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Title: Common Science

Author: Carleton Washburne

Editor: John W. Ritchie

Release date: August 28, 2009 [eBook #29838]
Most recently updated: January 5, 2021

Language: English

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

Transcriber's Note:

Minor inconsistencies in spelling, punctuation
and formatting are retained as in the original.
Where detailed corrections have been made on
the text these are listed at the end of this
document.

Disclaimer:

This is a work of historical interest only and
much of the scientific content has been
superseded. There are numerous experiments
described in this book which are hazardous and
should not be attempted. Advice given on
handling toxic substances, electrical apparatus
etc. should not be followed.

Do not try this at home!

COMMON SCIENCE

NEW-WORLD SCIENCE SERIES

Edited by John W. Ritchie

Science for Beginners
By Delos Fall

Trees, Stars, and Birds
By Edwin Lincoln Moseley
Common Science
By Carleton W. Washburne

Human Physiology
By John W. Ritchie

Sanitation and Physiology
By John W. Ritchie

Laboratory Manual for Use with "Human Physiology"
By Carl Hartman

Exercise and Review Book in Biology
By J. G. Blaisdell

Personal Hygiene and Home Nursing
By Louisa C. Lippitt

Science of Plant Life
By Edgar Nelson Transeau

Zoölogy
By T. D. A. Cockerell

Experimental Organic Chemistry
By Augustus P. West

NEW-WORLD SCIENCE SERIES

Edited by John W. Ritchie

COMMON SCIENCE
by

Carleton W. Washburne

Superintendent of Schools, Winnetka, Illinois
Formerly Supervisor in Physical Sciences and Instructor in Educational
Psychology
State Normal School
San Francisco, California

ILLUSTRATED

WITH PHOTOGRAPHS AND DRAWINGS

Yonkers-on-Hudson, New York

WORLD BOOK COMPANY

1921

WORLD BOOK COMPANY

THE HOUSE OF APPLIED KNOWLEDGE

Established, 1905, by Caspar W. Hodgson

Yonkers-on-Hudson, New York

2126 Prairie Avenue, Chicago

One of the results of the World War has been a widespread desire to see the
forces of science which proved so mighty in destruction employed generally
and systematically for the promotion of human welfare. World Book Company,
whose motto is The Application of the World's Knowledge to the World's
Needs, has been much in sympathy with the movement to make science an
integral part of our elementary education, so that all our people from the
highest to the lowest will be able to use it for themselves and to appreciate the
possibilities of ameliorating the conditions of human life by its application to
the problems that confront us. We count it our good fortune, therefore, that we
are able at this time to offer Common Science to the schools. It is one of the
new type of texts that are built on educational research and not by guess, and
we believe it to be a substantial contribution to the teaching of the subject.

NWSS:WCS-2

Copyright, 1920, by World Book Company
Copyright in Great Britain
All rights reserved

PREFACE
A collection of about 2000 questions asked by children forms the foundation
on which this book is built. Rather than decide what it is that children ought to
know, or what knowledge could best be fitted into some educational theory, an
attempt was made to find out what children want to know. The obvious way to
discover this was to let them ask questions.

The questions collected were asked by several hundred children in the upper
elementary grades, over a period of a year and a half. They were then sorted
and classified according to the scientific principles needed in order to answer
them. These principles constitute the skeleton of this course. The questions
gave a very fair indication of the parts of science in which children are most
interested. Physics, in simple, qualitative form,—not mathematical physics, of
course,—comes first; astronomy next; chemistry, geology, and certain forms of
physical geography (weather, volcanoes, earthquakes, etc.) come third;
biology, with physiology and hygiene, is a close fourth; and nature study, in
the ordinary school sense of the term, comes in hardly at all.

The chapter headings of this book might indicate that the course has to do with
physics and chemistry only. This is because general physical and chemical
principles form a unifying and inclusive matrix for the mass of applications.
But the examples and descriptions throughout the book include physical
geography and the life sciences. Descriptive astronomy and geology have,
however, been omitted. These two subjects can be best grasped in a reading
course and field trips, and they have been incorporated in separate books.

The best method of presenting the principles to the children was the next
problem. The study of the questions asked had shown that the children's
interests were centered in the explanation of a wide variety of familiar facts in
the world about them. It seemed evident, therefore, that a presentation of the
principles that would answer the questions asked would be most interesting to
the child. Experience with many different classes had shown that it is not
necessary to subordinate these explanations of what children really wish to
know to other methods of instruction of doubtful interest value.

Obviously the quantitative methods of the high school and college were
unsuitable for pupils of this age. We want children to be attracted to science,
not repelled by it. The assumption that scientific method can be taught to
children by making them perform uninteresting, quantitative experiments in an
effort to get a result that will tally with that given in the textbook is so palpably
unfounded that it is scarcely necessary to prove its failure by pointing to the
very unscientific product of most of our high school science laboratories.

