applied to the
mixture, an explosion would result. Modern science uses the force of such
exploding gases for the accomplishment of work, such as running of
automobiles and launches.
In connection with the
gasoline supply is a
carburetor or sprayer, from
which the cylinder C (Fig.
130) receives a fine mist of
gasoline vapor and air. This
mixture is ignited by an
automatic, electric sparking
FIG. 130.—The gas engine. device, and the explosion of
the gases drives the piston P
to the right. In the 4-cycle
type of gas engines (Fig. 130)—the kind used in automobiles—the four
strokes are as follows: 1. The mixture of gasoline and air enters the cylinder
as the piston moves to the right. 2. The valves being closed, the mixture is
compressed as the piston moves to the left. 3. The electric spark ignites the
compressed mixture and drives the piston to the right. 4. The waste gas is
expelled as the piston moves to the left. The exhaust valve is then closed,
the inlet valve opened, and another cycle of four strokes begins.
The use of gasoline in launches and automobiles is familiar to many. Not
only are launches and automobiles making use of gas power, but the
gasoline engine has made it possible to propel aëroplanes through the air.

CHAPTER XVIII
PUMPS AND THEIR VALUE TO MAN

181. "As difficult as for water to run up a hill!" Is there any one who has not
heard this saying? And yet most of us accept as a matter of course the
stream which gushes from our faucet, or give no thought to the ingenuity
which devised a means of forcing water upward through pipes. Despite the
fact that water flows naturally down hill, and not up, we find it available in
our homes and office buildings, in some of which it ascends to the fiftieth
floor; and we see great streams of it directed upon the tops of burning
buildings by firemen in the streets below.
In the country, where there are no great central pumping stations, water for
the daily need must be raised from wells, and the supply of each household
is dependent upon the labor and foresight of its members. The water may be
brought to the surface either by laboriously raising it, bucket by bucket, or
by the less arduous method of pumping. These are the only means possible;
even the windmill does not eliminate the necessity for the pump, but merely
replaces the energy used by man in working it.
In some parts of our country we have oil beds or wells. But if this
underground oil is to be of service to man, it must be brought to the surface,
and this is accomplished, as in the case of water, by the use of pumps.
An old tin can or a sponge may serve to bale out water from a leaking
rowboat, but such a crude device would be absurd if employed on our huge
vessels of war and commerce. Here a rent in the ship's side would mean
inevitable loss were it not possible to rid the ship of the inflowing water by
the action of strong pumps.
Another and very different use to which pumps are put is seen in the
compression of gases. Air is forced into the tires of bicycles and
automobiles until they become sufficiently inflated to insure comfort in

riding. Some present-day systems of artificial refrigeration (Section 93)
could not exist without the aid of compressed gases.
Compressed air has played a very important role in mining, being sent into
poorly ventilated mines to improve the condition of the air, and to supply to
the miners the oxygen necessary for respiration. Divers and men who work
under water carry on their backs a tank of compressed air, and take from it,
at will, the amount required.
There are many forms of pumps, and they serve widely different purposes,
being essential to the operation of many industrial undertakings. In the
following Sections some of these forms will be studied.
182. The Air as Man's Servant. Long
before man harnessed water for turbines,
or steam for engines, he made the air
serve his purpose, and by means of it
raised water from hidden underground
depths to the surface of the earth;
likewise, by means of it, he raised to his
dwelling on the hillside water from the
FIG. 131.—Carrying water home from stream in the valley below. Those who
the spring. live in cities where running water is
always present in the home cannot realize
the hardship of the days when this "ready-
made" supply did not exist, but when man laboriously carried to his
dwelling, from distant spring and stream, the water necessary for the daily
need.
What are the characteristics of the air which have enabled man to
accomplish these feats? They are well known to us and may be briefly
stated as follows:—
(1) Air has weight, and 1 cubic foot of air, at atmospheric pressure, weighs
1-1/4 ounces.
(2) The air around us presses with a force of about 15 pounds upon every
square inch of surface that it touches.

(3) Air is elastic; it can be compressed, as in the
balloon or bicycle tire, but it expands immediately
when pressure is reduced. As it expands and occupies
more space, its pressure falls and it exerts less force
against the matter with which it comes in contact. If,
for example, 1 cubic foot of air is allowed to expand
and occupy 2 cubic feet of space, the pressure which it
exerts is reduced one half. When air is compressed, its
pressure increases, and it exerts a greater force against
the matter with which it comes in contact. If 2 cubic
feet of air are compressed to 1 cubic foot, the pressure
of the compressed air is doubled. (See Section 89.)
183. The Common Pump or Lifting Pump. Place a
tube containing a close-fitting piston in a vessel of
water, as shown in Figure 132. Then raise the piston
FIG. 132.—The
with the hand and notice that the water rises in the
atmosphere pressing
piston tube. The rise of water in the piston tube is
similar to the raising of lemonade through a straw downward on a pushes
water after the rising
(Section 77). The atmosphere presses with a force of
piston b.
15 pounds upon every square inch of water in the large
vessel, and forces some of it into the space left vacant
by the retreating piston. The common pump works in a similar manner. It
consists of a piston or plunger which moves back and forth in an air-tight
cylinder, and contains an outward opening valve through which water and
air can pass. From the bottom of the cylinder a tube runs down into the well
or reservoir, and water from the well has access to the cylinder through
another outward-moving valve. In practice the tube is known as the suction
pipe, and its valve as the suction valve.
In order to understand the action of a pump, we will suppose that no water
is in the pump, and we will pump until a stream issues from the spout. The
various stages are represented diagrammatically by Figure 133. In (1) the
entire pump is empty of water but full of air at atmospheric pressure, and
both valves are closed. In (2) the plunger is being raised and is lifting the
column of air that rests on it. The air and water in the inlet pipe, being thus
partially relieved of downward pressure, are pushed up by the atmospheric
pressure on the surface of the water in the well. When the piston moves

downward as in (3), the valve in the pipe closes by its own weight, and the
air in the cylinder escapes through the valve in the plunger. In (4) the piston
is again rising, repeating the process of (2). In (5) the process of (3) is being
repeated, but water instead of air is escaping through the valve in the
plunger. In (6) the process of (2) is being repeated, but the water has
reached the spout and is flowing out.

FIG. 133. Diagram of the process of pumping.

After the pump is in condition (6), motion of the plunger is followed by a
more or less regular discharge of water through the spout, and the quantity
of water which gushes forth depends upon the speed with which the piston
is moved. A strong man giving quick strokes can produce a large flow; a
child, on the other hand, is able to produce only a thin stream. Whoever
pumps must exert sufficient force to lift the water from the surface of the
well to the spout exit. For this reason the pump has received the name of
lifting pump.
184. The Force Pump. In the common pump, water
cannot not be raised higher than the spout. In many
cases it is desirable to force water considerably above
the pump itself, as, for instance, in the fire hose; under
such circumstances a type of pump is employed which
has received the name of force pump. This differs but
little from the ordinary lift pump, as a reference to
Figure 134 will show. Here both valves are placed in
the cylinder, and the piston is solid, but the principle is
the same as in the lifting pump.
An upward motion of the plunger allows water to enter
the cylinder, and the downward motion of the plunger
drives water through E. (Is this true for the lift pump
as well?) Since only the downward motion of the
plunger forces water through E, the discharge is
intermittent and is therefore not practical for
commercial purposes. In order to convert this
FIG. 134.—Force pump.
intermittent discharge into a steady stream, an air
chamber is installed near the discharge tube, as in
Figure 135. The water forced into the air chamber by the downward-moving
piston compresses the air and increases its pressure. The pressure of the
confined air reacts against the water and tends to drive it out of the
chamber. Hence, even when the plunger is moving upward, water is forced
through the pipe because of the pressure of the compressed air. In this way a
continuous flow is secured.
The height to which the water can be forced in the pipe depends upon the
size and construction of the pump and upon the force with which the

plunger can be moved. The larger the stream
desired and the greater the height to be reached,
the stronger the force needed and the more
powerful the construction necessary.
The force pump gets its name from the fact that
the moving piston drives or forces the water
through the discharge tube.
185. Irrigation and Drainage. History shows
that the lifting pump has been used by man since
the fourth century before Christ; for many
present-day enterprises this ancient form of
pump is inconvenient and impracticable, and
hence it has been replaced in many cases by
more modern types, such as rotary and
centrifugal pumps (Fig. 136). In these forms,
rapidly rotating wheels lift the water and drive it
onward into a discharge pipe, from which it FIG. 135.—The air chamber A
issues with great force. There is neither piston insures a continuous flow of
nor valve in these pumps, and the quantity of water.
water raised and the force with which it is
driven through the pipes depends solely upon the size of the wheels and the
speed with which they rotate.
Irrigation, or the artificial
watering of land, is of the
greatest importance in those
parts of the world where the
land is naturally too dry for
farming. In the United States,
approximately two fifths of
the land area is so dry as to
be worthless for agricultural
purposes unless artificially
FIG. 136.—Centrifugal pump with part of the casing cut watered. In the West, several
away to show the wheel. large irrigating systems have
been built by the federal

government, and at present about ten million acres of land have been
converted from worthless farms into fields rich in crops. Many irrigating
systems use centrifugal pumps to force water over long distances and to
supply it in quantities sufficient for vast agricultural needs. In many
regions, the success of a farm or ranch depends upon the irrigation
furnished in dry seasons, or upon man's ability to drive water from a region
of abundance to a remote region of scarcity.

FIG. 137.—Agriculture made possible by irrigation.

The draining of land is also a matter of considerable importance; swamps
and marshes which were at one time considered useless have been drained
and then reclaimed and converted into good farming land. The surplus
water is best removed by centrifugal pumps, since sand and sticks which
would clog the valves of an ordinary pump are passed along without
difficulty by the rotating wheel.

FIG. 138.—Rice for its growth needs periodical flooding, and irrigation often supplies the necessary
water.

186. Camping.—Its Pleasures and its Dangers. The allurement of a
vacation camp in the heart of the woods is so great as to make many
campers ignore the vital importance of securing a safe water supply. A river
bank may be beautiful and teeming with diversions, but if the river is used
as a source of drinking water, the results will almost always be fatal to
some. The water can be boiled, it is true, but few campers are willing to
forage for the additional wood needed for this apparently unnecessary
requirement; then, too, boiled water does not cool readily in summer, and
hence is disagreeable for drinking purposes.
The only safe course is to abandon the river as a source of drinking water,
and if a spring cannot be found, to drive a well. In many regions, especially
in the neighborhood of streams, water can be found ten or fifteen feet below
the surface. Water taken from such a depth has filtered through a bed of
soil, and is fairly safe for any purpose. Of course the deeper the well, the
safer will be the water. With the use of such a pump as will be described,
campers can, without grave danger, throw dish water, etc., on the ground
somewhat remote from the camp; this may not injure their drinking water
because the liquids will slowly seep through the ground, and as they filter
downward will lose their dangerous matter. All the water which reaches the
well pipes will have filtered through the soil bed and therefore will
probably be safe.
But while the careless disposal of wastes may not spoil the drinking water
(in the well to be described), other laws of health demand a thoughtful
disposal of wastes. The malarial mosquito and the typhoid fly flourish in
unhygienic quarters, and the only way to guard against their dangers is to
allow them neither food nor breeding place.
The burning of garbage, the discharge of waters into cesspools, or, in
temporary camps, the discharge of wastes to distant points through the
agency of a cheap sewage pipe will insure safety to campers, will lessen the
trials of flies and mosquitoes, and will add but little to the expense.
187. A Cheap Well for Campers. A two-inch galvanized iron pipe with a
strong, pointed end containing small perforations is driven into the ground
with a sledge hammer. After it has penetrated for a few feet, another length
is added and the whole is driven down, and this is repeated until water is

reached. A cheap pump is then attached to
the upper end of the drill pipe and serves
to raise the water. During the drilling,
some soil particles get into the pipe
through the perforations, and these cloud
the water at first; but after the pipe has
once been cleaned by the upward-moving
water, the supply remains clear. The flow
from such a well is naturally small; first,
because water is not abundant near the
surface of the earth, and second, because
cheap pumps are poorly constructed and
cannot raise a large amount. But the
supply will usually be sufficient for the
needs of simple camp life, and many a
small farm uses this form of well, not only
for household purposes, but for watering
the cattle in winter.
If the cheapness of such pumps were
known, their use would be more general
for temporary purposes. The cost of FIG. 139—A driven well.
material need not exceed $5 for a 10-foot
well, and the driving of the pipe could be made as much a part of the
camping as the pitching of the tent itself. If the camping site is abandoned at
the close of the vacation, the pump can be removed and kept over winter for
use the following summer in another place. In this way the actual cost of the
water supply can be reduced to scarcely more than $3, the removable pump
being a permanent possession. In rocky or mountain regions the driven well
is not practicable, because the driving point is blunted and broken by the
rock and cannot pierce the rocky beds of land.

FIG. 140.—Diagram showing how supplying a city with good water lessens sickness and death. The
lines b show the relative number of people who died of typhoid fever before the water was filtered;
the lines a show the numbers who died after the water was filtered. The figures are the number of
typhoid deaths occurring yearly out of 100,000 inhabitants.

188. Our Summer Vacation. It has been asserted by some city health
officials that many cases of typhoid fever in cities can be traced to the
unsanitary conditions existing in summer resorts. The drinking water of
most cities is now under strict supervision, while that of isolated farms, of
small seaside resorts, and of scattered mountain hotels is left to the care of
individual proprietors, and in only too many instances receives no attention
whatever. The sewage disposal is often inadequate and badly planned, and
the water becomes dangerously contaminated. A strong, healthy person,
with plenty of outdoor exercise and with hygienic habits, may be able to
resist the disease germs present in the poor water supply; more often the
summer guests carry back with them to their winter homes the germs of
disease, and these gain the upper hand under the altered conditions of city
and business life. It is not too much to say that every man and woman
should know the source of his summer table water and the method of

sewage disposal. If the conditions are
unsanitary, they cannot be remedied at once, but
another resort can be found and personal danger
can be avoided. Public sentiment and the loss of
trade will go far in furthering an effort toward
better sanitation.
In the driven well, water cannot reach the spout
unless it has first filtered through the soil to the
depth of the driven pipe; after such a journey it
is fairly safe, unless very large quantities of
sewage are present; generally speaking, such a
depth of soil is able to filter satisfactorily the
drainage of the limited number of people which
a driven well suffices to supply.
Abundant water is rarely reached at less than 75
feet, and it would usually be impossible to drive
a pipe to such a depth. When a large quantity of
water is desired, strong machines drill into the
ground and excavate an opening into which a
wide pipe can be lowered. I recently spent a
summer in the Pocono Mountains and saw such
a well completed. The machine drilled to a
depth of 250 feet before much water was FIG. 141.—A deep well with the
reached and to over 300 feet before a flow was piston in the water.
obtained sufficient to satisfy the owner. The
water thus obtained was to be the sole water supply of a hotel
accommodating 150 persons; the proprietor calculated that the requirements
of his guests, for bath, toilet, laundry, kitchen, etc., and the domestics
employed to serve them, together with the livery at their disposal,
demanded a flow of 10 gallons per minute. The ground was full of rock and
difficult to penetrate, and it required 6 weeks of constant work for two
skilled men to drill the opening, lower the suction pipe, and install the
pump, the cost being approximately $700.

FIG. 142.—Showing how drinking water can be contaminated from cesspool (c) and wash water (w).

The water from such a well is safe and pure except under the conditions
represented in Figure 142. If sewage or slops be poured upon the ground in
the neighborhood of the well, the liquid will seep through the ground and
some may make its way into the pump before it has been purified by the
earth. The impure liquid will thus contaminate the otherwise pure water and
will render it decidedly harmful. For absolute safety the sewage discharge
should be at least 75 feet from the well, and in large hotels, where there is
necessarily a large quantity of sewage, the distance should be much greater.
As the sewage seeps through the ground it loses its impurities, but the
quantity of earth required to purify it depends upon its abundance; a small
depth of soil cannot take care of an indefinite amount of sewage. Hence, the
greater the number of people in a hotel, or the more abundant the sewage,
the greater should be the distance between well and sewer.
By far the best way to avoid contamination is to see to it that the sewage
discharges into the ground below the well; that is, to dig the well in such a
location that the sewage drainage will be away from the well.
In cities and towns and large summer communities, the sewage of
individual buildings drains into common tanks erected at public expense;
the contents of these are discharged in turn into harbors and streams, or are
otherwise disposed of at great expense, although they contain valuable
substances. It has been estimated that the drainage or sewage of England
alone would be worth $ 80,000,000 a year if used as fertilizer.