After a good deal of experimenting with children in a number of science
classes, the method followed in this book was developed. Briefly, it is as
follows:

At the head of each section are several of the questions which, in part,
prompted the writing of the section. The purpose of these is to let the children
know definitely what their goal is when they begin a section. The fact that the
questions had their origin in the minds of children gives reasonable assurance
that they will to some extent appeal to children. These questions in effect state
the problems which the section helps to solve.

Following the questions are some introductory paragraphs for arousing interest
in the problem at hand,—for motivating the child further. These paragraphs are
frequently a narrative description containing a good many dramatic elements,
and are written in conversational style. The purpose is to awaken the child's
imagination and to make clear the intimate part which the principle under
consideration plays in his own life. When a principle is universal, like gravity,
it is best brought out by imagining what would happen if it ceased to exist. If a
principle is particular to certain substances, like elasticity, it sometimes can be
brought out vividly by imagining what would happen if it were universal.
Contrast is essential to consciousness. To contrast a condition that is very
common with an imagined condition that is different brings the former into
vivid consciousness. Incidentally, it arouses real interest. The story-like
introduction to many sections is not a sugar coating to make the child swallow
a bitter pill. It is a psychologically sound method of bringing out the essential
and dramatic features of a principle which is in itself interesting, once the child
has grasped it.

Another means for motivating the work in certain cases consists in first doing
a dramatic experiment that will arouse the pupil's interest and curiosity. Still

another consists in merely calling the child's attention to the practical value of
the principle.

Following these various means for getting the pupil's interest, there are usually
some experiments designed to help him solve his problem. The experiments
are made as simple and interesting as possible. They usually require very
inexpensive apparatus and are chosen or invented both for their interest value
and their content value.

With an explanation of the experiments and the questions that arise, a principle
is made clear. Then the pupil is given an opportunity to apply the principle in
making intelligible some common fact, if the principle has only intelligence
value; or he is asked to apply the principle to the solution of a practical
problem where the principle also has utility value.

The "inference exercises" which follow each section after the first two consist
of statements of well-known facts explainable in terms of some of the
principles which precede them. They involve a constant review of the work
which has gone before, a review which nevertheless is new work—they review
the principles by giving them new applications. Furthermore, they give the
pupil very definite training in explaining the common things around him.

For four years a mimeographed edition of this book has been used in the
elementary department of the San Francisco State Normal School. During that
time various normal students have tried it in public school classes in and
around San Francisco and Oakland, and it has recently been used in Winnetka,
Illinois. It has been twice revised throughout in response to needs shown by
this use.

The book has proved itself adaptable to either an individual system of
instruction or the usual class methods.

TO THE TEACHER
Do not test the children on the narrative description which introduces most
sections, nor require them to recite on it. It is there merely to arouse their
interest, and that is likely to be checked if they think it is a lesson to be
learned. It is not at all necessary for them to know everything in the
introductory parts of each section. If the children are interested, they will
remember what is valuable to them; if they are not, do not prolong the agony.
The questions which accompany and follow the experiments, the applications
or required explanations at the ends of the sections, and the extensive inference
exercises, form an ample test of the child's grasp of the principles under
discussion.

It is not necessary to have the children write up their experiments. The
experiments are a means to an end. The end is the application of the principles
to everyday facts. If the children can make these applications, it does not
matter how much of the actual experiments they remember.

If possible, the experiments should be done by the pupils individually or in
couples, in a school laboratory. Where this cannot be done, almost all the
experiments can be demonstrated from the teacher's desk if electricity, water,
and gas are to be had. Alcohol lamps can be substituted for gas, but they are
less satisfactory.

It is a good plan to have pupils report additional exemplifications of each
principle from their home or play life, and in a quick oral review to let the rest
of the class name the principles back of each example.

This course is so arranged that it can be used according to the regular class
system of instruction, or according to the individual system where each child
does his own work at his natural rate of progress. The children can carry on the
work with almost no assistance from the teacher, if provision is made for their
doing the experiments themselves and for their writing the answers to the
inference exercises. When the individual system is used, the children may
write the inference exercises, or they may use them as a basis for study and
recite only a few to the teacher by way of test. In the elementary department of

the San Francisco State Normal School, where the individual system is used,
the latter method is in operation. The teacher has a card for each pupil, each
card containing a mimeographed list of the principles, with a blank after each.
Whenever a pupil correctly explains an example, a figure 1 is placed in the
blank following that principle; when he misapplies a principle, or fails to apply
it, an x is placed after it. When there are four successive 1's after any principle,
the teacher no longer includes that principle in testing that child. In this way
the number of inference exercises on which she hears any one individual recite
is greatly reduced. This plan would probably have to be altered in order to
adapt it to particular conditions.

The Socratic method can be employed to great advantage in handling difficult
inferences. The children discuss in class the principle under which an inference
comes, and the teacher guides the discussion, when necessary, by skillfully
placed questions designed to bring the essential problems into relief.1
Footnote 1: At the California State Normal School in San Francisco, this course in
general science is usually preceded by one in "introductory science."

The chapters and sections in this book are not of even length. In order to
preserve the unity of subject matter, it was felt desirable to divide the book
according to subjects rather than according to daily lessons. The varying
lengths of recitation periods in different schools, and the adaptation of the
course to individual instruction as well as to class work, also made a division
into lessons impracticable. Each teacher will soon discover about how much
matter her class, if she uses the class method, can take each day. Probably the
average section will require about 2 days to cover; the longest sections, 5 days.
The entire course should easily be covered in one year with recitations of
about 25 minutes daily. Two 50-minute periods a week give a better division of
time and also ought to finish the course in a year. Under the individual system,
the slowest diligent children finish in 7 or 8 school months, working 4 half-
hours weekly. The fastest do it in about one third that time.

Upon receipt of 20 cents, the publishers will send a manual prepared by the
author, containing full instructions as to the organization and equipment of the
laboratory or demonstration desk, complete lists of apparatus and material
needed, and directions for the construction of a chemical laboratory.

The latter is a laboratory course in which the children are turned loose
among all sorts of interesting materials and apparatus,—kaleidoscope,

microscope, electric bell, toy motor, chemicals that effervesce or change
color when put together, soft glass tubing to mold and blow, etc. The
teacher demonstrates various experiments from time to time to show the
children what can be done with these things, but the children are left free
to investigate to their heart's content. There is no teaching in this
introductory course other than brief answers to questions. The astronomy
and geology reading usually accompany the work in introductory science.

ACKNOWLEDGMENTS
To Frederic Burk, president of the San Francisco State Normal School, I am
most under obligation in connection with the preparation of this book. His
ideas inspired it, and his dynamic criticism did much toward shaping it. My
wife, Heluiz Chandler Washburne, gave invaluable help throughout the work,
especially in the present revision of the course. One of my co-workers on the
Normal School faculty, Miss Louise Mohr, rendered much assistance in the
classification and selection of inferences. Miss Beatrice Harper assisted in the
preparation of the tables of supplies and apparatus, published in the manual to
accompany this book. And I wish to thank the children of the Normal School
for their patience and cooperation in posing for the photographs. The
photographs are by Joseph Marron.

CONTENTS
CHAPTER PAGE
1. Gravitation 1
1. A real place where things weigh nothing and where there
1
is no up or down
2. "Water seeks its own level" 6
3. The sea of compressed air in which we live: Air pressure 10
4. Sinking and floating: Displacement 23
5. How things are kept from toppling over: Stability 29
2. Molecular Attraction 36
6. How liquids are absorbed: Capillary attraction 36
7. How things stick to one another: Adhesion 41
8. The force that makes a thing hold together: Cohesion 44
9. Friction 49
3. Conservation of Energy 57
10. Levers 57
11. Inertia 66
12. Centrifugal force 72
13. Action and reaction 77
14. Elasticity 82
4. Heat 88
15. Heat makes things expand 88
16. Cooling from expansion 94
17. Freezing and melting 96
18. Evaporation 100
19. Boiling and condensing 107
20. Conduction of heat and convection 116
5. Radiant Heat and Light 122

21. How heat gets here from the sun; why things glow
122
when they become very hot
22. Reflection 129
23. The bending of light: Refraction 136
24. Focus 142
25. Magnification 150
26. Scattering of light: Diffusion of light 158
27. Color 161
6. Sound 174
28. What sound is 174
29. Echoes 183
30. Pitch 185
7. Magnetism and Electricity 190
31. Magnets; the compass 190
32. Static electricity 196
8. Electricity 203
33. Making electricity flow 203
34. Conduction of electricity 213
35. Complete circuits 219
36. Grounded circuits 225
37. Resistance 229
38. The electric arc 233
39. Short circuits and fuses 240
40. Electromagnets 247
9. Mingling of Molecules 259
41. Solutions and emulsions 259
42. Crystals 265
43. Diffusion 268
44. Clouds, rain, and dew: Humidity 274
45. Softening due to oil or water 290
10. Chemical Change and Energy 293
46. What things are made of: Elements 293

47. Burning: Oxidation 312
48. Chemical change caused by heat 323
49. Chemical change caused by light 326
50. Chemical change caused by electricity 335
51. Chemical change releases energy 340
52. Explosions 342
11. Solution and Chemical Action 349
53. Chemical change helped by solution 349
54. Acids 351
55. Bases 355
56. Neutralization 360
57. Effervescence 365
12. Analysis 370
58. Analysis 370
Appendixes:
A. The Electrical Apparatus 379
B. Construction of the Cigar-box Telegraph 381
Index 383

COMMON SCIENCE

CHAPTER ONE
GRAVITATION

Section 1. A real place where things weigh nothing and where there is no up or
down.

Why is it that the oceans do not flow off the earth?
What is gravity?
What is "down," and what is "up"?