A few cities, such as Columbus and Cleveland, Ohio, realize the need of
utilizing this source of wealth, and by chemical means deodorize their
sewage and change it into substances useful for agricultural and industrial
purposes. There is still a great deal to be learned on this subject, and it is
possible that chemically treated sewage may be made a source of income to
a community rather than an expense.
189. Pumps which Compress Air. The pumps considered in the preceding
Sections have their widest application in agricultural districts, where by
means of them water is raised to the surface of the earth or is pumped into
elevated tanks. From a commercial and industrial standpoint a most
important class of pump is that known as the compression type; in these, air
or any other gas is compressed rather than rarefied.
Air brakes and self-opening and self-closing doors on cars are operated by
means of compression pumps. The laying of bridge and pier foundations, in
fact all work which must be done under water, is possible only through the
agency of compression pumps. Those who have visited mines, and have
gone into the heart of the underground labyrinth, know how difficult it is for
fresh air to make its way to the miners. Compression pumps have
eliminated this difficulty, and to-day fresh air is constantly pumped into the
mines to supply the laborers there. Agricultural methods also have been
modified by the compression pump. The spraying of trees (Fig. 143),
formerly done slowly and laboriously, is now a relatively simple matter.
190. The Bicycle Pump. The bicycle pump is the best known of all
compression pumps. Here, as in other pumps of its type, the valves open
inward rather than outward. When the piston is lowered, compressed air is
driven through the rubber tubing, pushes open an inward-opening valve in
the tire, and thus enters the tire. When the piston is raised, the lower valve
closes, the upper valve is opened by atmospheric pressure, and air from
outside enters the cylinder; the next stroke of the piston drives a fresh
supply of air into the tire, which thus in time becomes inflated. In most
cheap bicycle pumps, the piston valve is replaced by a soft piece of leather
so attached to the piston that it allows air to slip around it and into the
cylinder, but prevents its escape from the cylinder (Fig. 144).

191.
How a
Man
works
under
Water.
Place
one end
of a
piece of
glass
FIG. 143.—Spraying trees by means of a tube in a
compression pump. vessel of
water
and notice that the water rises in the tube (Fig. FIG. 144.—The bicycle foot
145). Blow into the tube and see whether you pump.
can force the water wholly or partially down the
tube. If the tube is connected to a small
compression pump, sufficient air can be sent into the tube to cause the water
to sink and to keep the tube permanently clear of water. This is, in brief, the
principle employed for work under water. A compression pump forces air
through a tube into the chamber in which men are to work (Fig. 146). The
air thus furnished from above supplies the workmen with oxygen, and by its
pressure prevents water from entering the chamber. When the task has been
completed, the chamber is raised and later lowered to a new position.
Figure 147 shows men at work on a bridge foundation. Workmen, tools, and
supplies are lowered in baskets through a central tube BC provided with an
air chamber L, having air-tight gates at A and A'. The gate A is opened and
workmen enter the air chamber. The gate A is then closed and the gate A' is
opened slowly to give the men time to get accustomed to the high pressure
in B, and then the men are lowered to the bottom. Excavated earth is
removed in a similar manner. Air is supplied through a tube DD. Such an
arrangement for work under water is called a caisson. It is held in position
by a mass of concrete EE.
In many cases men work in diving suits rather than in caissons; these suits
are made of rubber except for the head piece, which is of metal provided

with
transpar
ent
eyepiece
s. Air is
supplied
through
a
flexible
tube by
a
compres
FIG. 146—The principle of work under water.
sion
FIG. 145.—Water does not enter
pump.
the tube as long as we blow into
The diver sometimes carries on his back a tank
it.
of compressed air, from which the air escapes
through a tube to the space between
the body and the suit. When the air
has become foul, the diver opens a
valve in his suit and allows it to
pass into the water, at the same time
admitting a fresh supply from the
tank. The valve opens outward from
the body, and hence will allow of
the exit of air but not of the
entrance of water. When the diver
ceases work and desires to rise to
the surface, he signals and is drawn
up by a rope attached to the suit.
192. Combination of Pumps. In
many cases the combined use of
FIG. 147—Showing how men can work under both exhaust and compression
water. pumps is necessary to secure the
desired result; as, for example, in
pneumatic dispatch tubes. These are employed in the transportation of
letters and small packages from building to building or between parts of the

same building. A pump removes air from the part of the tube ahead of the
package, and thus reduces the resistance, while a compression pump forces
air into the tube behind the package and thus drives it forward with great
speed.

CHAPTER XIX
THE WATER PROBLEM OF A LARGE CITY

193. It is by no means unusual for the residents of a large city or town to
receive through the newspapers a notification that the city water supply is
running low and that economy should be exercised in its use. The problem
of supplying a large city with an abundance of pure water is among the
most difficult tasks which city officials have to perform, and is one little
understood and appreciated by the average citizen.
Intense interest in personal and domestic affairs is natural, but every citizen,
rich or poor, should have an interest in civic affairs as well, and there is no
better or more important place to begin than with the water supply. One of
the most stirring questions in New York to-day has to do with the
construction of huge aqueducts designed to convey to the residents of the
city, water from the distant Catskill Mountains. The growth of the
population has been so phenomenally rapid that the combined output of all
available near-by sources does not suffice to meet the increasing
consumption.
Where does your city obtain its water? Does it bring it to its reservoirs in
the most economic way possible, and is there any legitimate excuse for the
scarcity of water which many communities face in dry seasons?
194. Two Possibilities. Sometimes a city is fortunate enough to be situated
near hills and mountains through which streams flow, and in that case the
water problem is simple. In such a case all that is necessary is to run pipes,
usually underground, from the elevated lakes or streams to the individual
houses, or to common reservoirs from which it is distributed to the various
buildings.

FIG. 148.—The elevated mountain lake serves as a source of water.

Figure 148 illustrates in a simple way the manner in which a mountain lake
may serve to supply the inhabitants of a valley. The city of Denver, for
example, is surrounded by mountains abounding in streams of pure, clear
water; pipes convey the water from these heights to the city, and thus a
cheap and adequate flow is obtained. Such a system is known as the gravity
system. The nearer and steeper the elevation, the greater the force with
which the water flows through the valley pipes, and hence the stronger the
discharge from the faucets.
Relatively few cities and towns are so favorably situated as regards water;
more often the mountains are too distant, or the elevation is too slight, to be
of practical value. Cities situated in plains and remote from mountains are
obliged to utilize the water of such streams as flow through the land,
forcing it to the necessary height by means of pumps. Streams which flow
through populated regions are apt to be contaminated, and hence water from
them requires public filtration. Cities using such a water supply thus have
the double expense of pumping and filtration.
195. The Pressure of Water. No practical business man would erect a
turbine or paddle wheel without calculating in advance the value of his
water power. The paddle wheel might be so heavy that the stream could not
turn it, or so frail in comparison with the water force that the stream would
destroy it. In just as careful a manner, the size and the strength of municipal
reservoirs and pumps must be calculated. The greater the quantity of water
to be held in the reservoir, the heavier are the walls required; the greater the

elevation of the houses, the stronger must be the pumps and the engines
which run them.
In order to understand how these calculations are made, we must study the
physical characteristics of water just as we studied the physical
characteristics of air.
When we measure water, we find that 1 cubic foot of it weighs about 62.5
pounds; this is equivalent to saying that water 1 foot deep presses on the
bottom of the containing vessel with a force of 62.5 pounds to the square
foot. If the water is 2 feet deep, the load supported by the vessel is doubled,
and the pressure on each square foot of the bottom of the vessel will be 125
pounds, and if the water is 10 feet deep, the load borne by each square foot
will be 625 pounds. The deeper the water, the greater will be the weight
sustained by the confining vessel and the greater the pressure exerted by the
water.
Since the pressure borne by 1 square foot
of surface is 62.5 pounds, the pressure
supported by 1 square inch of surface is
1/144 of 62.5 pounds, or .43 pound,
nearly 1/2 pound. Suppose a vessel held
water to the depth of 10 feet, then upon
every square inch of the bottom of that
vessel there would be a pressure of 4.34
pounds. If a one-inch tap were inserted in
the bottom of the vessel so that the water
flowed out, it would gush forth with a
force of 4.34 pounds. If the water were 20
feet deep, the force of the outflowing
water would be twice as strong, because
the pressure would be doubled. But the
flow would not remain constant, because
FIG. 149.—Water 1 foot deep exerts a
as the water leaves the outlet, less and less
pressure of 62.5 pounds a square foot.
of it remains in the vessel, and hence the
[209
pressure gradually sinks and the flow
drops correspondingly.

In seasons of prolonged
drought, the streams which
feed a city reservoir are apt to
contain less than the usual
amount of water, hence the
level of the water supply
sinks, the pressure at the
outlet falls, and the force of
the outflowing water is
lessened (Fig. 150).
196. Why the Water Supply
is not uniform in All Parts
of the City. In the preceding
Section, we saw that the flow
from a faucet depends upon
the height of the reserve
water above the tap. Houses
on a level with the main
supply pipes (Figs. 148 and
151) have a strong flow
FIG. 150.—The pressure at an outlet decreases as the
because the water is under the
level of the water supply sinks.
pressure of a column A;
houses situated on elevation
B have less flow, because the water is under the pressure of a shorter
column B; and houses at a considerable elevation C have a less rapid flow
corresponding to the diminished depth (C).
Not only does the flow vary with the elevation of the house, but it varies
with the location of the faucet within the house. Unless the reservoir is very
high, or the pumps very powerful, the flow on the upper floors is noticeably
less than that in the cellar, and in the upper stories of some high building the
flow is scarcely more than a feeble trickle.

FIG. 151.—Water pressure varies in different parts of a water system.

When the respective flows at A, B, and C (Fig. 151) are measured, they are
found to be far lower than the pressures which columns of water of the
heights A, B, and C have been shown by actual demonstration to exert. This
is because water, in flowing from place to place, expends force in
overcoming the friction of the pipes and the resistance of the air. The
greater the distance traversed by the water in its journey from reservoir to
faucet, the greater the waste force and the less the final flow.
In practice, large mains lead from the reservoir to the city, smaller mains
convey the water to the various sections of the city, and service pipes lead
to the individual house taps. During this long journey, considerable force is
expended against friction, and hence the flow at a distance from the
reservoir falls to but a fraction of its original strength. For this reason,
buildings situated near the main supply have a much stronger flow (Fig.
152) than those on the same level but remote from the supply. Artificial
reservoirs are usually constructed on the near outskirts of a town in order
that the frictional force lost in transmission may be reduced to a minimum.

FIG. 152.—The more distant the fountain, the weaker the flow.

In the case of a natural reservoir, such as an elevated lake or stream, the
distance cannot be planned or controlled. New York, for example, will
secure an abundance of pure water from the Catskill Mountains, but it will
lose force in transmission. Los Angeles is undertaking one of the greatest
municipal projects of the day. Huge aqueducts are being built which will
convey pure mountain water a distance of 250 miles, and in quantities
sufficient to supply two million people. According to calculations, the force
of the water will be so great that pumps will not be needed.
197. Why Water does not always flow from a Faucet.
Most of us have at times been annoyed by the inability to
secure water on an upper story, because of the drawing
off of a supply on a lower floor. During the working
hours of the day, immense quantities of water are drawn
off from innumerable faucets, and hence the quantity in
the pipes decreases considerably unless the supply
station is able to drive water through the vast network of
pipes as fast as it is drawn off. Buildings at a distance
from the reservoir suffer under such circumstances,
because while the diminished pressure is ordinarily
powerful enough to supply the lower floors, it is
frequently too weak to force a continuous stream to high
levels. At night, however, and out of working hours, few
faucets are open, less water is drawn off at any one time,
and the intricate pipes are constantly full of water under
high pressure. At such times, a good flow is obtainable
even on the uppermost floors.
FIG. 153.—A In order to overcome the disadvantage of a decrease in
standpipe. flow during the day, standpipes (Fig. 153) are sometimes
placed in various sections. These are practically small
steel reservoirs full of water and connecting with the city pipes. During
"rush" hours, water passes from these into the communicating pipes and
increases the available supply, while during the night, when the faucets are
turned off, water accumulates in the standpipe against the next emergency
(Figs. 151 and 154). The service rendered by the standpipe is similar to that
of the air cushion discussed in Section 184.

198. The Cost of Water. In the gravity system, where an elevated lake or
stream serves as a natural reservoir, the cost of the city's waterworks is
practically limited to the laying of pipes. But when the source of the supply
is more or less on a level with the surrounding land, the cost is great,
because the supply for the entire city must either be pumped into an
artificial reservoir, from which it can be distributed, or else must be driven
directly through the mains (Fig. 154).

FIG. 154.—Water must be got to the houses by means of pumps.

A gallon of water weighs approximately 8.3 pounds, and hence the work
done by a pump in raising a gallon of water to the top of an average house,
an elevation of 50 feet, is 8.3 × 50, or 415 foot pounds. A small
manufacturing town uses at least 1,000,000 gallons daily, and the work
done by a pump in raising that amount to an elevation of 50 feet would be
8.3 × 1,000,000 × 50, or 415,000,000 foot pounds.
The total work done during the day by the pump, or the engine driving the
pump, is 415,000,000 foot pounds, and hence the work done during one
hour would be 1/24 of 415,000,000, or 17,291,666 foot pounds; the work
done in one minute would be 1/60 of 17,291,666, or 288,194 foot pounds,
and the work done each second would be 1/60 of 288,194, or 4803 foot
pounds.
A 1-H.P. engine does 550 foot pounds of work each second, and therefore if
the pump is to be operated by an engine, the strength of the latter would
have to be 8.7 H.P. An 8.7-H.P. pumping engine working at full speed every
second of the day and night would be able to supply the town with the
necessary amount of water. When, however, we consider the actual height
to which the water is raised above the pumping station, and the extra
pumping which must be done in order to balance the frictional loss, it is

easy to understand that in actual practice a much more powerful engine
would be needed. The larger the piston and the faster it works, the greater is
the quantity of water raised at each stroke, and the stronger must be the
engine which operates the pump.
In many large cities there is no one single pumping station from which
supplies run to all parts of the city, but several pumping stations are
scattered throughout the city, and each of them supplies a restricted
territory.
199. The Bursting of Dams and Reservoirs. The construction of a safe
reservoir is one of the most important problems of engineers. In October,
1911, a town in Pennsylvania was virtually wiped out of existence because
of the bursting of a dam whose structure was of insufficient strength to
resist the strain of the vast quantity of water held by it. A similar breakage
was the cause of the fatal Johnstown flood in 1889, which destroyed no less
than seven towns, and in which approximately 2000 persons are said to
have lost their lives.
Water presses not only on the bottom of a vessel, but upon the sides as well;
a bucket leaks whether the hole is in its side or its bottom, showing that
water presses not only downward but outward. Usually a leak in a dam or
reservoir occurs near the bottom. Weak spots at the top are rare and easily
repaired, but a leak near the bottom is usually fatal, and in the case of a
large reservoir the outflowing water carries death and destruction to
everything in its path.
If the leak is near the surface, as at a (Fig. 155), the water issues as a feeble
stream, because the pressure against the sides at that level is due solely to
the relatively small height of water above a (Section 195). If the leak is
lower, as at b, the issuing stream is stronger and swifter, because at that
level the outward pressure is much greater than at a, the increase being due
to the fact that the height of the water above b is greater than that above a.
If the leak is quite low, as at c, the issuing stream has a still greater speed
and strength, and gushes forth with a force determined by the height of the
water above c.
The dam at Johnstown was nearly 1/2 mile wide, and 40 feet high, and so
great was the force and speed of the escaping stream that within an hour

after the break had occurred, the water
had traveled a distance of 18 miles, and
had destroyed property to the value of
millions of dollars.
If a reservoir has a depth of 100 feet, the
pressure exerted upon each square foot of
its floor is 62.5 × 100, or 6250 pounds;
the weight therefore to be sustained by
every square foot of the reservoir floor is
somewhat more than 3 tons, and hence
strong foundations are essential. The
outward lateral pressure at a depth of 25 FIG. 155.—The flow from an opening
feet would be only one fourth as great as depends upon the height of water above
that on the bottom—hence the strain on the opening.
the sides at that depth would be relatively
slight, and a less powerful construction would suffice. But at a depth of 50
feet the pressure on the sides would be one half that of the floor pressure, or
1-1/2 tons. At a depth of 75 feet, the pressure on the sides would be three
quarters that on the bottom, or 2-1/4 tons. As the bottom of the reservoir is
approached, the pressure against the sides increases, and more powerful
construction becomes necessary.
Small elevated tanks, like those of the windmill, frequently have heavy iron
bands around their lower portion as a protection against the extra strain.
Before erecting a dam or reservoir, the maximum pressure to be exerted
upon every square inch of surface should be accurately calculated, and the
structure should then be built in such a way that the varying pressure of the
water can be sustained. It is not sufficient that the bottom be strong; the
sides likewise must support their strain, and hence must be increased in
strength with depth. This strengthening of the walls is seen clearly in the
reservoir shown in Figure 152. The bursting of dams and reservoirs has
occasioned the loss of so many lives, and the destruction of so much
property, that some states are considering the advisability of federal
inspection of all such structures.

200. The Relation of Forests to
the Water Supply. When heavy
rains fall on a bare slope, or when
snow melts on a barren hillside, a
small amount of the water sinks
into the ground, but by far the
greater part of it runs off quickly
and swells brooks and streams, thus
causing floods and freshets.
When, however, rain falls on a
FIG. 156.—The lock gates must be strong in
wooded slope, the action is
order to withstand the great pressure of the water
reversed; a small portion runs off,
against them.
while the greater portion sinks into
the soft earth. This is due partly to
the fact that the roots of trees by their constant growth keep the soil loose
and open, and form channels, as it were, along which the water can easily
run. It is due also to the presence on the ground of decaying leaves and
twigs, or humus. The decaying vegetable matter which covers the forest
floor acts more or less as a sponge, and quickly absorbs falling rain and
melting snow. The water which thus passes into the humus and the soil
beneath does not remain there, but slowly seeps downward, and finally after
weeks and months emerges at a lower level as a stream. Brooks and springs
formed in this way are constant feeders of rivers and lakes.
In regions where the land has been deforested, the rivers run low in season
of prolonged drought, because the water which should have slowly seeped
through the soil, and then supplied the rivers for weeks and months, ran off
from the barren slopes in a few days.
Forests not only lessen the danger of floods, but they conserve our
waterways, preventing a dangerous high-water mark in the season of heavy
rains and melting snows, and then preventing a shrinkage in dry seasons
when the only feeders of the rivers are the underground sources. In the
summer of 1911, prolonged drought in North Carolina lowered the rivers to
such an extent that towns dependent upon them suffered greatly. The city of
Charlotte was reduced for a time to a practically empty reservoir; washing
and bathing were eliminated, machinery dependent upon water-power and

steam stood idle, and every glass of water drunk was carefully reckoned.
Thousands of gallons of water were brought in tanks from neighboring
cities, and were emptied into the empty reservoir from whence it trickled
slowly through the city mains. The lack of water caused not only personal
inconvenience and business paralysis, but it occasioned real danger of
disease through unflushed sewers and insufficiently drained pipes.
The conservation of the forest means the conservation of our waterways,
whether these be used for transportation or as sources of drinking water.

CHAPTER XX
MAN'S CONQUEST OF SUBSTANCES

201. Chemistry. Man's mechanical inventions have been equaled by his
chemical researches and discoveries, and by the application he has made of
his new knowledge.
The plain cotton frock of our grandmothers had its death knell sounded a
few years ago, when John Mercer showed that cotton fabrics soaked in
caustic soda assumed under certain conditions a silky sheen, and when dyed
took on beautiful and varied hues. The demonstration of this simple fact
laid the foundation for the manufacture of a vast variety of attractive dress
materials known as mercerized cotton.
Possibly no industry has been more affected by chemical discovery than
that of dyeing. Those of us who have seen the old masterpieces in painting,
or reproductions of them, know the softness, the mellowness, the richness
of tints employed by the old masters. But if we look for the brilliancy and
variety of color seen in our own day, the search will be fruitless, because
these were unknown until a half century ago. Up to that time, dyes were
few in number and were extracted solely from plants, principally from the
indigo and madder plants. But about the year 1856 it was discovered that
dyes in much greater variety and in purer form could be obtained from coal
tar. This chemical production of dyes has now largely supplanted the
original method, and the industry has grown so rapidly that a single firm
produced in one year from coal tar a quantity of indigo dye which under the
natural process of plant extraction would have required a quarter million
acres of indigo plant.
The abundance and cheapness of newspapers, coarse wrapping papers, etc.,
is due to the fact that man has learned to substitute wood for rags in the
manufacture of paper. Investigation brought out the fact that wood
contained the substance which made rags valuable for paper making. Since

the supply of rags was far less than the demand, the problem of the
extraction from wood of the paper-forming substance was a vital one. From
repeated trials, it was found that caustic soda when heated with wood chips
destroyed everything in the wood except the desired substance, cellulose;
this could be removed, bleached, dried, and pressed into paper. The
substitution of wood for rags has made possible the daily issue of
newspapers, for the making of which sufficient material would not
otherwise have been available. When we reflect that a daily paper of wide
circulation consumes ten acres of wood lot per day, we see that all the rags
in the world would be inadequate to meet this demand alone, to say nothing
of periodicals, books, tissue paper, etc.
Chemistry plays a part in every phase of life; in the arts, the industries, the
household, and in the body itself, where digestion, excretion, etc., result
from the action of the bodily fluids upon food. The chemical substances of
most interest to us are those which affect us personally rather than
industrially; for example, soap, which cleanses our bodies, our clothing, our
household possessions; washing soda, which lightens laundry work; lye,
which clears out the drain pipe clogged with grease; benzine, which
removes stains from clothing; turpentine, which rids us of paint spots left by
careless workmen; and hydrogen peroxide, which disinfects wounds and
sores.
In order to understand the action of several of these substances we must
study the properties of two groups of chemicals—known respectively as
acids and bases; the first of these may be represented by vinegar, sulphuric
acid, and oxalic acid; and the second, by ammonia, lye, and limewater.
202. Acids. All of us know that vinegar and lemon juice have a sour taste,
and it is easy to show that most acids are characterized by a sour taste. If a
clean glass rod is dipped into very dilute acid, such as acetic, sulphuric, or
nitric acid, and then lightly touched to the tongue, it will taste sour. But the
best test of an acid is by sight rather than by taste, because it has been found
that an acid is able to discolor a plant substance called litmus. If paper is
soaked in a litmus solution until it acquires the characteristic blue hue of the
plant substance, and is then dried thoroughly, it can be used to detect acids,
because if it comes in contact with even the minutest trace of acid, it loses
its blue color and assumes a red tint. Hence, in order to detect the presence

of acid in a substance, one has merely to put some of the substance on blue
litmus paper, and note whether or not the latter changes color. This test
shows that many of our common foods contain some acid; for example,
fruit, buttermilk, sour bread, and vinegar.
The damage which can be done by strong acids is well known; if a jar of
sulphuric acid is overturned, and some of it falls on the skin, it eats its way
into the flesh and leaves an ugly sore; if it falls on carpet or coat, it eats its
way into the material and leaves an unsightly hole. The evil results of an
accident with acid can be lessened if we know just what to do and do it
quickly, but for this we must have a knowledge of bases, the second group
of chemicals.
203. Bases. Substances belonging to this group usually have a bitter taste
and a slimy, soapy feeling. For our present purposes, the most important
characteristic of a base is that it will neutralize an acid and in some measure
hinder the damage effected by the former. If, as soon as an acid has been
spilled on cloth, a base, such as ammonia, is applied to the affected region,
but little harm will be done. In your laboratory experiments you may be
unfortunate enough to spill acid on your body or clothing; if so, quickly
apply ammonia. If you delay, the acid does its work, and there is no remedy.
If soda (a base) touches black material, it discolors it and leaves an ugly
brown spot; but the application of a little acid, such as vinegar or lemon
juice, will often restore the original color and counteract the bad effects of
the base. Limewater prescribed by physicians in cases of illness is a well-
known base. This liquid neutralizes the too abundant acids present in a
weak system and so quiets and tones the stomach.
The interaction of acids and bases may be observed in another way. If blue
litmus paper is put into an acid solution, its color changes to red; if now the
red litmus paper is dipped into a base solution, caustic soda, for example, its
original color is partially restored. What the acid does, the base undoes,
either wholly or in part. Bases always turn red litmus paper blue.
Bases, like acids, are good or bad according to their use; if they come in
contact with cloth, they eat or discolor it, unless neutralized by an acid. But
this property of bases, harmful in one way, is put to advantage in the home,
where grease is removed from drainpipe and sink by the application of lye,

a strong base. If the lye is too concentrated, it will not only eat the grease,
but will corrode the metal piping; it is easy, however, to dilute base
solutions to such a degree that they will not affect piping, but will remove
grease. Dilute ammonia is used in almost every home and is an
indispensable domestic servant; diluted sufficiently, it is invaluable in the
washing of delicate fabrics and in the removing of stains, and in a more
concentrated form it is helpful as a smelling salt in cases of fainting.
Some concentrated bases are so powerful in their action on grease, cloth,
and metal that they have received the designation caustic, and are ordinarily
known as caustic soda, caustic potash (lye), and caustic lime. These more
active bases are generally called alkalies in distinction from the less active
ones.
204. Neutral Substances. To any acid solution add gradually a small
quantity of a base, and test the mixture from time to time with blue litmus
paper; at first the paper will turn red quickly, but as more and more of the
base is added to the solution, it has less and less effect on the blue litmus
paper, and finally a point is reached when a fresh strip of blue paper will not
be affected. Such a result indicates infallibly the absence of any acid
qualities in the solution. If now red litmus paper is tested in the same
solution, its color also will remain unchanged; such a result indicates
infallibly the absence of any basic quality. The solution has the
characteristic property of neither acid nor base and is said to be neutral.
If to the neutral solution an extra portion of base is added, so that there is an
excess of base over acid, the neutralization is overbalanced and the red
paper turns blue. If to the neutral solution an extra portion of acid is added,
so that there is an excess of acid over base, the neutralization is
overbalanced in the opposite direction, and the solution acquires acid
characteristics.
Most acids and bases will eat and corrode and discolor, while neutral
substances will not; it is for this reason that soap, a slightly alkaline
substance, is the safest cleansing agent for laundry, bath, and general work.
Good soaps, being carefully made, are so nearly neutral that they will not
fade the color out of clothing; the cheap soaps are less carefully prepared

and are apt to have a strong excess of the base ingredient; such soaps are
not safe for delicate work.
205. Soap. If we gather together scrapings of lard, butter, bits of tallow
from burned-out candles, scraps of waste fat, or any other sort of grease,
and pour a strong solution of lye over the mass, a soft soapy substance is
formed. In colonial times, every family made its own supply of soap,
utilizing, for that purpose, household scraps often regarded by the
housekeeper of to-day as worthless. Grease and fat were boiled with water
and hardwood ashes, which are rich in lye, and from the mixture came the
soft soap used by our ancestors. In practice, the wood ashes were boiled in
water, which was then strained off, and the resulting filtrate, or lye, was
mixed with the fats for soap making.
Most fats contain a substance of an acid nature, and are decomposed by the
action of bases such as caustic soda and caustic potash. The acid component
of the grease partially neutralizes the base, and a new substance is formed,
namely, soap.
With the advance of civilization the labor of soap making passed from the
home to the factory, very much as bread making has done in our own day.
Different varieties of soaps appeared, of which the hard soap was the most
popular, owing to the ease with which it could be transported. Within the
last few years liquid soaps have come into favor, especially in schools,
railroad stations, and other public places, where a cake of soap would be
handled by many persons. By means of a simple device (Fig. 157), the soap
escapes from a receptacle when needed. The mass of the soap does not
come in contact with the skin, and hence the spread of contagious skin
diseases is lessened.
Commercial soaps are made from a great variety of substances, such as
tallow, lard, castor oil, coconut oil, olive oil, etc.; or in cheaper soaps, from
rosin, cottonseed oil, and waste grease. The fats which go to waste in our
garbage could be made a source of income, not only to the housewife, but
to the city. In Columbus, Ohio, garbage is used as a source of revenue; the
grease from the garbage being sold for soap making, and the tankage
(Section 188) for fertilizer.

206. Why Soap Cleans. The natural oil of the skin
catches and retains dust and dirt, and makes a greasy
film over the body. This cannot be removed by water
alone, but if soap is used and a generous lather is
applied to the skin, the dirt is "cut" and passes from
the body into the water. Soap affects a grease film and
water very much as the white of an egg affects oil and
water. These two liquids alone do not mix, the oil
remaining separate on the surface of the water; but if a
small quantity of white of egg is added, an emulsion is
formed, the oil separating into minute droplets which
spread through the water. In the same way, soap acts
on a grease film, separating it into minute droplets
which leave the skin and spread through the water, FIG. 157.—Liquid soap
container.
carrying with them the dust and dirt particles. The
warmer the water, the better will be the emulsion, and
hence the more effective the removal of dirt and grease. This explanation
holds true for the removal of grease from any surface, whether of the body,
clothing, furniture, or dishes.
207. Washing Powders. Sometimes soap refuses to form a lather and
instead cakes and floats as a scum on the top of the water; this is not the
fault of the soap but of the water. As water seeps through the soil or flows
over the land, it absorbs and retains various soil constituents which modify
its character and, in some cases, render it almost useless for household
purposes. Most of us are familiar with the rain barrel of the country house,
and know that the housewife prefers rain water for laundry and general
work. Rain water, coming as it does from the clouds, is free from the
chemicals gathered by ground water, and is hence practically pure. While
foreign substances do not necessarily injure water for drinking purposes
(Section 69), they are often of such a nature as to prevent soap from
forming an emulsion, and hence from doing its work. Under such
circumstances the water is said to be hard, and soap used with it is wasted.
Even if water is only moderately hard, much soap is lost. The substances
which make water hard are calcium and magnesium salts. When soap is put
into water containing one or both of these, it combines with the salts to
form sticky insoluble scum. It is therefore not free to form an emulsion and

to remove grease. As a cleansing agent it is valueless. The average city
supply contains so little hardness that it is satisfactory for toilet purposes;
but in the laundry, where there is need for the full effect of the soap, and
where the slightest loss would aggregate a great deal in the course of time,
something must be done to counteract the hardness. The addition of soda, or
sodium carbonate to the water will usually produce the desired effect.
Washing soda combines with calcium and magnesium and prevents them
from uniting with soap. The soap is thus free to form an emulsion, just as in
ordinary water. Washing powders are sometimes used instead of washing
soda. Most washing powders contain, in addition to a softening agent, some
alkali, and hence a double good is obtained from their use; they not only
soften the water and allow the soap to form an emulsion, but they also,
through their alkali content, cut the grease and themselves act as cleansers.
In some cities where the water is very hard, as in Columbus, Ohio, it is
softened and filtered at public expense, before it leaves the reservoirs. But
even under these circumstances, a moderate use of washing powder is
general in laundry work.
If washing powder is put on clothes dry, or is thrown into a crowded tub, it
will eat the clothes before it has a chance to dissolve in the water. The only
safe method is to dissolve the powder before the clothes are put into the tub.
The trouble with our public laundries is that many of them are careless
about this very fact, and do not take time to dissolve the powder before
mixing it with the clothes.
The strongest washing powder is soda, and this cheap form is as good as
any of the more expensive preparations sold under fancy names. Borax is a
milder powder and is desirable for finer work.
One of the most disagreeable consequences of the use of hard water for
bathing is the unavoidable scum which forms on the sides of bathtub and
washbowl. The removal of the caked grease is difficult, and if soap alone is
used, the cleaning of the tub requires both patience and hard scrubbing. The
labor can be greatly lessened by moistening the scrubbing cloth with
turpentine and applying it to the greasy film, which immediately dissolves
and thus can be easily removed. The presence of the scum can be largely
avoided by adding a small amount of liquid ammonia to the bath water. But

many persons object to this; hence it is well to have some other easy
method of removing the objectionable matter.
208. To remove Stains from Cloth. While soap is, generally speaking, the
best cleansing agent, there are occasions when other substances can be used
to better advantage. For example, grease spots on carpet and non-washable
dress goods are best removed by the application of gasoline or benzine.
These substances dissolve the grease, but do not remove it from the
clothing; for that purpose a woolen cloth should be laid under the stain in
readiness to absorb the benzine and the grease dissolved in it. If the grease
is not absorbed while in solution, it remains in the clothing and after the
evaporation of the benzine reappears in full force.
Cleaners frequently clean suits by laying a blotter over a grease spot and
applying a hot iron; the grease, when melted by the heat, takes the easiest
way of spreading itself and passes from cloth to blotter.
209. Salts. A neutral liquid formed as in Section 204, by the action of
hydrochloric acid and the alkali solution of caustic soda, has a brackish,
salty taste, and is, in fact, a solution of salt. This can be demonstrated by
evaporating the neutral liquid to dryness and examining the residue of solid
matter, which proves to be common salt.
When an acid is mixed with a base, the result is a substance more or less
similar in its properties to common salt; for this reason all compounds
formed by the neutralization of an acid with a base are called salts. If,
instead of hydrochloric acid (HCl), we use an acid solution of potassium
tartrate, and if instead of caustic soda we use bicarbonate of soda (baking
soda), the result is a brackish liquid as before, but the salt in the liquid is not
common salt, but Rochelle salt. Different combinations of acids and bases
produce different salts. Of all the vast group of salts, the most abundant as
well as the most important is common salt, known technically as sodium
chloride because of its two constituents, sodium and chlorine.
We are not dependent upon neutralization for the enormous quantities of
salt used in the home and in commerce. It is from the active, restless seas of
the present, and from the dead seas of the prehistoric past that our vast
stores of salt come. The waters of the Mediterranean and of our own Great
Salt Lake are led into shallow basins, where, after evaporation by the heat

of the sun, they leave a residue of salt. By far the largest quantity of salt,
however, comes from the seas which no longer exist, but which in far
remote ages dried up and left behind them their burden of salt. Deposits of
salt formed in this way are found scattered throughout the world, and in our
own country are found in greatest abundance in New York. The largest salt
deposit known has a depth of one mile and exists in Germany.
Salt is indispensable on our table and in our kitchen, but the amount of salt
used in this way is far too small to account for a yearly consumption of
4,000,000 tons in the United States alone. The manufacture of soap, glass,
bleaching powders, baking powders, washing soda, and other chemicals
depends on salt, and it is for these that the salt beds are mined.
210. Baking Soda. Salt is by all odds the most important sodium
compound. Next to it come the so-called carbonates: first, sodium
carbonate, which is already familiar to us as washing soda; and second,
sodium bicarbonate, which is an ingredient of baking powders. These are
both obtained from sodium chloride by relatively simple means; that is, by
treating salt with the base, ammonia, and with carbon dioxide.
Washing soda has already been discussed. Since baking powders in some
form are used in almost all homes for the raising of cake and pastry dough,
it is essential that their helpful and harmful qualities be clearly understood.
The raising of dough by means of baking soda—bicarbonate of soda—is a
very simple process. When soda is heated, it gives off carbon dioxide gas;
you can easily prove this for yourself by burning a little soda in a test tube,
and testing the escaping gas in a test tube of limewater. When flour and
water alone are kneaded and baked in loaves, the result is a mass so
compact and hard that human teeth are almost powerless to crush and chew
it. The problem is to separate the mass of dough or, in other words, to cause
it to rise and lighten. This can be done by mixing a little soda in the flour,
because the heat of the oven causes the soda to give off bubbles of gas, and
these in expanding make the heavy mass slightly porous. Bread is never
lightened with soda because the amount of gas thus given off is too small to
convert heavy compact bread dough into a spongy mass; but biscuit and
cake, being by nature less compact and heavy, are sufficiently lightened by
the gas given off from soda.