There is a place where nothing has weight; where there is no "up" or "down";
where nothing ever falls; and where, if people were there, they would float
about with their heads pointing in all directions. This is not a fairy tale; every
word of it is scientifically true. If we had some way of flying straight toward
the sun about 160,000 miles, we should really reach this strange place.

Let us pretend that we can do it. Suppose we have built a machine that can fly
far out from the earth through space (of course no one has really ever invented
such a machine). And since the place is far beyond the air that surrounds the
earth, let us imagine that we have fitted out the air-tight cabin of our machine
with plenty of air to breathe, and with food and everything we need for living.
We shall picture it something like the cabin of an ocean steamer. And let us
pretend that we have just arrived at the place where things weigh nothing:

When you try to walk, you glide toward the ceiling of the cabin and do not
stop before your head bumps against it. If you push on the ceiling, you float
back toward the floor. But you cannot tell whether the floor is above or below,
because you have no idea as to which way is up and which way is down.

As a matter of fact there is no up or down. You discover this quickly enough
when you try to pour a glass of water. You do not know where to hold the glass
or where to hold the pitcher. No matter how you hold them, the water will not
pour—point the top of the pitcher toward the ceiling, or the floor, or the wall, it
makes no difference. Finally you have to put your hand into the pitcher and
pull the water out. It comes. Not a drop runs between your fingers—which

way can it run, since there is no down? The big lump of water stays right on
your hand. This surprises you so much that you let go of the pitcher. Never
mind; the pitcher stays poised in mid-air. But how are you going to get a
drink? It does not seem reasonable to try to drink a large lump of water. Yet
when you hold the lump to your lips and suck it you can draw the water into
your mouth, and it is as wet as ever; then you can force it on down to (or rather
toward) your throat with your tongue. Still you have left in your hand a big
piece of water that will not flow off. You throw it away, and it sails through the
air of the cabin in a straight line until it splashes against the wall. It wets the
wall as much as water on the earth would, but it does not run off. It sticks
there, like a splash-shaped piece of clear, colorless gelatin.

Suppose that for the sake of experimenting you have brought an elephant along
on this trip. You can move under him (or over him—anyway between him and
the floor), brace your feet on the floor, and give him a push. (If he happens to
step on your toes while you are doing this, you do not mind in the least,
because he does not weigh anything, you know.) If you push hard enough to
get the elephant started, he rises slowly toward the ceiling. When he objects on
the way, and struggles and kicks and tries to get back to the floor, it does not
help him at all. His bulky, kicking body floats steadily on till it crashes into the
ceiling.

No chairs or beds are needed in this place. You can lie or sit in mid-air, or cling
to a fixture on a wall, resting as gently there as a feather might. There is no
need to set the table for meals—just lay the dishes with the food on them in
space and they stay there. If the top of your cup of chocolate is toward the
ceiling, and your plate of food is turned the other way, no harm is done. Your
feet may happen to point toward the ceiling, while some one else's point
toward the floor, as you sit in mid-air, eating. There is some difficulty in
getting the food on the dishes, so probably you do not wish to bother with
dishes, after all. Do you want some mashed potatoes? All right, here it is—and
the cook jerks the spoon away from the potatoes, leaving them floating before
you, ready to eat.

It is literally a topsy-turvy place.

Do you want to know why all this would happen? Here is the reason: There is
a great force known as gravitation. It is the pull that everything in the universe
has on everything else. The more massive a thing is, the more gravitational

pull it has on other objects; but the farther apart things are, the less pull they
have on each other.

The earth is very massive, and we live right on its surface; so it pulls us
strongly toward it. Therefore we say that we weigh something. And since
every time we roll off a bed, for instance, or jump off a chair, the earth pulls us
swiftly toward it, we say that the earth is down. "Down" simply means toward
the thing that is pulling us. If we were on the surface of the moon, the moon
would pull us. "Down" would then be under our feet or toward the center of
the moon, and the earth would be seen floating up in the sky. For "up" means
away from the thing which is pulling us.

Why water does not flow off the earth. It was because people did not know
about gravitation that they laughed at Columbus when he said the earth was
round. "Why, if the earth were round," they argued, "the water would all flow
off on the other side." They did not know that water flows downhill because
the earth is pulling it toward its center by gravitation, and that it does not make
the slightest difference on which side of the earth water is, since it is still
pulled toward the center.

Why the world does not fall down. And people used to wonder "what held
the earth up." The answer, as you can see, is easy. There simply is no up or
down in space. The earth cannot fall down, because there is no down to fall to.
"Down" merely means toward the earth, and the earth cannot very well fall
toward itself, can it? The sun is pulling on it, though; so the earth could fall
into the sun, and it would do so, if it were not swinging around the sun so fast.
You will see how this keeps it from falling into the sun when you come to the
section on centrifugal force.