But there is one great objection to the use of soda alone as a leavening
agent. After baking soda has lost its carbon dioxide gas, it is no longer
baking soda, but is transformed into its relative, washing soda, which has a
disagreeable taste and is by no means desirable for the stomach.
Man's knowledge of chemicals and their effect on each other has enabled
him to overcome this difficulty and, at the same time, to retain the
leavening effect of the baking soda.
211. Baking Powders. If some cooking soda is put into lemon juice or
vinegar, or any acid, bubbles of gas immediately form and escape from the
liquid. After the effervescence has ceased, a taste of the liquid will show
you that the lemon juice has lost its acid nature, and has acquired in
exchange a salty taste. Baking soda, when treated with an acid, is
transformed into carbon dioxide and a salt. The various baking powders on
the market to-day consist of baking soda and some acid substance, which
acts upon the soda, forces it to give up its gas, and at the same time unites
with the residue to form a harmless salt.
Cream of tartar contains sufficient acid to act on baking soda, and is a
convenient and safe ingredient for baking powder. When soda and cream of
tartar are mixed dry, they do not react on each other, neither do they
combine rapidly in cold moist dough, but as soon as the heat of the oven
penetrates the doughy mass, the cream of tartar combines with the soda and
sets free the gas needed to raise the dough. The gas expands with the heat of
the oven, raising the dough still more. Meanwhile, the dough itself is
influenced by the heat and is stiffened to such an extent that it retains its
inflated shape and spongy nature.
Many housewives look askance at ready-made baking powders and prefer
to bake with soda and sour milk, soda and buttermilk, or soda and cream of
tartar. Sour milk and buttermilk are quite as good as cream of tartar, because
the lactic acid which they contain combines with the soda and liberates
carbon dioxide, and forms a harmless residue in the dough.
The desire of manufacturers to produce cheap baking powders led to the use
of cheap acids and alkalies, regardless of the character of the resulting salt.
Alum and soda were popular for some time; but careful examination proved
that the particular salt produced by this combination was not readily

absorbed by the stomach, and that its retention there was injurious to health.
For this reason, many states have prohibited the use of alum in baking
powders.
It is not only important to choose the ingredients carefully; it is also
necessary to calculate the respective quantities of each, otherwise there will
be an excess of acid or alkali for the stomach to take care of. A standard
powder contains twice as much cream of tartar as of bicarbonate of soda,
and the thrifty housewife who wishes to economize, can make for herself, at
small cost, as good a baking powder as any on the market, by mixing tartar
and soda in the above proportions and adding a little corn starch to keep the
mixture dry.
The self-raising flour, so widely advertised by grocers, is flour in which
these ingredients or their equivalent have been mixed by the manufacturer.
212. Soda Mints. Bicarbonate of soda is practically the sole ingredient of
the soda mints popularly sold for indigestion. These correct a tendency to
sour stomach because they counteract the surplus acid in the stomach, and
form with it a safe neutral substance.
Seidlitz powder is a simple remedy consisting of two powders, one
containing bicarbonate of soda, and the other, some acid such as cream of
tartar. When these substances are dissolved in water and mixed,
effervescence occurs, carbon dioxide escapes, and a solution of Rochelle
salt remains.
212a. Source of Soda. An enormous quantity of sodium carbonate, or soda,
as it is usually called, is needed in the manufacture of glass, soap, bleaching
powders, and other commercial products. Formerly, the supply of soda was
very limited because man was dependent upon natural deposits and upon
ashes of sea plants for it. Common salt, sodium chloride, is abundant, and in
1775 a prize was offered to any one who would find a way to obtain soda
from salt. As a result of this, soda was soon manufactured from common
salt. In the most recent methods of manufacture, salt, water, ammonia, and
carbon dioxide are made to react. Baking soda is formed from the reaction.
The baking soda is then heated and decomposed into washing soda or the
soda of commerce.

CHAPTER XXI
FERMENTATION

213. While baking powder is universally used for biscuits and cake, it is
seldom, if ever, used for bread, because it does not furnish sufficient gas to
lighten the tough heavy mass of bread dough. Then, too, most people prefer
the taste of yeast-raised bread. There is a reason for this widespread
preference, but to understand it, we must go somewhat far afield, and must
study not only the bread of to-day, but the bread of antiquity, and the wines
as well.
If grapes are crushed, they yield a liquid which tastes like the grapes; but if
the liquid is allowed to stand in a warm place, it loses its original character,
and begins to ferment, becoming, in the course of a few weeks, a strongly
intoxicating drink. This is true not only of grape juice but also of the juice
of all other sweet fruits; apple juice ferments to cider, currant juice to
currant wine, etc. This phenomenon of fermentation is known to practically
all races of men, and there is scarcely a savage tribe without some kind of
fermented drink; in the tropics the fermented juice of the palm tree serves
for wine; in the desert regions, the fermented juice of the century plant; and
in still other regions, the root of the ginger plant is pressed into service.
The fermentation which occurs in bread making is similar to that which is
responsible for the transformation of plant juices into intoxicating drinks.
The former process is not so old, however, since the use of alcoholic
beverages dates back to the very dawn of history, and the authentic record
of raised or leavened bread is but little more than 3000 years old.
214. The Bread of Antiquity. The original method of bread making and the
method employed by savage tribes of to-day is to mix crushed grain and
water until a paste is formed, and then to bake this over a camp fire. The
result is a hard compact substance known as unleavened bread. A
considerable improvement over this tasteless mass is self-raised bread. If

dough is left standing in a warm place a number of hours, it swells up with
gas and becomes porous, and when baked, is less compact and hard than the
savage bread. Exposure to air and warmth brings about changes in dough as
well as in fruit juices, and alters the character of the dough and the bread
made from it. Bread made in this way would not seem palatable to civilized
man of the present day, accustomed, as he is, to delicious bread made light
and porous by yeast; but to the ancients, the least softening and lightening
was welcome, and self-fermented bread, therefore, supplanted the original
unleavened bread.
Soon it was discovered that a pinch of this fermented dough acted as a
starter on a fresh batch of dough. Hence, a little of the fermented dough was
carefully saved from a batch, and when the next bread was made, the
fermented dough, or leaven, was worked into the fresh dough and served to
raise the mass more quickly and effectively than mere exposure to air and
warmth could do in the same length of time. This use of leaven for raising
bread has been practiced for ages.
Grape juice mixed with millet ferments quickly and strongly, and the
Romans learned to use this mixture for bread raising, kneading a very small
amount of it through the dough.
215. The Cause of Fermentation. Although alcoholic fermentation, and
the fermentation which goes on in raising dough, were known and utilized
for many years, the cause of the phenomenon was a sealed book until the
nineteenth century. About that time it was discovered, through the use of the
microscope, that fermenting liquids contain an army of minute plant
organisms which not only live there, but which actually grow and multiply
within the liquid. For growth and multiplication, food is necessary, and this
the tiny plants get in abundance from the fruit juices; they feed upon the
sugary matter and as they feed, they ferment it, changing it into carbon
dioxide and alcohol. The carbon dioxide, in the form of small bubbles,
passes off from the fermenting mass, while the alcohol remains in the
liquid, giving the stimulating effect desired by imbibers of alcoholic drinks.
The unknown strange organisms were called yeast, and they were the
starting point of the yeast cakes and yeast brews manufactured to-day on a
large scale, not only for bread making but for the commercial production of
beer, ale, porter, and other intoxicating drinks.

The grains, rye, corn, rice, wheat, from which meal is made, contain only a
small quantity of sugar, but, on the other hand, they contain a large quantity
of starch which is easily convertible into sugar. Upon this the tiny yeast
plants in the dough feed, and, as in the case of the wines, ferment the sugar,
producing carbon dioxide and alcohol. The dough is thick and sticky and
the gas bubbles expand it into a spongy mass. The tiny yeast plants multiply
and continue to make alcohol and gas, and in consequence, the dough
becomes lighter and lighter. When it has risen sufficiently, it is kneaded and
placed in an oven; the heat of the oven soon kills the yeast plants and drives
the alcohol out of the bread; at the same time it expands the imprisoned gas
bubbles and causes them to lighten and swell the bread still more.
Meanwhile, the dough has become stiff enough to support itself. The result
of the fermentation is a light, spongy loaf.
216. Where does Yeast come From? The microscopic plants which we call
yeast are widely distributed in the air, and float around there until chance
brings them in contact with a substance favorable to their growth, such as
fruit juices and moist warm batter. Under the favorable conditions of
abundant moisture, heat, and food, they grow and multiply rapidly, and
cause the phenomenon of fermentation. Wild yeast settles on the skin of
grapes and apples, but since it does not have access to the fruit juices
within, it remains inactive very much as a seed does before it is planted. But
when the fruit is crushed, the yeast plants get into the juice, and feeding on
it, grow and multiply. The stray yeast plants which get into the sirup are
relatively few, and hence fermentation is slow; it requires several weeks for
currant wine to ferment, and several months for the juice of grapes to be
converted into wine.
Stray yeast finds a favorable soil for growth in the warmth and moisture of
a batter; but although the number of these stray plants is very large, it is
insufficient to cause rapid fermentation, and if we depended upon wild
yeast for bread raising, the result would not be to our liking.
When our remote ancestors saved a pinch of dough as leaven for the next
baking, they were actually cultivating yeast, although they did not know it.
The reserved portion served as a favorable breeding place to the yeast
plants within it; they grew and reproduced amazingly, and became so

numerous, that the small mass of old dough in which they were gathered
served to leaven the entire batch at the next baking.
As soon as man learned that yeast plants caused fermentation in liquors and
bread, he realized that it would be to his advantage to cultivate yeast and to
add it to bread and to plant juices rather than to depend upon accidental and
slow fermentation from wild yeast. Shortly after the discovery of yeast in
the nineteenth century, man commenced his attempt to cultivate the tiny
organisms. Their microscopic size added greatly to his trouble, and it was
only after years of careful and tedious investigation that he was able to
perfect the commercial yeast cakes and yeast brews universally used by
bakers and brewers. The well-known compressed yeast cake is simply a
mass of live and vigorous yeast plants, embedded in a soft, soggy material,
and ready to grow and multiply as soon as they are placed under proper
conditions of heat, moisture, and food. Seeds which remain on our shelves
do not germinate, but those which are planted in the soil do; so it is with the
yeast plants. While in the cake they are as lifeless as the seed; when placed
in dough, or fruit juice, or grain water, they grow and multiply and cause
fermentation.

CHAPTER XXII
BLEACHING

217. The beauty and the commercial value of uncolored fabrics depend
upon the purity and perfection of their whiteness; a man's white collar and a
woman's white waist must be pure white, without the slightest tinge of
color. But all natural fabrics, whether they come from plants, like cotton
and linen, or from animals, like wool and silk, contain more or less coloring
matter, which impairs the whiteness. This coloring not only detracts from
the appearance of fabrics which are to be worn uncolored, but it seriously
interferes with the action of dyes, and at times plays the dyer strange tricks.
Natural fibers, moreover, are difficult to spin and weave unless some
softening material such as wax or resin is rubbed lightly over them. The
matter added to facilitate spinning and weaving generally detracts from the
appearance of the uncolored fabric, and also interferes with successful
dyeing. Thus it is easy to see that the natural coloring matter and the added
foreign matter must be entirely removed from fabrics destined for
commercial use. Exceptions to this general fact are sometimes made,
because unbleached material is cheaper and more durable than the bleached
product, and for some purposes is entirely satisfactory; unbleached
cheesecloth and sheeting are frequently purchased in place of the more
expensive bleached material. Formerly, the only bleaching agent known
was the sun's rays, and linen and cotton were put out to sun for a week; that
is, the unbleached fabrics were spread on the grass and exposed to the
bleaching action of sun and dew.

218. An Artificial Bleaching
Agent. While the sun's rays are
effective as a bleaching agent, the
process is slow; moreover, it would
be impossible to expose to the sun's
rays the vast quantity of fabrics
used in the civilized world of to-
day, and the huge and numerous
bolts of material which daily come
from our looms and factories must
therefore be whitened by artificial
means. The substance almost
universally used as a rapid artificial
bleaching agent is chlorine, best
known to us as a constituent of
common salt. Chlorine is never free
in nature, but is found in
FIG. 158.—Preparing chlorine from hydrochloric combination with other substances,
acid and manganese dioxide. as, for example, in combination
with sodium in salt, or with
hydrogen in hydrochloric acid.
The best laboratory method of securing free chlorine is to heat in a water
bath a mixture of hydrochloric acid and manganese dioxide, a compound
containing one part of manganese and two parts of oxygen. The heat causes
the manganese dioxide to give up its oxygen, which immediately combines
with the hydrogen of the hydrochloric acid and forms water. The manganese
itself combines with part of the chlorine originally in the acid, but not with
all. There is thus some free chlorine left over from the acid, and this passes
off as a gas and can be collected, as in Figure 158. Free chlorine is heavier
than air, and hence when it leaves the exit tube it settles at the bottom of the
jar, displacing the air, and finally filling the bottle.
Chlorine is a very active substance and combines readily with most
substances, but especially with hydrogen; if chlorine comes in contact with
steam, it abstracts the hydrogen and unites with it to form hydrochloric
acid, but it leaves the oxygen free and uncombined. This tendency of

chlorine to combine with hydrogen makes it valuable as a bleaching agent.
In order to test the efficiency of chlorine as a bleaching agent, drop a wet
piece of colored gingham or calico into the bottle of chlorine, and notice the
rapid disappearance of color from the sample. If unbleached muslin is used,
the moist strip loses its natural yellowish hue and becomes a clear, pure
white. The explanation of the bleaching power of chlorine is that the
chlorine combines with the hydrogen of the water and sets oxygen free; the
uncombined free oxygen oxidizes the coloring matter in the cloth and
destroys it.
Chlorine has no effect on dry material, as may be seen if we put dry
gingham into the jar; in this case there is no water to furnish hydrogen for
combination with the chlorine, and no oxygen to be set free.
219. Bleaching Powder. Chlorine gas has a very injurious effect on the
human body, and hence cannot be used directly as a bleaching agent. It
attacks the mucous membrane of the nose and lungs, and produces the
effect of a severe cold or catarrh, and when inhaled, causes death. But
certain compounds of chlorine are harmless, and can be used instead of
chlorine for destroying either natural or artificial dyes. One of these
compounds, namely, chloride of lime, is the almost universal bleaching
agent of commerce. It comes in the form of powder, which can be dissolved
in water to form the bleaching solution in which the colored fabrics are
immersed. But fabrics immersed in a bleaching powder solution do not lose
their color as would naturally be expected. The reason for this is that the
chlorine gas is not free to do its work, but is restricted by its combination
with the other substances. By experiment it has been found that the addition
to the bleaching solution of an acid, such as vinegar or lemon juice or
sulphuric acid, causes the liberation of the chlorine. The chlorine thus set
free reacts with the water and liberates oxygen; this in turn destroys the
coloring matter in the fibers, and transforms the material into a bleached
product.
The acid used to liberate the chlorine from the bleaching powder, and the
chlorine also, rot materials with which they remain in contact for any length
of time. For this reason, fabrics should be removed from the bleaching
solution as soon as possible, and should then be rinsed in some solution,
such as ammonia, which is capable of neutralizing the harmful substances;

finally the fabric should be thoroughly rinsed in water in order that all
foreign matter may be removed. The reason home bleaching is so seldom
satisfactory is that most amateurs fail to realize the necessity of immediate
neutralization and rinsing, and allow the fabric to remain too long in the
bleaching solution, and allow it to dry with traces of the bleaching
substances present in the fibers. Material treated in this way is thoroughly
bleached, but is at the same time rotten and worthless. Chloride of lime is
frequently used in laundry work; the clothes are whiter than when cleaned
with soap and simple washing powders, but they soon wear out unless the
precaution has been taken to add an "antichlor" or neutralizer to the
bleaching solution.
220. Commercial Bleaching. In commercial bleaching the material to be
bleached is first moistened with a very weak solution of sulphuric acid or
hydrochloric acid, and is then immersed in the bleaching powder solution.
As the moist material is drawn through the bleaching solution, the acid on
the fabric acts upon the solution and releases chlorine. The chlorine
liberates oxygen from the water. The oxygen in turn attacks the coloring
matter and destroys it.

FIG. 159.—The material to be bleached is drawn through an acid a, then through a bleaching solution
b, and finally through a neutralizing solution c.

The bleached material is then immersed in a neutralizing bath and is finally
rinsed thoroughly in water. Strips of cotton or linen many miles long are
drawn by machinery into and out of the various solutions (Fig. 159), are
then passed over pressing rollers, and emerge snow white, ready to be dyed
or to be used as white fabric.
221. Wool and Silk Bleaching. Animal fibers like silk, wool, and feathers,
and some vegetable fibers like straw, cannot be bleached by means of
chlorine, because it attacks not only the coloring matter but the fiber itself,

and leaves it shrunken and inferior. Cotton and linen fibers, apart from the
small amount of coloring matter present in them, contain nothing but
carbon, oxygen, and hydrogen, while animal fibers contain in addition to
these elements some compounds of nitrogen. The presence of these nitrogen
compounds influences the action of the chlorine and produces
unsatisfactory results. For animal fibers it is therefore necessary to discard
chlorine as a bleaching agent, and to substitute a substance which will have
a less disastrous action upon the fibers. Such a substance is to be had in
sulphurous acid. When sulphur burns, as in a match, it gives off
disagreeable fumes, and if these are made to bubble into a vessel containing
water, they dissolve and form with the water a substance known as
sulphurous acid. That this solution has bleaching properties is shown by the
fact that a colored cloth dipped into it loses its color, and unbleached fabrics
immersed in it are whitened. The harmless nature of sulphurous acid makes
it very desirable as a bleaching agent, especially in the home.
Silk, lace, and wool when bleached with chlorine become hard and brittle,
but when whitened with sulphurous acid, they retain their natural
characteristics.
This mild form of a bleaching substance has been put to uses which are now
prohibited by the pure food laws. In some canneries common corn is
whitened with sulphurous acid, and is then sold under false representations.
Cherries are sometimes bleached and then colored with the bright shades
which under natural conditions indicate freshness.
Bleaching with chlorine is permanent, the dyestuff being destroyed by the
chlorine; but bleaching with sulphurous acid is temporary, because the
milder bleach does not actually destroy the dyestuff, but merely modifies it,
and in time the natural yellow color of straw, cotton, and linen reappears.
The yellowing of straw hats during the summer is familiar to everyone; the
straw is merely resuming its natural color which had been modified by the
sulphurous acid solution applied to the straw when woven.
222. Why the Color Returns. Some of the compounds formed by the
sulphurous acid bleaching process are gradually decomposed by sunlight,
and in consequence the original color is in time partially restored. The
portion of a hat protected by the band retains its fresh appearance because

the light has not had access to it. Silks and other fine fabrics bleached in
this way fade with age, and assume an unnatural color. One reason for this
is that the dye used to color the fabric requires a clear white background,
and loses its characteristic hues when its foundation is yellow instead of
white. Then, too, dyestuffs are themselves more or less affected by light,
and fade slowly under a strong illumination.
Materials which are not exposed directly to an intense and prolonged
illumination retain their whiteness for a long time, and hence dress
materials and hats which have been bleached with sulphurous acid should
be protected from the sun's glare when not in use.
223. The Removal of Stains. Bleaching powder is very useful in the
removal of stains from white fabrics. Ink spots rubbed with lemon juice and
dipped in bleaching solution fade away and leave on the cloth no trace of
discoloration. Sometimes these stains can be removed by soaking in milk,
and where this is possible, it is the better method.
Bleaching solution, however, while valuable in the removal of some stains,
is unable to remove paint stains, because paints owe their color to mineral
matter, and on this chlorine is powerless to act. Paint stains are best
removed by the application of gasoline followed by soap and water.

CHAPTER XXIII
DYEING

224. Dyes. One of the most important and lucrative industrial processes of
the world to-day is that of staining and dyeing. Whether we consider the
innumerable shades of leather used in shoes and harnesses and upholstery;
the multitude of colors in the paper which covers our walls and reflects light
ranging from the somber to the gay, and from the delicate to the gorgeous;
the artificial scenery which adorns the stage and by its imitation of trees and
flowers and sky translates us to the Forest of Arden; or whether we consider
the uncounted varieties of color in dress materials, in carpets, and in
hangings, we are dealing with substances which owe their beauty to dyes
and dyestuffs.
The coloring of textile fabrics, such as cotton, wool, and silk, far outranks
in amount and importance that of leather, paper, etc., and hence the former
only will be considered here; but the theories and facts relative to textile
dyeing are applicable in a general way to all other forms as well.
225. Plants as a Source of Dyes. Among the most beautiful examples of
man's handiwork are the baskets and blankets of the North American
Indians, woven with a skill which cannot be equaled by manufacturers, and
dyed in mellow colors with a few simple dyes extracted from local plants.
The magnificent rugs and tapestries of Persia and Turkey, and the silks of
India and Japan, give evidence that a knowledge of dyes is widespread and
ancient. Until recently, the vegetable world was the source of practically all
coloring matter, the pulverized root of the madder plant yielding the reds,
the leaves and stems of the indigo plant the blues, the heartwood of the
tropical logwood tree the blacks and grays, and the fruit of certain palm and
locust trees yielding the soft browns. So great was the commercial demand
for dyestuffs that large areas of land were given over to the exclusive
cultivation of the more important dye plants. Vegetable dyes are now,

however, rarely used because about the year 1856 it was discovered that
dyes could be obtained from coal tar, the thick sticky liquid formed as a by-
product in the manufacture of coal gas. These artificial coal-tar, or aniline,
dyes have practically undisputed sway to-day, and the vast areas of land
formerly used for the cultivation of vegetable dyes are now free for other
purposes.
226. Wool and Cotton Dyeing. If a piece of wool is soaked in a solution of
a coal-tar dye, such as magenta, the fiber of the cloth draws some of the dye
out of the solution and absorbs it, becoming in consequence beautifully
colored. The coloring matter becomes "part and parcel," as it were, of the
wool fiber, because repeated washing of the fabric fails to remove the newly
acquired color; the magenta coloring matter unites chemically with the fiber
of the wool, and forms with it a compound insoluble in water, and hence
fast to washing.
But if cotton is used instead of wool, the acquired color is very faint, and
washes off readily. This is because cotton fibers possess no chemical
substance capable of uniting with the coloring matter to form a compound
insoluble in water.
If magenta is replaced by other artificial dyes,—for example, scarlets,—the
result is similar; in general, wool material absorbs dye readily, and uniting
with it is permanently dyed. Cotton material, on the other hand, does not
combine chemically with coloring matter and therefore is only faintly
tinged with color, and loses this when washed. When silk and linen are
tested, it is found that the former behaves in a general way as did wool,
while the linen has more similarity to the cotton. That vegetable fibers, such
as cotton and linen, should act differently toward coloring matter from
animal fibers, such as silk and wool, is not surprising when we consider that
the chemical nature of the two groups is very different; vegetable fibers
contain only oxygen, carbon, and hydrogen, while animal fibers always
contain nitrogen in addition, and in many cases sulphur as well.
227. The Selection of Dyes. When silk and wool, cotton and linen, are
tested in various dye solutions, it is found that the former have, in general, a
great affinity for coloring matter and acquire a permanent color, but that

cotton and linen, on the other hand, have little affinity for dyestuffs. The
color acquired by vegetable fibers is, therefore, usually faint.
There are, of course, many exceptions to the general statement that animal
fibers dye readily and vegetable fibers poorly, because certain dyes fail
utterly with woolen and silk material and yet are fairly satisfactory when
applied to cotton and linen fabrics. Then, too, a dye which will color silk
may not have any effect on wool in spite of the fact that wool, like silk, is
an animal fiber; and certain dyestuffs to which cotton responds most
beautifully are absolutely without effect on linen.
The nature of the material to be dyed determines the coloring matter to be
used; in dyeing establishments a careful examination is made of all textiles
received for dyeing, and the particular dyestuffs are then applied which long
experience has shown to be best suited to the material in question. Where
"mixed goods," such as silk and wool, or cotton and wool, are concerned,
the problem is a difficult one, and the countless varieties of gorgeously
colored mixed materials give evidence of high perfection in the art of
dyeing and weaving.
Housewives who wish to do successful home dyeing should therefore not
purchase dyes indiscriminately, but should select the kind best suited to the
material, because the coloring principle which will remake a silk waist may
utterly ruin a woolen skirt or a linen suit. Powders designed for special
purposes may be purchased from druggists.
228. Indirect Dyeing. We have seen that it is practically impossible to color
cotton and linen in a simple manner with any degree of permanency,
because of the lack of chemical action between vegetable fibers and
coloring matter. But the varied uses to which dyed articles are put make
fastness of color absolutely necessary. A shirt, for example, must not be
discolored by perspiration, nor a waist faded by washing, nor a carpet
dulled by sweeping with a dampened broom. In order to insure permanency
of dyes, an indirect method was originated which consisted of adding to the
fibers a chemical capable of acting upon the dye and forming with it a
colored compound insoluble in water, and hence "safe." For example,
cotton material dyed directly in logwood solution has almost no value, but
if it is soaked in a solution of oxalic acid and alum until it becomes

saturated with the chemicals, and is then transferred to a logwood bath, the
color acquired is fast and beautiful.
This method of indirect dyeing is known as the mordanting process; it
consists of saturating the fabric to be dyed with chemicals which will unite
with the coloring matter to form compounds unaffected by water. The
chemicals are called mordants.
229. How Variety of Color is Secured. The color which is fixed on the
fabric as a result of chemical action between mordant and dye is frequently
very different from that of the dye itself. Logwood dye when used alone
produces a reddish brown color of no value either for beauty or
permanence; but if the fabric to be dyed is first mordanted with a solution
of alum and oxalic acid and is then immersed in a logwood bath, it acquires
a beautiful blue color.
Moreover, since the color acquired depends upon the mordant as well as
upon the dye, it is often possible to obtain a wide range of colors by varying
the mordant used, the dye remaining the same. For example, with alum and
oxalic acid as a mordant and logwood as a dye, blue is obtained; but with a
mordant of ferric sulphate and a dye of logwood, blacks and grays result.
Fabrics immersed directly in alizarin acquire a reddish yellow tint; when,
however, they are mordanted with certain aluminium compounds they
acquire a brilliant Turkey red, when mordanted with chromium compounds,
a maroon, and when mordanted with iron compounds, the various shades of
purple, lilac, and violet result.
230. Color Designs in Cloth. It is thought that the earliest attempts at
making "fancy materials" consisted in painting designs on a fabric by
means of a brush. In more recent times the design was cut in relief on hard
wood, the relief being then daubed with coloring matter and applied by
hand to successive portions of the cloth. The most modern method of
design-making is that of machine or roller printing. In this, the relief blocks
are replaced by engraved copper rolls which rotate continuously and in the
course of their rotation automatically receive coloring matter on the
engraved portion. The cloth is to be printed is then drawn uniformly over
the rotating roll, receiving color from the engraved design; in this way, the
color pattern is automatically printed on the cloth with perfect regularity. In

cases where the fabrics do not unite directly with the coloring matter, the
design is supplied with a mordant and the impression made on the fabric is
that of the mordant; when the fabric is later transferred to a dye bath, the
mordanted portions, represented by the design, unite with the coloring
matter and thus form the desired color patterns.
Unless the printing is well done, the coloring matter does not thoroughly
penetrate the material, and only a faint blurred design appears on the back
of the cloth; the gaudy designs of cheap calicoes and ginghams often do not
show at all on the under side. Such carelessly made prints are not fast to
washing or light, and soon fade. But in the better grades of material the
printing is well done, and the color designs are fairly fast, and a little care in
the laundry suffices to eliminate any danger of fading.
Color designs of the greatest durability are produced by the weaving
together of colored yarns. When yarn is dyed, the coloring matter penetrates
to every part of the fiber, and hence the patterns formed by the weaving
together of well-dyed yarns are very fast to light and water.
If the color designs to be woven in the cloth are intricate, complex
machinery is necessary and skillful handwork; hence, patterns formed by
the weaving of colored yarns are expensive and less common than printed
fabrics.

CHAPTER XXIV
CHEMICALS AS DISINFECTANTS AND PRESERVATIVES

231. The prevention of disease epidemics is one of the most striking
achievements of modern science. Food, clothing, furniture, and other
objects contaminated in any way by disease germs may be disinfected by
chemicals or by heat, and widespread infection from persons suffering with
a contagious disease may be prevented.
When disease germs are
within the body, the problem
is far from simple, because
chemicals which would
effectively destroy the germs
would be fatal to life itself.
But when germs are outside
the body, as in water or milk,
or on clothing, dishes, or
furniture, they can be easily
killed. One of the best
methods of destroying germs
is to subject them to intense
FIG. 160.—Pasteurizing apparatus, an arrangement by
heat. Contaminated water is
which milk is conveniently heated to destroy disease
made safe by boiling for a
germs.
few minutes, because the
strong heat destroys the
disease-producing germs. Scalded or Pasteurized milk saves the lives of
scores of babies, because the germs of summer complaint which lurk in
poor milk are killed and rendered harmless in the process of scalding.
Dishes used by consumptives, and persons suffering from contagious
diseases, can be made harmless by thorough washing in thick suds of
almost boiling water.

The bedding and clothing of persons suffering with diphtheria, tuberculosis,
and other germ diseases should always be boiled and hung to dry in the
bright sunlight. Heat and sunshine are two of the best disinfectants.
232. Chemicals. Objects, such as furniture, which cannot be boiled, are
disinfected by the use of any one of several chemicals, such as sulphur,
carbolic acid, chloride of lime, corrosive sublimate, etc.
One of the simplest methods of disinfecting consists in burning sulphur in a
room whose doors, windows, and keyholes have been closed, so that the
burning fumes cannot escape, but remain in the room long enough to
destroy disease germs. This is probably the most common means of
fumigation.
For general purposes, carbolic acid is one of the very best disinfectants, but
must be used with caution, as it is a deadly poison except when very dilute.
Chloride of lime when exposed to the air and moisture slowly gives off
chlorine, and can be used as a disinfectant because the gas thus set free
attacks germs and destroys them. For this reason chloride of lime is an
excellent disinfectant of drainpipes. Certain bowel troubles, such as
diarrhœa, are due to microbes, and if the waste matter of a person suffering
from this or similar diseases is allowed passage through the drainage
system, much damage may be done. But a small amount of chloride of lime
in the closet bowl will insure disinfection.
233. Personal Disinfection. The hands may gather germs from any
substances or objects with which they come in contact; hence the hands
should be washed with soap and water, and especially before eating.
Physicians who perform operations wash not only their hands, but their
instruments, sterilizing the latter by placing them in boiling water for
several minutes.
Cuts and wounds allow easy access to the body; a small cut has been known
to cause death because of the bacteria which found their way into the open
wound and produced disease. In order to destroy any germs which may
have entered into the cut from the instrument, it is well to wash out the
wound with some mild disinfectant, such as very dilute carbolic acid or

hydrogen peroxide, and then to bind the wound with a clean cloth, to
prevent later entrance of germs.
234. Chemicals as Food Preservatives. The spoiling of meats and soups,
and the souring of milk and preserves, are due to germs which, like those
producing disease, can be destroyed by heat and by chemicals.
Milk heated to the boiling point does not sour readily, and successful
canning consists in cooking fruits and vegetables until all the germs are
killed, and then sealing the cans so that germs from outside cannot find
entrance and undo the work of the canner.
Some dealers and manufacturers have learned that certain chemicals will
act as food preservatives, and hence they have replaced the safe method of
careful canning by the quicker and simpler plan of adding chemicals to
food. Catchup, sauces, and jellies are now frequently preserved in this way.
But the chemicals which destroy bacteria frequently injure the consumer as
well. And so much harm has been done by food preservatives that the pure
food laws require that cans and bottles contain a labeled statement of the
kind and quantity of chemicals used.
Even milk is not exempt, but is doctored to prevent souring, the
preservative most generally used by milk dealers being formaldehyde. The
vast quantity of milk consumed by young and old, sick and well, makes the
use of formaldehyde a serious menace to health, because no constitution
can endure the injury done by the constant use of preservatives.
The most popular and widely used preservatives of meats are borax and
boric acid. These chemicals not only arrest decay, but partially restore to old
and bad meat the appearance of freshness; in this way unscrupulous dealers
are able to sell to the public in one form or other meats which may have
undergone partial decomposition; sausage frequently contains partially
decomposed meat, restored as it were by chemicals.
In jams and catchups there is abundant opportunity for preservatives; badly
or partially decayed fruits are sometimes disinfected and used as the basis
of foods sold by so-called good dealers. Benzoate of soda, and salicylic acid
are the chemicals most widely employed for this purpose, with coal-tar dyes
to simulate the natural color of the fruit.