Why there is a place where things weigh nothing. Now about the place
where gravitation has no effect. Since an object near the sun is pulled more by
the sun than it is by the earth, and since down here near the earth an object is
pulled harder by the earth than by the sun, it is clear that there must be a place
between the sun and the earth where their pulls just balance; and where the sun
pulls just as hard one way as the earth pulls the other way, things will not fall
either way, but will float. The place where the pulls of the sun and the earth are
equal is not halfway between the earth and the sun, because the sun is so much
larger and pulls so much more powerfully than the earth, that the place where
their pulls balance is much nearer the earth than it is to the sun. As a matter of

fact, it can be easily calculated that this spot is somewhere near 160,000 miles
from the earth.

There are other spots like it between every two stars, and in the center of the
earth, and in the center of every other body. You see, in the center of the earth
there is just as much of the earth pulling one way as there is pulling the other,
so again there is no up or down.

Application 1. Explain why the people on the other side of the earth do
not fall off; why you have weight; why rivers run downhill; why the
world does not fall down.

Section 2. "Water seeks its own level."

Why does a spring bubble up from the ground?
What makes the water come up through the pipe into your
house?
Why is a fire engine needed to pump water up high?

You remember that up where the pull of the earth and the sun balance each
other, water could not flow or flatten out. Let us try to imagine that water, here
on the earth, has lost its habit of flattening out whenever possible—that, like
clay, it keeps whatever shape it is given.

First you notice that the water fails to run out of the faucets. (For in most
places in the world as it really is, the water that comes through faucets is
simply flowing down from some high reservoir.) People all begin to search for
water to drink. They rush to the rivers and begin to dig the water out of them.
It looks queer to see a hole left in the water wherever a person has scooped up
a pailful. If some one slips into the river while getting water, he does not
drown, because the water cannot close in over his head; there is just a deep
hole where he has fallen through, and he breathes the air that comes down to
him at the bottom of the hole. If you try to row on the water, each stroke of the
oars piles up the water, and the boat makes a deep furrow wherever it goes so
that the whole river begins to look like a rough, plowed field.

When the rivers are used up, people search in vain for springs. (No springs
could flow in our everyday world if water did not seek its own level; for the
waters of the springs come from hills or mountains, and the higher water, in

trying to flatten out, forces the lower water up through the ground on the
hillsides or in the valleys.) So people have to get their water from underground
or go to lakes for it. And these lakes are strange sights. Storms toss up huge
waves, which remain as ridges and furrows until another storm tears them
down and throws up new ones.

But with no rivers flowing into them, the lakes also are used up in time. The
only fresh water to be had is what is caught from the rain. Even wells soon
become useless; because as soon as you pump up the water surrounding the
pump, no more water flows in around it; and if you use a bucket to raise the
water, the well goes dry as soon as the supply of water standing in it has been
drawn.

You will understand more about water seeking its own level if you do this
experiment:

Experiment 1. Put one end of a rubber tube over the narrow neck of a
funnel (a glass funnel is best), and put the other end of the tube over a
piece of glass tubing not less than 5 or 6 inches long. Hold up the glass
tube and the funnel, letting the rubber tube sag down between them as in
Figure 1. Now fill the funnel three fourths full of water. Raise the glass
tube higher if the water starts to flow out of it. If no water shows in the
glass tube, lower it until it does. Gradually raise and lower the tube, and
notice how high the water goes in it whenever it is held still.

This same thing would happen with any shape of tube or funnel. You have
another example of it when you fill a teakettle: the water rises in the spout just
as high as it does in the kettle.

Fig. 1. The water in the tube rises
to the level of the water in the
funnel.

Why water flows up into your house. It is because water seeks its own level
that it comes up through the pipes in your house. Usually the water for a city is
pumped into a reservoir that is as high as the highest house in the city. When it
flows down from the reservoir, it tends to rise in any pipe through which it
flows, to the height at which the water in the reservoir stands. If a house is
higher than the surface of the water in the reservoir, of course that house will
get no running water.

Why fire engines are needed to force water high. In putting out a fire, the
firemen often want to throw the water with a good deal of force. The tendency
of the water to seek its own level does not always give a high enough or
powerful enough stream from the fire hose; so a fire engine is used to pump
the water through the hose, and the stream flows with much more force than if
it were not pumped.

Application 2. A. C. Wheeler of Chicago bought a little farm in Indiana,
and had a windmill put up to supply the place with water. But at first he
was not sure where he should put the tank into which the windmill was to
pump the water and from which the water should flow into the kitchen,
bathroom, and barn. The barn was on a knoll, so that its floor was almost
as high as the roof of the house. Which would have been the best place
for the tank: high up on the windmill (which stood on the knoll by the
barn), or the basement of the house, or the attic of the house?

Fig. 2. Where is the best location for the
tank?

Application 3. A man was about
to open a garage in San
Francisco. He had a large oil
tank and wanted a simple way of
telling at a glance how full it
was. One of his workmen
suggested that he attach a long
piece of glass tubing to the side
of the tank, connecting it with an
extra faucet near the bottom of
Fig. 3. When the tank is full, will the the tank. A second workman
oil overflow the top of the tube? said, "No, that won't do. Your
tank holds ever so much more
than the tube would hold, so the

oil in the tank would force the oil up over the top of the tube, even when
the tank was not full." Who was right?