Many of the cheap candies sold by street venders are not fit for
consumption, since they are not only made of bad material, but are
frequently in addition given a light dipping in varnish as a protection
against the decaying influences of the atmosphere.
The only wise preservatives are those long known and employed by our
ancestors; salt, vinegar, and spices are all food preservatives, but they are at
the same time substances which in small amounts are not injurious to the
body. Smoked herring and salted mackerel are chemically preserved foods,
but they are none the less safe and digestible.
235. The Preservation of Wood and Metal. The decaying of wood and the
rusting of metal are due to the action of air and moisture. When wood and
metal are surrounded with a covering which neither air nor moisture can
penetrate, decay and rust are prevented. Paint affords such a protective
covering. The main constituent of paint is a compound of white lead or
other metallic substance; this is mixed with linseed oil or its equivalent in
order that it may be spread over wood and metal in a thin, even coating.
After the mixture has been applied, it hardens and forms a tough skin fairly
impervious to weathering. For the sake of ornamentation, various colored
pigments are added to the paint and give variety of effect.
Railroad ties and street paving blocks are ordinarily protected by oil rather
than paint. Wood is soaked in creosote oil until it becomes thoroughly
saturated with the oily substance. The pores of the wood are thus closed to
the entrance of air and moisture, and decay is avoided. Wood treated in this
way is very durable. Creosote is poisonous to insects and many small
animals, and thus acts as a preservation not only against the elements but
against animal life as well.

CHAPTER XXV
DRUGS AND PATENT MEDICINES

236. Stimulants and Narcotics. Man has learned not only the action of
substances upon each other, such as bleaching solution upon coloring
matter, washing soda upon grease, acids upon bases, but also the effect
which certain chemicals have upon the human body.
Drugs and their varying effects upon the human system have been known to
mankind from remote ages; in the early days, familiar leaves, roots, and
twigs were steeped in water to form medicines which served for the
treatment of all ailments. In more recent times, however, these simple herb
teas have been supplanted by complex drugs, and now medicines are
compounded not only from innumerable plant products, but from animal
and mineral matter as well. Quinine, rhubarb, and arnica are examples of
purely vegetable products; iron, mercury, and arsenic are equally well
known as distinctly mineral products, while cod-liver oil is the most
familiar illustration of an animal remedy. Ordinarily a combination of
products best serves the ends of the physician.
Substances which, like cod-liver oil, serve as food to a worn-out body, or,
like iron, tend to enrich the blood, or, like quinine, aid in bringing an
abnormal system to a healthy condition, are valuable servants and cannot be
entirely dispensed with so long as man is subject to disease.
But substances which, like opium, laudanum, and alcohol, are not required
by the body as food, or as a systematic, intelligent aid to recovery, but are
taken solely for the stimulus aroused or for the insensibility induced, are
harmful to man, and cannot be indulged in by him without ultimate mental,
moral, and physical loss. Substances of the latter class are known as
narcotics and stimulants.

237. The Cost of Health. In the physical as in the financial world, nothing
is to be had without a price. Vigor, endurance, and mental alertness are
bought by hygienic living; that is, by proper food, fresh air, exercise,
cleanliness, and reasonable hours. Some people wish vigor, endurance, etc.,
but are unwilling to live the life which will develop these qualities. Plenty
of sleep, exercise, and simple food all tend to lay the foundations of health.
Many, however, are not willing to take the care necessary for healthful
living, because it would force them to sacrifice some of the hours of
pleasure. Sooner or later, these pleasure-seekers begin to feel tired and
worn, and some of them turn to drugs and narcotics for artificial strength.
At first the drugs seem to restore the lost energy, and without harm;
however, the cost soon proves to be one of the highest Nature ever
demands.
238. The Uncounted Cost. The first and most obvious effect of opium, for
example, is to deaden pain and to arouse pleasure; but while the drug is
producing these soothing sensations, it interferes with bodily functions.
Secretion, digestion, absorption of food, and the removal of waste matters
are hindered. Continued use of the drug leads to headache, exhaustion,
nervous depression, and heart weakness. There is thus a heavy toll reckoned
against the user, and the creditor is relentless in demanding payment.
Moreover, the respite allowed by a narcotic is exceedingly brief, and a
depression which is long and deep inevitably follows. In order to overcome
this depression, recourse is usually had to a further dose, and as time goes
on, the intervals of depression become more frequent and lasting, and the
necessity to overcome them increases. Thus without intention one finds
one's self bound to the drug, its fast victim. The sanatoria of our country are
crowded with people who are trying to free themselves of a drug habit into
which they have drifted unintentionally if not altogether unknowingly. What
is true of opium is equally applicable to other narcotics.
239. The Right Use of Narcotics. In the hands of the physician, narcotics
are a great blessing. In some cases, by relieving pain, they give the system
the rest necessary for overcoming the cause of the pain. Only those who
know of the suffering endured in former times can fully appreciate the
decrease in pain brought about by the proper use of narcotics.

240. Patent Medicines, Cough Sirups. A reputable physician is solicitous
regarding the permanent welfare of his patient and administers carefully
chosen and harmless drugs. Mere medicine venders, however, ignore the
good of mankind, and flood the market with cheap patent preparations
which delude and injure those who purchase, but bring millions of dollars to
those who manufacture.
Practically all of these patent, or proprietary, preparations contain a large
proportion of narcotics or stimulants, and hence the benefit which they
seem to afford the user is by no means genuine; examination shows that the
relief brought by them is due either to a temporary deadening of
sensibilities by narcotics or to a fleeting stimulation by alcohol and kindred
substances.
Among the most common ailments of both young and old are coughs and
colds; hence many patent cough mixtures have been manufactured and
placed on the market for the consumption of a credulous public. Such
"quick cures" almost invariably contain one or more narcotic drugs, and not
only do not relieve the cold permanently, but occasion subsequent
disorders. Even lozenges and pastilles are not free from fraud, but have a
goodly proportion of narcotics, containing in some cases chloroform,
morphine, and ether.
The widespread use of patent cough medicines is due largely to the fact that
many persons avoid consulting a physician about so trivial an ailment as an
ordinary cold, or are reluctant to pay a medical fee for what seems a slight
indisposition and hence attempt to doctor themselves.
Catarrh is a very prevalent disease in America, and consequently numerous
catarrh remedies have been devised, most of which contain in a disguised
form the pernicious drug, cocaine. Laws have been enacted which require
on the labels a declaration of the contents of the preparation, both as to the
kind of drug used and the amount, and the choice of accepting or refusing
such mixtures is left to the individual. But the great mass of people are
ignorant of the harmful nature of drugs in general, and hence do not even
read the self-accusing label, or if they do glance at it, fail to comprehend the
dangerous nature of the drugs specified there. In order to safeguard the
uninformed purchaser and to restrict the manufacture of harmful patent

remedies, some states limit the sale of all preparations containing narcotics
and thus give free rein to neither consumer nor producer.
241. Soothing Sirups; Soft Drinks. The development of a race is limited
by the mental and physical growth of its children, and yet thousands of its
children are annually stunted and weakened by drugs, because most colic
cures, teething concoctions, and soothing syrups are merely agreeably
flavored drug mixtures. Those who have used such preparations freely,
know that a child usually becomes fretful and irritable between doses, and
can be quieted only by larger and more frequent supplies. A habit formed in
this way is difficult to overcome, and many a child when scarcely over its
babyhood had a craving which in later years may lead to systematic drug
taking. And even though the pernicious drug craving is not created,
considerable harm is done to the child, because its body is left weak and
non-resistant to diseases of infancy and childhood.
Many of our soft drinks contain narcotics. The use of the coca leaf and the
kola nut for such preparations has increased very greatly within the last few
years, and doubtless legislation will soon be instituted against the
indiscriminate sale of soft drinks.
242. Headache Powders. The stress and strain of modern life has opened
wide the door to a multitude of bodily ills, among which may be mentioned
headache. Work must be done and business attended to, and the average
sufferer does not take time from his vocation to investigate the cause of the
headache, but unthinkingly grasps at any remedy which will remove the
immediate pain, and utterly disregards later injury. The relief afforded by
most headache mixtures is due to the presence of antipyrin or acetanilid,
and it has been shown conclusively that these drugs weaken heart action,
diminish circulation, reduce the number of red corpuscles in the blood, and
bring on a condition of chronic anemia. Pallid cheeks and blue lips are
visible evidence of the too frequent use of headache powders.
The labels required by law are often deceptive and convey no adequate idea
of the amount of drug consumed; for example, 240 grains of acetanilid to an
ounce seems a small quantity of drug for a powder, but when one considers
that there are only 480 grains in an ounce, it will be seen that each powder
is one half acetanilid.

Powders taken in small quantities and at rare intervals are apparently
harmless; but they never remove the cause of the trouble, and hence the
discomfort soon returns with renewed force. Ordinarily, hygienic living will
eliminate the source of the trouble, but if it does not, a physician should be
consulted and medicine should be procured from him which will restore the
deranged system to its normal healthy condition.
243. Other Deceptions. Nearly all patent medicines contain some alcohol,
and in many, the quantity of alcohol is far in excess of that found in the
strongest wines. Tonics and bitters advertised as a cure for spring fever and
a worn-out system are scarcely more than cheap cocktails, as one writer has
derisively called them, and the amount of alcohol in some widely advertised
patent remedies is alarmingly large and almost equal to that of strong
whisky.

FIG. 161.—Diagram showing the amount of alcohol in some alcoholic drinks and in one much used
patent medicine.

Some conscientious persons who would not touch beer, wine, whisky, or
any other intoxicating drink consume patent remedies containing large
quantities of alcohol and thus unintentionally expose themselves to mental
and physical danger. In all cases of bodily disorder, the only safe course is
to consult a physician who has devoted himself to the study of the body and
the methods by which a disordered system may be restored to health.

CHAPTER XXVI
NITROGEN AND ITS RELATION TO PLANTS

244. Nitrogen. A substance which plays an important part in animal and
plant life is nitrogen. Soil and the fertilizers which enrich it, the plants
which grow on it, and the animals which feed on these, all contain nitrogen
or nitrogenous compounds. The atmosphere, which we ordinarily think of
as a storehouse of oxygen, contains far more nitrogen than oxygen, since
four fifths of its whole weight is made up of this element.
Nitrogen is colorless, odorless, and tasteless. Air is composed chiefly of
oxygen and nitrogen; if, therefore, the oxygen in a vessel filled with air can
be made to unite with some other substance or can be removed, there will
be a residue of nitrogen. This can be done by floating on water a light dish
containing phosphorus, then igniting the phosphorus, and placing an
inverted jar over the burning substance. The phosphorus in burning unites
with the oxygen of the air and hence the gas that remains in the jar is
chiefly nitrogen. It has the characteristics mentioned above and, in addition,
does not combine readily with other substances.
245. Plant Food. Food is the course of energy in every living thing and is
essential to both animal and plant life. Plants get their food from the lifeless
matter which exists in the air and in the soil; while animals get their food
from plants. It is true that man and many other animals eat fleshy foods and
depend upon them for partial sustenance, but the ultimate source of all
animal food is plant life, since meat-producing animals live upon plant
growth.
Plants get their food from the air, the soil, and moisture. From the air, the
leaves take carbon dioxide and water and transform them into starchy food;
from the soil, the roots take water rich in mineral matters dissolved from the
soil. From the substances thus gathered, the plant lives and builds up its
structure.

A food substance necessary to plant life and growth is nitrogen. Since a vast
store of nitrogen exists in the air, it would seem that plants should never
lack for this food, but most plants are unable to make use of the boundless
store of atmospheric nitrogen, because they do not possess the power of
abstracting nitrogen from the air. For this reason, they have to depend solely
upon nitrogenous compounds which are present in the soil and are soluble
in water. The soluble nitrogenous soil compounds are absorbed by roots and
are utilized by plants for food.
246. The Poverty of the Soil. Plant roots are constantly taking nitrogen and
its compounds from the soil. If crops which grow from the soil are removed
year after year, the soil becomes poorer in nitrogen, and finally possesses
too little of it to support vigorous and healthy plant life. The nitrogen of the
soil can be restored if we add to it a fertilizer containing nitrogen
compounds which are soluble in water. Decayed vegetable matter contains
large quantities of nitrogen compounds, and hence if decayed vegetation is
placed upon soil or is plowed into soil, it acts as a fertilizer, returning to the
soil what was taken from it. Since man and all other animals subsist upon
plants, their bodies likewise contain nitrogenous substances, and hence
manure and waste animal matter is valuable as a fertilizer or soil restorer.
247. Bacteria as Nitrogen Gatherers. Soil from which crops are removed
year after year usually becomes less fertile, but the soil from which crops of
clover, peas, beans, or alfalfa have been removed is richer in nitrogen rather
than poorer. This is because the roots of these plants often have on them
tiny swellings, or tubercles, in which millions of certain bacteria live and
multiply. These bacteria have the remarkable power of taking free nitrogen
from the air in the soil and of combining it with other substances to form
compounds which plants can use. The bacteria-made compounds dissolve in
the soil water and are absorbed into the plant by the roots. So much
nitrogen-containing material is made by the root bacteria of plants of the
pea family that the soil in which they grow becomes somewhat richer in
nitrogen, and if plants which cannot make nitrogen are subsequently planted
in such a soil, they find there a store of nitrogen. A crop of peas, beans, or
clover is equivalent to nitrogenous fertilizer and helps to make ready the
soil for other crops.

248. Artificial Fertilizers. Plants
need other foods besides nitrogen,
and they exhaust the soil not only of
nitrogen, but also of phosphorus
and potash, since large quantities of
these are necessary for plant life.
There are many other substances
absorbed from the soil by the plant,
namely, iron, sodium, calcium,
magnesium, but these are used in
smaller quantities and the supply in
the soil does not readily become
exhausted. FIG. 162.—Roots of soy bean having tubercle-
bearing bacteria.
Commercial fertilizers generally
contain nitrogen, phosphorus, and
potash in amounts varying with the requirements of the soil. Wheat requires
a large amount of phosphorus and quickly exhausts the ground of that food
stuff; a field which has supported a crop of wheat is particularly poor in
phosphorus, and a satisfactory fertilizer for that land would necessarily
contain a large percentage of phosphorus. The fertilizer to be used in a soil
depends upon the character of the soil and upon the crops previously grown
on it.
The quantity of fertilizer needed by the farmers of the world is enormous,
and the problem of securing the necessary substances in quantities sufficient
to satisfy the demand bids fair to be serious. But modern chemistry is at
work on the problem, and already it is possible to make some nitrogen
compounds on a commercial scale. When nitrogen gas is in contact with
heated calcium carbide, a reaction takes place which results in the
formation of calcium nitride, a compound suitable for enriching the soil.
There are other commercial methods for obtaining nitrogen compounds
which are suitable for absorption by plant roots.
Phosphorus is obtained from bone ash and from phosphate rock which is
widely distributed over the surface of the earth. Bone ash and thousands of
tons of phosphate rock are treated with sulphuric acid to form a phosphorus

compound which is soluble in soil water and which,
when added to soil, will be usable by the plants
growing there.
The other important ingredient of most fertilizers is
potash. Wood ashes are rich in potash and are a
valuable addition to the soil. But the amount of potash
thus obtained is far too limited to supply the needs of
agriculture; and to-day the main sources of potash are
the vast deposits of potassium salts found in Prussia.
Although Germany now furnishes the American
farmer with the bulk of his potash, she may not do so
much longer. In 1911 an indirect potash tax was levied
by Germany on her best customer, the United States,
FIG. 163.—Water to whom 15 million dollars' worth of potash had been
cultures of buckwheat: sold the preceding year. This led Americans to inquire
1, with all the food whether potash could not be obtained at home.
elements; 2, without
Geologists say that long ages ago Germany was
potash; 3, without
submerged, that the waters slowly evaporated and that
nitrates.
the various substances in the sea water were deposited
in thick layers. The deposits thus left by the
evaporation of the sea water gradually became hidden by sediment and soil,
and lost to sight. From such deposits, potash is obtained. Geologists tell us
that our own Western States were once submerged, and that the waters
evaporated and disappeared from our land very much as they did from
Germany. The Great Salt Lake of Utah is a relic of a great body of water. If
it be true that waters once covered our Western States, there may be buried
deposits of potash there, and to-day the search for the hidden treasure is
going on with the energy and enthusiasm characteristic of America.
Another probable source of potash is seaweed. The sea is a vast reservoir of
potash, and seaweed, especially the giant kelp, absorbs large quantities of
this potash. A ton of dried kelp (dried by sun and wind) contains about 500
pounds of pure potash. The kelps are abundant, covering thousands of
square miles in the Pacific Ocean, from Mexico to the Arctic Ocean.