Section 3. The sea of compressed air in which we live: Air pressure.

Does a balloon explode if it goes high in the air?

What is suction?

Why does soda water run up a straw when you draw on the straw?

Why will evaporated milk not flow freely out of a can in which there is
only one hole?

Why does water gurgle when you pour it out of a bottle?

We are living in a sea of compressed air. Every square inch of our bodies has
about 15 pounds of pressure against it. The only reason we are not crushed is
that there is as strong pressure inside of our bodies pushing out as there is
outside pushing in. There is compressed air in the blood and all through the
body. If you were to lie down on the ground and have all the air pumped out
from under you, the air above would crush you as flat as a pancake. You might
as well let a dozen big farm horses trample on you, or let a huge elephant roll
over you, as let the air press down on you if there were no air underneath and
inside your body to resist the pressure from above. It is hard to believe that the
air and liquids in our bodies are pressing out with a force great enough to resist
this crushing weight of air. But if you were suddenly to go up above the earth's
atmosphere, or if you were to stay down here and go into a room from which
the air were to be pumped all at once, your body would explode like a torpedo.

When you suck the air out of a bottle, the surrounding air pressure forces the
bottle against your tongue; if the bottle is a small one, it will stick there. And
the pressure of the air and blood in your tongue will force your tongue down
into the neck of the bottle from which part of the air has been taken.

In the same way, when you force the air out of a rubber suction cap, such as is
used to fasten reading lamps to the head of a bed, the air pressure outside holds
the suction cap tightly to the object against which you first pressed it, making
it stick there.

We can easily experiment with air pressure because we can get almost entirely
rid of it in places and can then watch what happens. A place from which the air
is practically all pumped out is called a vacuum. Here are some interesting
experiments that will show what air pressure does:

Fig. 4. When the point is knocked off the
electric lamp, the water is forced into the
vacuum.

Experiment 2. Hold a burned-out electric lamp in a basin of water, break
its point off, and see what happens.

All the common electric lamps (less than 70 watts) are made with vacuums
inside. The reason for this is that the fine wire would burn up if there were any
air in the lamps. When you knock the point off the globe, it leaves a space into
which the water can be pushed. Since the air is pressing hard on the surface of
the water except in the one place where the vacuum in the lamp globe is, the
water is forced violently into this empty space.

It really is a good deal like the way mud comes up between your toes when
you are barefoot. Your foot is pressing on the mud all around except in the
spaces between your toes, and so the mud is forced up into these spaces. The
air pressure on the water is like your foot on the mud, and the space in the
lamp globe is like the space between your toes. Since wherever there is air it is

pressing hard, the only space into which it can force water or anything else is
into a place from which all the air has been removed, like the inside of the
lamp globe.

The reason that the water does not run out of the globe is this: the hole is too
small to let the air squeeze up past the water, and therefore no air can take the
place of the water that might otherwise run out. In order to flow out, then, the
water would have to leave an empty space or vacuum behind it, and the air
pressure would not allow this.

Why water gurgles when it pours out of a bottle. You have often noticed
that when you pour water out of a bottle it gurgles and gulps instead of flowing
out evenly. The reason for this is that when a little water gets out and leaves an
empty space behind, the air pushing against the water starts to force it back up;
but since the mouth of the bottle is fairly wide, the air itself squeezes past the
water and bubbles up to the top.

Experiment 3. Put a straw or a piece of glass tube down into a glass of
water. Hold your finger tightly over the upper end, and lift the tube out of
the water. Notice how the water stays in the tube. Now remove your
finger from the upper end.

The air holds the water up in the tube because there is no room for it to bubble
up into the tube to take the place of the water; and the water, to flow out of the
tube, would have to leave a vacuum, which the air outside does not allow. But
when you take your finger off the top of the straw or tube, the air from above
takes the place of the water as rapidly as it flows out; so there is no tendency to
form a vacuum, and the water leaves the tube. Now do you see why you make
two holes in the top of a can of evaporated milk when you wish to pour the
milk out evenly?

Fig. 5. The water is held in the tube by air
pressure.

Experiment 4. Push a rubber suction cap firmly against the inside of the
bell jar of an air pump. Try to pull the suction cap off. If it comes off,
press it on again; place the bell jar on the plate of the air pump, and pump
the air out of the jar. What must have been holding the suction cap against
the inside of the jar? Does air press up and sidewise as well as down? Test
this further in the following experiment:

Fig. 6. An air pump.

Experiment 5. Put a cork into an empty bottle. Do not use a new cork,
but one that has been fitted into the bottle many times and has become
shaped to the neck. Press the cork in rather firmly, so that it is air-tight,
but do not jam it in. Set the bottle on the plate of the air pump, put the bell
jar over it, and pump the air out of the jar. What makes the cork fly out of
the bottle? What was really in the "empty" bottle? Why could it not push
the cork out until you had pumped the air out of the jar?