CHAPTER XXVII
SOUND

249. The Senses. All the information which we possess of the world around
us comes to us through the use of the senses of sight, hearing, taste, touch,
and smell. Of the five senses, sight and hearing are generally considered the
most valuable. In preceding Chapters we studied the important facts relative
to light and the power of vision; it remains for us to study Sound as we
studied Light, and to learn what we can of sound and the power to hear.
250. How Sound is Produced. If one investigates the source of any sound,
he will always find that it is due to motion of some kind. A sudden noise is
traced to the fall of an object, or to an explosion, or to a collision; in fact, is
due to the motion of matter. A piano gives out sound whenever a player
strikes the keys and sets in motion the various wires within the piano;
speech and song are caused by the motion of chest, vocal cords, and lips.
If a large dinner bell is rung, its motion or vibration may be felt on touching
it with the finger. If a tuning fork is made to give forth sound by striking it
against the knee, or hitting it with a rubber hammer, and is then touched to
the surface of water, small sprays of water will be thrown out, showing that
the prongs of the fork are in rapid motion. (A rubber hammer is made by
putting a piece of glass tubing through a rubber cork.)
If a light cork ball on the end of a thread is brought in contact with a
sounding fork, the ball does not remain at rest, but vibrates back and forth,
being driven by the moving prongs.
These simple facts lead us to conclude that all sound is due to the motion of
matter, and that a sounding body of any kind is in rapid motion.
251. Sound is carried by Matter. In most cases sound reaches the ear
through the air; but air is not the only medium through which sound is

carried. A loud
noise will
startle fish,
and cause
them to dart
away, so we
conclude that
the sound
must have FIG. 165.—The ball does not remain at
reached them rest
through the
water. An Indian puts his ear to the ground in
order to detect distant footsteps, because sounds
too faint to be heard through the air are
comparatively clear when transmitted through
FIG. 164.—Sprays of water the earth. A gentle tapping at one end of a long
show that the fork is in motion. table can be distinctly heard at the opposite end
if the ear is pressed against the table; if the ear is
removed from the wood, the sound of tapping is
much fainter, showing that wood transmits sound more readily than air. We
see therefore that sound can be transmitted to the ear by solids, liquids, or
gases.
Matter of any kind can transmit sound to the ear. The following experiments
will show that matter is necessary for transmission. Attach a small toy bell
to a glass rod (Fig. 166) by means of a rubber tube and pass the rod through
one of two openings in a rubber cork. Insert the cork in a strong flask
containing a small quantity of water and shake the bell, noting the sound
produced. Then heat the flask, allowing the water to boil briskly, and after
the boiling has continued for a few minutes remove the flame and instantly
close up the second opening by inserting a glass stopper. Now shake the
flask and note that the sound is very much fainter than at first. As the flask
was warmed, air was rapidly expelled; so that when the flask was shaken
the second time, less air was present to transmit the sound. If the glass
stopper is removed and the air is allowed to reenter the flask, the loudness
of the sound immediately increases.

Since the sound of the bell grows fainter as air is
removed, we infer that there would be no sound if all
the air were removed from the flask; that is to say,
sound cannot be transmitted through empty space or a
vacuum. If sound is to reach our ears, it must be
through the agency of matter, such as wood, water, or
air, etc.
252. How Sound is transmitted through Air. We saw
in Section 250 that sound can always be traced to the
motion or vibration of matter. It is impossible to
conceive of an object being set into sudden and
continued motion without disturbing the air
immediately surrounding it. A sounding body always
disturbs and throws into vibration the air around it,
FIG. 166.—Sound is and the air particles which receive motion from a
carried by the air. sounding body transmit their motion to neighboring
particles, these in turn to the next adjacent particles,
and so on until the motion has traveled to very great
distances. The manner in which vibratory motion is transmitted by the
atmosphere must be unusual in character, since no motion of the air is
apparent, and since in the stillness of night when "not a breath of air" is
stirring, the shriek of a railroad whistle miles distant may be heard with
perfect clearness. Moreover, the most delicate notes of a violin can be heard
in the remotest corners of a concert hall, when not the slightest motion of
the air can be seen or felt.
In our study of the atmosphere we saw that air can be compressed and
rarefied; in other words, we saw that air is very elastic. It can be shown
experimentally that whenever an elastic body in motion comes in contact
with a body at rest, the moving body transfers its motion to the second body
and then comes to rest itself. Let two billiard balls be suspended in the
manner indicated in Figure 167. If one of the balls is drawn aside and is
then allowed to fall against the other, the second ball is driven outward to
practically the height from which the first ball fell and the first ball comes
to rest.

If a number of balls
are arranged in line
as in Figure 168 or
Figure 169, and the
end ball is raised and
then allowed to fall,
or if A is pushed
against C, the last
FIG. 168.—Suspended billiard balls. ball B will move
outward alone, with FIG. 167.—Elastic balls.
a force nearly equal to that originally possessed by A
and to a distance nearly equal to that through which A moved. But there will
be no visible motion of the intervening balls. The force of the moving ball A
is given to the second ball, and the second ball in turn gives the motion to
the third, and so on throughout the entire number, until B is reached. But B
has no ball to give its motion to, hence B itself moves outward, and moves
with a force nearly equal to that originally imparted by A and to a distance
nearly equal to that through which A fell. Motion at A is transmitted to B
without any perceptible motion of the balls lying between these points.
Similarly the particles of air set into motion by a sounding body impart their
motion to each other, the motion being transmitted onward without any
perceptible motion of the air itself. When this motion reaches the ear, it sets
the drum of the ear into vibration, and these vibrations are in turn
transmitted to the auditory nerves, which interpret the motion as sound.

FIG. 169.—Elastic balls transmit motion.

FIG. 170.—When a ball meets more than one ball, it divides its motion.

253. Why Sound dies away with Distance. Since the last ball B is driven
outward with a force nearly equal to that possessed by A, it would seem that
the effect on the ear drum should be independent of distance and that a
sound should be heard as distinctly when remote as when near. But we
know from experience that this is not true, because the more distant the
source of sound, the fainter the impression; and finally, if the distance
between the source of sound and the hearer becomes too great, the sound
disappears entirely and nothing is heard. The explanation of this well-
known fact is found in a further study of the elastic balls (Fig. 170). If A
hits two balls instead of one, the energy possessed by A is given in part to
one ball, and in part to the other, so that neither obtains the full amount.
These balls, having each received less than the original energy, have less to
transmit; each of these balls in turn meets with others, and hence the motion
becomes more and more distributed, and distant balls receive less and less
impetus. The energy finally given becomes too slight to affect neighboring
balls, and the system comes to rest. This is what occurs in the atmosphere; a
moving air particle meets not one but many adjacent air particles, and each
of these receives a portion of the original energy and transmits a portion.
When the original disturbance becomes scattered over a large number of air
particles, the energy given to any one air particle becomes correspondingly
small, and finally the energy becomes so small that further particles are not
affected; beyond this limit the sound cannot be heard.
If an air particle transmitted motion only to those air particles directly in
line with it, we should not be able to detect sound unless the ear were in
direct line with the source. The fact that an air particle divides its motion
among all particles which it touches, that is, among those on the sides as
well as those in front, makes it possible to hear sound in all directions. A

good speaker is heard not only by those directly in front of him, but by
those on the side, and even behind him.
254. Velocity of Sound. The transmission of motion from particle to
particle does not occur instantaneously, but requires time. If the distance is
short, so that few air particles are involved, the time required for
transmission is very brief, and the sound is heard at practically the instant it
is made. Ordinarily we are not conscious that it requires time for sound to
travel from its source to our ears, because the distance involved is too short.
At other times we recognize that there is a delay; for example, thunder
reaches our ears after the lightning which caused the thunder has
completely disappeared. If the storm is near, the interval of time between
the lightning and the thunder is brief, because the sound does not have far to
travel; if the storm is distant, the interval is much longer, corresponding to
the greater distance through which the sound travels. Sound does not move
instantaneously, but requires time for its transmission. The report of a
distant cannon is heard after the flash and smoke are seen; the report of a
near cannon is heard the instant the flash is seen.
The speed with which sounds travels through the air, or its velocity, was
first measured by noting the interval (54.6 seconds) which elapsed between
the flash of a cannon and the sound of the report. The distance of the
cannon from the observer was measured and found to be 61,045 feet, and
by dividing this distance by the number of seconds, we find that the
distance traveled by sound in one second is approximately 1118 feet.
High notes and low notes, soft notes and shrill notes, all travel at the same
rate. If bass notes traveled faster or slower than soprano notes, or if the
delicate tones of the violin traveled faster or slower than the tones of a
drum, music would be practically impossible, because at a distance from the
source of sound the various tones which should be in unison would be out
of time—some arriving late, some early.
255. Sound Waves. Practically everyone knows that a hammock hung with
long ropes swings or vibrates more slowly than one hung with short ropes,
and that a stone suspended by a long string swings more slowly than one
suspended by a short string. No two rocking chairs vibrate in the same way
unless they are exactly alike in shape, size, and material. An object when

disturbed vibrates in a manner peculiar to itself, the vibration being slow, as
in the case of the long-roped swing, or quick, as in the case of the short-
roped swing. The time required for a single swing or vibration is called the
period of the body, and everything that can vibrate has a characteristic
period. Size and shape determine to a large degree the period of a body; for
example, a short, thick tuning fork vibrates more rapidly than a tall slender
fork.
Some tuning forks when struck
vibrate so rapidly that the prongs
move back and forth more than
5000 times per second, while other
tuning forks vibrate so slowly that
the vibrations do not exceed 50 per
second. In either case the distance
through which the prongs move is
very small and the period is very
short, so that the eye can seldom
detect the movement itself. That the FIG. 171.—The two hammocks swing
prongs are in motion, however, is differently.
seen by the action of a pith ball
when brought in contact with the
prongs (see Section 250).
The disturbance created by a vibrating body is called a
wave.
256. Waves. While the disturbance which travels out
from a sounding body is commonly called a wave, it is
by no means like the type of wave best known to us,
namely, the water wave.
If a closely coiled heavy wire is suspended as in
Figure 173 and the weight is drawn down and then
released, the coil will assume the appearance shown;
FIG. 172.—The pitch
there is clearly an overcrowding or condensation in
given out by a fork
some places, and a spreading out or rarefaction in
depends upon its shape.
other places. The pulse of condensation and

rarefaction which travels the length of the wire is called a wave, although it
bears little or no resemblance to the familiar water wave. Sound waves are
similar to the waves formed in the stretched coil.
Sound waves may be said to consist of a series of
condensations and rarefactions, and the distance
between two consecutive condensations and
rarefactions may be defined as the wave length.
257. How One Sounding Body produces Sound in
Another Body. In Section 255 we saw that any object
when disturbed vibrates in a manner peculiar to itself,
—its natural period,—a long-roped hammock
vibrating slowly and a short-roped hammock vibrating
rapidly. From observation we learn that it requires but
little force to cause a body to vibrate in its natural
period. If a sounding body is near a body which has
the same period as itself, the pulses of air produced by
the sounding body will, although very small, set the
second body into motion and cause it to make a faint
sound. When a piano is being played, we are often
startled to find that a window pane or an ornament
responds to some note of the piano. If two tuning forks
of exactly identical periods (that is, of the same
frequency) are placed on a table as in Figure 174, and
one is struck so as to give forth a clear sound, the
FIG. 173.—Waves in a second fork will likewise vibrate, even though the two
coiled wire. forks may be separated by several feet of air. We can
readily see that the second fork is in motion, although
it has not been struck, because it will set in motion a pith ball suspended
beside it; at first the pith ball does not move, then it moves slightly, and
finally bounces rapidly back and forth. If the periods of the two forks are
not identical, but differ in the slightest degree, the second fork will not
respond to the first fork, no matter how long or how loud the sound of the
first fork. If we suppose that the fork vibrates 256 times each second, then
256 gentle pulses of air are produced each second, and these, traveling
outward through the air, reach the silent fork and tend to set it in motion. A
single pulse of air could not move the solid, heavy prongs, but the

accumulated action of 256 vibrations per second soon makes itself felt, and
the second fork begins to vibrate, at first gently, then gradually stronger, and
finally an audible tone is given forth.
The cumulative power of feeble
forces acting frequently at definite
intervals is seen in many ways in
everyday life. A small boy can
easily swing a much larger boy,
provided he gives the swing a
gentle push in the right direction
every time it passes him. But he
must be careful to push at the
proper instant, since otherwise his
effort does not count for much; if he
FIG. 174.—When the first fork vibrates, the
pushes forward when the swing is
second responds.
moving backward, he really hinders
the motion; if he waits until the
swing has moved considerably forward, his push counts for little. He must
push at the proper instant; that is, the way in which his hand moves in
giving the push must correspond exactly with the way in which the swing
would naturally vibrate. A very striking experiment can be made by
suspending from the ceiling a heavy weight and striking this weight gently
at regular, properly timed intervals with a small cork hammer. Soon the
pendulum, or weight, will be set swinging.
258. Borrowed Sound. Picture frames and ornaments
sometimes buzz and give forth faint murmurs when a
piano or organ is played. The waves sent out by a
sounding body fall upon all surrounding objects and
by their repeated action tend to throw these bodies into
vibration. If the period of any one of the objects
corresponds with the period of the sounding body, the
gentle but frequent impulses affect the object, which
responds by emitting a sound. If, however, the periods
FIG. 175.—The hollow
do not correspond, the action of the sound waves is not
sufficiently powerful to throw the object into
vibration, and no sound is heard. Bodies which

wooden box reënforces respond in this way are said to be sympathetic and the
the sound. response produced is called resonance. Seashells when
held to the ear seem to contain the roar of the sea; this
is because the air within the shell is set into sympathetic vibrations by some
external tone. If the seashell were held to the ear in an absolutely quiet
room, no sound would be heard, because there would be no external forces
to set into vibration the air within the shell.
Tuning forks do not produce strong tones unless mounted on hollow
wooden boxes (Fig. 175), whose size and shape are so adjusted that
resonance occurs and strengthens the sound. When a human being talks or
sings, the air within the mouth cavity is thrown into sympathetic vibration
and strengthens the otherwise feeble tone of the speaker.
259. Echo. If one shouts in a forest, the sound is sometimes heard a second
time a second or two later. This is because sound is reflected when it strikes
a large obstructing surface. If the sound waves resulting from the shout
meet a cliff or a mountain, they are reflected back, and on reaching the ear
produce a later sensation of sound.
By observation it has been found that the ear cannot distinguish sounds
which are less than one tenth of a second apart; that is, if two sounds follow
each other at an interval less than one tenth of a second, the ear recognizes
not two sounds, but one. This explains why a speaker can be heard better
indoors than in the open air. In the average building, the walls are so close
that the reflected waves have but a short distance to travel, and hence reach
the ear at practically the same time as those which come directly from the
speaker. In the open, there are no reflecting walls or surfaces, and the
original sound has no reënforcement from reflection.
If the reflected waves reach the ear too late to blend with the original sound,
that is, come later than one tenth of a second after the first impression, an
echo is heard. What we call the rolling of thunder is really the reflection and
re-reflection of the original thunder from cloud and cliff.
Some halls are so large that the reflected sounds cause a confusion of
echoes, but this difficulty can be lessened by hanging draperies, which
break the reflection.