Experiment 6. Wax the rims of the two Magdeburg hemispheres (see Fig.
7). Screw the lower section into the hole in the plate of the air pump. Be
sure that the stop valve in the neck of the hemisphere is open. (The little
handle should be vertical.) Fit the other section on to the first, and pump
out as much air as you can. Close the stop valve. Unscrew the
hemispheres from the air pump. Try to pull them apart—pull straight out,
taking care not to slide the parts. If you wish, let some one else take one
handle, and see if the two of you can pull it apart.

Fig. 7. The experiment with the Magdeburg
hemispheres.

Before you pumped the air out of the hemisphere, the compressed air inside of
them (you remember all the air down here is compressed) was pushing them
apart just as hard as the air outside of them was pushing them together. When
you pumped the air out, however, there was hardly any air left inside of them
to push outward. So the strong pressure of the outside air against the
hemispheres had nothing to oppose it. It therefore pressed them very tightly
together and held them that way.

This experiment was first tried by a man living in Magdeburg, Germany. The
first set of hemispheres he used proved too weak, and when the air in them was
partly pumped out, the pressure of the outside air crushed them like an egg
shell. The second set was over a foot in diameter and much stronger. After he
had pumped the air out, it took sixteen horses, eight pulling one way and eight
the opposite way, to pull the hemispheres apart.

Experiment 7. Fill a bottle (or flask) half full of water. Through a one-
hole stopper that will fit the bottle, put a bent piece of glass tubing that
will reach down to the bottom of the bottle. Set the bottle, thus stoppered,
on the plate of the air pump, with a beaker or tumbler under the outer end
of the glass tube. Put the bell jar over the bottle and glass, and pump the
air out of the jar. What is it that forces the water up and out of the bottle?
Why could it do this when the air was pumped out of the bell jar and not
before?

How a seltzer siphon works. A seltzer siphon works on the same principle.
But instead of the ordinary compressed air that is all around us, there is in the
seltzer siphon a gas (carbon dioxid) which has been much more compressed
than ordinary air. This strongly compressed gas forces the seltzer water out
into the less compressed air, exactly as the compressed air in the upper part of
the bottle forced the water out into the comparative vacuum of the bell jar in
Experiment 7.

Experiment 8. Fill a toy balloon partly full of air by blowing into it, and
close the neck with a rubber band so that no air can escape. Lay a saucer
over the hole in the plate of the air pump, so that the rubber of the balloon
cannot be sucked down the hole. Lay the balloon on top of this saucer, put
the bell jar over it, and pump the air out of the jar. What makes the
balloon expand? What is in it? Why could it not expand before you
pumped the air out from around it?

A toy balloon expands for the same reason when it goes high in the air. Up
there the air pressure is not so strong outside the balloon, and so the gas inside
makes the balloon expand until it bursts.

Fig. 8. A siphon. The air pushes the water
over the side of the pan.

Experiment 9. Lay a rubber tube flat in the bottom of a pan of water, so
that the tube will be filled with water. Let one end stay under water, but
pinch the other end tightly shut with your thumb and finger and lift it out
of the pan. Lower this closed end into a sink or empty pan that is lower
than the pan of water. Now stop pinching the tube shut. This device is
called a siphon (Fig. 8).

Experiment 10. Put the mouth of a small syringe, or better, of a glass
model lift pump, under water. Draw the handle up. Does the water follow
the plunger up, stand still, or go down in the pump?

When you pull up the plunger, you leave an empty space; you shove the air out
of the pump or syringe ahead of the plunger. The air outside, pressing on the
water, forces it up into this empty space from which the air has been pushed.
But air pressure cannot force water up even into a perfect vacuum farther than
about 33 feet. If your glass pump were, say, 40 feet long, the water would
follow the plunger up for a little over 30 feet, but nothing could suck it higher;

for by the time it reaches that height it is pushing down with its own weight as
hard as the air is pressing on the water below. No suction pump, or siphon,
however perfect, will ever lift water more than about 33 feet, and it will do
well if it draws water up 28 or 30 feet. This is because a perfect vacuum cannot
be made. There is always some water vapor formed by the water evaporating a
little, and there is always a small amount of air that has been dissolved in
water, both of which partly fill the space above the water and press down a
little on the water within the pump.

Fig. 9. A glass model suction
pump.

If you had a straw over 33 feet long, and if some one held a glass of lemonade
for you down near the sidewalk while you leaned over from the roof of a three-
story building with your long straw, you could not possibly drink the
lemonade. The air pressure would not be great enough to lift it so high, no
matter how hard you sucked,—that is, no matter how perfect a vacuum you
made in the upper part of the straw. The lemonade would rise part way, and
then your straw would be flattened by the pressure outside.