260. Motion does not always produce Sound. While we know that all
sound can be traced to motion, we know equally well that motion does not
always produce sound. The hammock swinging in the breeze does not give
forth a sound; the flag floating in the air does not give forth a sound unless
blown violently by the wind; a card moved slowly through the air does not
produce sound, but if the card is moved rapidly back and forth, a sound
becomes audible.
Motion, in order to produce sound, must be rapid; a ball attached to a string
and moved slowly through the air produces no sound, but the same ball,
whirled rapidly, produces a distinct buzz, which becomes stronger and
stronger the faster the ball is whirled.
261. Noise and Music. When the rapid motions which produce sound are
irregular, we hear noise; when the motions are regular and definite, we have
a musical tone; the rattling of carriage wheels on stones, the roar of waves,
the rustling of leaves are noise, not music. In all these illustrations we have
rapid but irregular motion; no two stones strike the wheel in exactly the
same way, no two waves produce pulses in the air of exactly the same
character, no two leaves rustle in precisely the same way. The disturbances
which reach the ear from carriage, waves, and leaves are irregular both in
time and strength, and irritate the ear, causing the sensation which we call
noise.
The tuning fork is musical. Here we have rapid, regular motion; vibrations
follow each other at perfectly definite intervals, and the air disturbance
produced by one vibration is exactly like the disturbance produced by a
later vibration. The sound waves which reach the ear are regular in time and
kind and strength, and we call the sensation music.
To produce noise a body must vibrate in such a way as to give short, quick
shocks to the air; to produce music a body must not only impart short, quick
shocks to the air, but must impart these shocks with unerring regularity and
strength. A flickering light irritates the eye; a flickering sound or noise
irritates the ear; both are painful because of the sudden and abrupt changes
in effect which they cause, the former on the eye, the latter on the ear.
The only thing essential for the production of a musical sound is that the
waves which reach the ear shall be rapid and regular; it is immaterial how

these waves are produced. If a toothed wheel is mounted and slowly
rotated, and a stiff card is held against the teeth of the wheel, a distinct tap
is heard every time the card strikes the wheel. But if the wheel is rotated
rapidly, the ear ceases to hear the various taps and recognizes a deep
continuous musical tone. The blending of the individual taps, occurring at
regular intervals, has produced a sustained musical tone. A similar result is
obtained if a card is drawn slowly and then rapidly over the teeth of a comb.
That musical tones are due to a succession of
regularly timed impulses is shown most clearly by
means of a rotating disk on which are cut two sets
of holes, the outer set equally spaced, and the inner
set unequally spaced (Fig. 176).
If, while the disk is rotating rapidly, a tube is held
over the outside row and air is blown through the
tube, a sustained musical tone will be heard. If,
however, the tube is held, during the rotation of the
disk, over the inner row of unequally spaced holes, FIG. 176.—A rotating disk.
the musical tone disappears, and a series of noises
take its place. In the first case, the separate puffs of
air followed each other regularly and blended into one tone; in the second
case, the separate puffs of air followed each other at uncertain and irregular
intervals and the result was noise.
Sound possesses a musical quality only when the waves or pulses follow
each other at absolutely regular intervals.
262. The Effect of the Rapidity of Motion on the Musical Tone
Produced. If the disk is rotated so slowly that less than about 16 puffs are
produced in one second, only separate puffs are heard, and a musical tone is
lacking; if, on the other hand, the disk is rotated in such a way that 16 puffs
or more are produced in one second, the separate puffs will blend together
to produce a continuous musical note of very low pitch. If the speed of the
disk is increased so that the puffs become more frequent, the pitch of the
resulting note rises; and at very high speeds the notes produced become so
shrill and piercing as to be disagreeable to the ear. If the speed of the disk is
lessened, the pitch falls correspondingly; and if the speed again becomes so

low that less than 16 puffs are formed per second, the sustained sound
disappears and a series of intermittent noises is produced.
263. The Pitch of a Note. By means of an apparatus called the siren, it is
possible to calculate the number of vibrations producing any given musical
note, such, for example, as middle C on the piano. If air is forced
continuously against the disk as it rotates, a series of puffs will be heard
(Fig. 177).
If the disk turns fast enough, the puffs blend into a musical sound, whose
pitch rises higher and higher as the disk moves faster and faster, and
produces more and more puffs each second.

FIG. 177.—A siren.

The instrument is so constructed that clockwork at the top registers the
number of revolutions made by the disk in one second. The number of holes
in the disk multiplied by the number of revolutions a second gives the
number of puffs of air produced in one second. If we wish to find the
number of vibrations which correspond to middle C on the piano, we
increase the speed of the disk until the note given forth by the siren agrees
with middle C as sounded on the piano, as nearly as the ear can judge; we
then calculate the number of puffs of air which took place each second at
that particular speed of the disk. In this way we find that middle C is due to
about 256 vibrations per second; that is, a piano string must vibrate 256

times per second in order for the resultant note to be of pitch middle C. In a
similar manner we determine the following frequencies:—

do re mi fa sol la si do
C D E F G A B C
256 288 320 341 384 427 480 512

The pitch of pianos, from the lowest bass note to the very highest treble,
varies from 27 to about 3500 vibrations per second. No human voice,
however, has so great a range of tone; the highest soprano notes of women
correspond to but 1000 vibrations a second, and the deepest bass of men
falls but to 80 vibrations a second.
While the human voice is limited in its production of sound,—rarely falling
below 80 vibrations a second and rarely exceeding 1000 vibrations a
second,—the ear is by no means limited to that range in hearing. The
chirrup of a sparrow, the shrill sound of a cricket, and the piercing shrieks
of a locomotive are due to far greater frequencies, the number of vibrations
at times equaling 38,000 per second or more.
264. The Musical Scale. When we talk, the pitch of the voice changes
constantly and adds variety and beauty to conversation; a speaker whose
tone, or pitch, remains too constant is monotonous and dull, no matter how
brilliant his thoughts may be.
While the pitch of the voice changes constantly, the changes are normally
gradual and slight, and the different tones merge into each other
imperceptibly. In music, however, there is a well-defined interval between
even consecutive notes; for example, in the musical scale, middle C (do)
with 256 vibrations is followed by D (re) with 288 vibrations, and the
interval between these notes is sharp and well marked, even to an untrained
ear. The interval between two notes is defined as the ratio of the
frequencies; hence, the interval between C and D (do and re) is 288/256, or
9/8. Referring to Section 263, we see that the interval between C and E is
320/256, or 5/4, and the interval between C and C' is 512/256, or 2; the
interval between any note and its octave is 2.

The successive notes in one octave of the musical scale are related as
follows:—

Keys of C C D E F G A B C'
No. of vibrations
256 288 320 341 384 427 480 512
per sec.
Interval 9/8 5/4 4/3 3/2 5/3 15/8 2

The intervals of F and A are not strictly 4/3 and 5/3, but are nearly so; if F
made 341.3 vibrations per second instead of 341; and if A made 426.6
instead of 427, then the intervals would be exactly 4/3 and 5/3. Since the
real difference is so slight, we can assume the simpler ratios without
appreciable error.
Any eight notes whose
frequencies are in the
ratio of 9/8, 5/4, etc.,
will when played in
succession give the
familiar musical scale;
for example, the
deepest bass voice
starts a musical scale
whose notes have the
frequencies 80, 90,
100, 107, 120, 133,
150, 160, but the
intervals here are
identical with those of
a higher scale; the FIG. 178.—A song as sung by three voices of different pitch.
interval between C and
D, 80 and 90, is 9/8, just as it was before when the frequencies were much
greater; that is, 256 and 288. In singing "Home, Sweet Home," for example,
a bass voice may start with a note vibrating only 132 times a second; while
a tenor may start at a higher pitch, with a note vibrating 198 times per
second, and a soprano would probably take a much higher range still, with

an initial frequency of 528 vibrations per second. But no matter where the
voices start, the intervals are always identical. The air as sung by the bass
voice would be represented by A. The air as sung by the tenor voice would
be represented by B. The air as sung by the soprano voice would be
represented by C.

CHAPTER XXVIII
MUSICAL INSTRUMENTS

265. Musical instruments maybe divided into three groups according to the
different ways in which their tones are produced:—
First. The stringed instruments in which sound is produced by the vibration
of stretched strings, as in the piano, violin, guitar, mandolin.
Second. The wind instruments in which sound is produced by the vibrations
of definite columns of air, as in the organ, flute, cornet, trombone.
Third. The percussion instruments, in which sound is produced by the
motion of stretched membranes, as in the drum, or by the motion of metal
disks, as in the tambourines and cymbals.
266. Stringed Instruments. If the lid of a piano is opened, numerous wires
are seen within; some long, some short, some coarse, some fine. Beneath
each wire is a small felt hammer connected with the keys in such a way that
when a key is pressed, a string is struck by a hammer and is thrown into
vibration, thereby producing a tone.
If we press the lowest key, that is, the key giving forth the lowest pitch, we
see that the longest wire is struck and set into vibration; if we press the
highest key, that is, the key giving the highest pitch, we see that the shortest
wire is struck. In addition, it is seen that the short wires which produce the
high tones are fine, while the long wires which produce the low tones are
coarse. The shorter and finer the wire, the higher the pitch of the tone
produced. The longer and coarser the wire, the lower the pitch of the tone
produced.

FIG. 179.—Piano wires seen from the back.

The constant striking of the hammers against the strings stretches and
loosens them and alters their pitch; for this reason each string is fastened to
a screw which can be turned so as to tighten the string or to loosen it if
necessary. The tuning of the piano is the adjustment of the strings so that
each shall produce a tone of the right pitch. When the strings are tightened,
the pitch rises; when the strings are loosened, the pitch falls.
What has been said of the piano applies as well to the violin, guitar, and
mandolin. In the latter instruments the strings are few in number, generally
four, as against eighty-eight in the piano; the hammer of the piano is
replaced in the violin by the bow, and in the guitar by the fingers; varying
pitches on any one string are obtained by sliding a finger of the left hand
along the wire, and thus altering its length.
Frequent tuning is necessary, because the fine adjustments are easily
disturbed. The piano is the best protected of all the stringed instruments,
being inclosed by a heavy framework, even when in use.

FIG. 180.—Front view of an open piano.

267. Strings and their Tones. Fasten a violin string to a wooden frame or
box, as shown in Figure 181, stretching it by means of some convenient
weight; then lay a yardstick along the box in order that the lengths may be
determined accurately. If the stretched string is plucked with the fingers or
bowed with the violin bow, a clear musical sound of definite pitch will be
produced. Now divide the string into two equal parts by inserting the bridge
midway between the two ends; and pluck either half as before. The note
given forth is of a decidedly higher pitch, and if by means of the siren we
compare the pitches in the two cases, we find that the note sounded by the
half wire is the octave of the note sounded by the entire wire; the frequency
has been doubled by halving the length. If now the bridge is placed so that
the string is divided into two unequal portions such as 1:3 and 2:3, and the
shorter portion is plucked, the pitch will be still higher; the shorter the
length plucked, the higher the pitch produced. This movable bridge
corresponds to the finger of the violinist; the finger slides back and forth
along the string, thus changing the length of the bowed portion and
producing variations in pitch.
If there were but one string, only one pitch could be sounded at any one
time; the additional strings of the violin allow of the simultaneous

production of several tones.
268. The Freedom of a
String. Some stringed
instruments give forth tones
which are clear and sweet,
but withal thin and lacking in
richness and fullness. The FIG. 181.—The length of a string influences the pitch.
tones sounded by two
different strings may agree in
pitch and loudness and yet produce quite different effects on the ear,
because in one case the tone may be much more pleasing than in the other.
The explanation of this is, that a string may vibrate in a number of different
ways.

FIG. 182.—Only one half of the string is bowed, but both halves vibrate.

Touch the middle of a wire with the finger or a pencil (Fig. 182), thus
separating it into two portions and draw a violin bow across the center of
either half. Only one half of the entire string is struck, but the motion of this
half is imparted to the other half and throws it into similar motion, and if a
tiny A-shaped piece of paper or rider is placed upon the unbowed half, it is
hurled off.

FIG. 183.—The string vibrates in three portions.

If the wire is touched at a distance of one third its length and a bow is
drawn across the middle of the smaller portion, the string will vibrate in
three parts; we cannot always see these various motions in different parts of
the string, but we know of their existence through the action of the riders.
Similarly, touching the wire one fourth of its length from an end makes it
vibrate in four segments; touching it one fifth of its length makes it vibrate
in five segments.
In the first case, the string vibrated as a whole string and also as two strings
of half the length; hence, three tones must have been given out, one tone
due to the entire string and two tones due to the segments. But we saw in
Section 267 that halving the length of a string doubles the pitch of the
resulting tone, and produces the octave of the original tone; hence a string
vibrating as in Figure 183 gives forth three tones, one of which is the
fundamental tone of the string, and two of which are the octave of the
fundamental tone. Hence, the vibrating string produces two sensations, that
of the fundamental note and of its octave.

FIG. 184.—When a string vibrates as a whole, it gives out the fundamental note.

When a string is plucked in the middle without being held, it vibrates
simply as a whole (Fig. 184), and gives forth but one note; this is called the
fundamental. If the string is made to vibrate in two parts, it gives forth two
notes, the fundamental, and a note one octave higher than the fundamental;
this is called the first overtone. When the string is made to move as in
Figure 183, three distinct motions are called forth, the motion of the entire
string, the motion of the portion plucked, and the motion of the remaining
unplucked portion of the string. Here, naturally, different tones arise,
corresponding to the different modes of vibration. The note produced by the
vibration of one third of the original string is called the second overtone.
The above experiments show that a string is able to vibrate in a number of
different ways at the same time, and to emit simultaneously a number of
different tones; also that the resulting complex sound consists of the
fundamental and one or more overtones, and that the number of overtones
present depends upon how and where the string is plucked.
269. The Value of Overtones. The presence of overtones determines the
quality of the sound produced. If the string vibrates as a whole merely, the
tone given out is simple, and seems dull and characterless. If, on the other
hand, it vibrates in such a way that overtones are present, the tone given
forth is full and rich and the sensation is pleasing. A tuning fork cannot
vibrate in more than one way, and hence has no overtones, and its tone,
while clear and sweet, is far less pleasing than the same note produced by a
violin or piano. The untrained ear is not conscious of overtones and
recognizes only the strong dominant fundamental. The overtones blend in
with the fundamental and are so inconspicuously present that we do not

realize their existence;
it is only when they are
absent that we become
aware of the beauty
which they add to the
music. A song played
on tuning forks instead
of on strings would be
lifeless and
unsatisfying because of
the absence of
overtones.
It is not necessary to
hold finger or pencil at
the points 1:3, 1:4, etc.,
in order to cause the
string to vibrate in
various ways; if a
string is merely
FIG. 185.—A string can vibrate in a number of different ways plucked or bowed at
simultaneously, and can produce different notes simultaneously. those places, the result
will be the same. It is
important to remember that no matter where a string of definite length is
bowed, the note most distinctly heard will be the fundamental; but the
quality of the emitted tone will vary with the bowing. For example, if a
string is bowed in the middle, the effect will be far less pleasing than
though it were bowed near the end. In the piano, the hammers are arranged
so as to strike near one end of the string, at a distance of about 1:7 to 1:9;
and hence a large number of overtones combine to reënforce and enrich the
fundamental tone.
270. The Individuality of Instruments. It has been shown that a piano
string when struck by a hammer, or a violin string when bowed, or a
mandolin string when plucked, vibrates not only as a whole, but also in
segments, and as a result gives forth not a simple tone, as we are
accustomed to think, but a very complex tone consisting of the fundamental
and one or more overtones. If the string whose fundamental note is lower C

(128 vibrations per second) is thrown into vibration, the tone produced may
contain, in addition to the prominent fundamental, any one or more of the
following overtones: C', G'', C'', E'', C''', etc.
The number of overtones actually present depends upon a variety of
circumstances: in the piano, it depends largely upon the location of the
hammer; in the violin, upon the place and manner of bowing. Mechanical
differences in construction account for prominent and numerous overtones
in some instruments and for feeble and few overtones in others. The oboe,
for example, is so constructed that only the high overtones are present, and
hence the sound gives a "pungent" effect; the clarinet is so constructed that
the even-numbered overtones are killed, and the presence of only odd-
numbered overtones gives individuality to the instrument. In these two
instruments we have vibrating air columns instead of vibrating strings, but
the laws which govern vibrating strings are applicable to vibrating columns
of air, as we shall see later. It is really the presence or absence of overtones
which enables us to distinguish the note of the piano from that of the violin,
flute, or clarinet. If overtones could be eliminated, then middle C, or any
other note on the piano, would be indistinguishable from that same note
sounded on any other instrument. The fundamental note in every instrument
is the same, but the overtones vary with the instrument and lend
individuality to each. The presence of high overtones in the oboe and the
presence of odd-numbered overtones in the clarinet enable us to distinguish
without fail the sounds given out by these instruments.
The richness and individuality of an instrument are due, not only to the
overtones which accompany the fundamental, but also to the "forced"
vibrations of the inclosing case, or of the sounding board. If a vibrating
tuning fork is held in the hand, the sound will be inaudible except to those
quite near; if, however, the base of the fork is held against the table, the
sound is greatly intensified and becomes plainly audible throughout the
room.
The vibrations of the fork are transmitted to the table top and throw it into
vibrations similar to its own, and these additional vibrations intensify the
original sound. Any fork, no matter what its frequency, can force the surface
of the table into vibration, and hence the sound of any fork will be
intensified by contact with a table or box.