Some days the air can force water up farther in a tube than it can on other days.
If it can force the water up 33 feet today, it will perhaps be able to force it up
only 30 feet immediately before a storm. And if it forces water up 33 feet at
sea level, it may force it up only 15 or 20 feet on a high mountain, for on a
mountain there is much less air above to make pressure. The pressure of the air
is different in different places; where the air is heavy and pressing hard, we say
the pressure is high; where the air is light and not pressing so hard, we call the
pressure low. A place where the air is heavy is called an area of high pressure;
where it is light, an area of low pressure. (See Section 44.)

What makes winds? It is because the air does not press equally all the time
and everywhere that we have winds. Naturally, if the air is pressing harder in
one place than in another, the lower air will be pushed sidewise in the areas of
high pressure and will rush to the areas where there is less pressure. And air
rushing from one place to another is called wind.

Fig. 10.

Application 4. A man had two water reservoirs, which stood at the same
level, one on each side of a hill. The hill between them was about 50 feet
high. One reservoir was full, and the other was empty. He wanted to get
some of the water from the full reservoir into the empty one. He did not
have a pump to force the water from one to the other, but he did have a
long hose, and could have bought more. His hose was long enough to
reach over the top of the hill, but not long enough to go around it. Could
he have siphoned the water from one reservoir to the other? Would he
have had to buy more hose?

Application 5. Two boys were out hiking and were very thirsty. They
came to a deserted farm and found a deep well; it was about 40 feet down
to the water. They had no pump, but there was a piece of hose about 50

feet long. One boy suggested that they drop one end of the hose down to
the water and suck the water up, but the other said that that would not
work—the only way would be to lower the hose into the water, close the
upper end, pull the hose out and let the water pour out of the lower end of
the hose into their mouths. A stranger came past while the boys were
arguing, and said that neither way would work; that although the hose was
long enough, the water was too far down to be raised in either way. He
advised the boys to find a bucket and to use the hose as a rope for
lowering it. Who was right?

Inference Exercise

Explanatory Note. In the inference exercises in this book, there is a
group of facts for you to explain. They can always be explained by one or
more of the principles studied, like gravitation, water seeking its own
level, or air pressure. If asked to explain why sucking through a straw
makes soda water come up into your mouth, for instance, you should not
merely say "air pressure," but should tell why you think it is air pressure
that causes the liquid to rise through the straw. The answer should be
something like this: "The soda water comes up into your mouth because
the sucking takes the air pressure away from the top of the soda water that
is in the straw. This leaves the air pressing down only on the surface of
the soda water in the glass. Therefore, the air pressure pushes the soda
water up into the straw and into your mouth where the pressure has been
removed by sucking." Sometimes, when you have shown that you
understand the principles very well, the teacher may let you take a short
cut and just name the principle, but this will be done only after you have
proved by a number of full answers that you thoroughly understand each
principle named.

Some of the following facts are accounted for by air pressure; some by
water seeking its own level; others by gravitation. See if you can tell
which of the three principles explains each fact:

1. Rain falls from the clouds.

2. After rain has soaked into the sides of mountains it runs underground
and rises, at lower levels, in springs.

3. When there are no springs near, people raise the water from
underground with suction pumps.

4. As fast as the water is pumped away from around the bottom of a
pump, more water flows in to replace it.

5. After you pump water up, it flows down into your pail from the spout
of the pump.

6. You can drink lemonade through a straw.

7. If a lemon seed sticks to the bottom of your straw, the straw flattens out
when you suck.

8. When you pull your straw out to remove the seed, there is no hole left
in the lemonade; it closes right in after the straw.

9. If you drop the seed, it falls to the floor.

10. If you tip the glass to drink the lemonade, the surface of the lemonade
does not tip with the glass, but remains horizontal.

Section 4. Sinking and floating: Displacement.

What keeps a balloon up?

What makes an iceberg float?

Why does cork float on the water and why do heavier substances sink?

If iron sinks, why do iron ships not sink?

Again let us imagine ourselves up in the place where gravitation has no effect.
Suppose we lay a nail on the surface of a bowl of water. It stays there and does
not sink. This does not seem at all surprising, of course, since the nail no
longer has weight. But when we put a cork in the midst of the water, it stays
there instead of floating to the surface. This seems peculiar, because the less a
thing weighs the more easily it floats. So when the cork weighs nothing at all,
it seems that it should float better than ever. Of course there is some difficulty
in deciding whether it ought to float toward the part of the water nearest the

floor or toward the part nearest the ceiling, since there is no up or down; but
one would think that it ought somehow to get to the outside of the water and
not stay exactly in the middle. If put on the outside, however, it stays there as
well.

A toy balloon, in the same way, will not go toward either the ceiling or the
floor, but just stays where it is put, no matter how light a gas it is filled with.

The explanation is as follows: For an object to float on the water or in the air,
the water or air must be heavier than the object. It is the water or air being
pulled under the object by gravity, that pushes it up. Therefore, if the air and
water themselves weighed nothing, of course they would be no heavier than
the balloon or the cork; the air or water would then not be pulled in under the
balloon or cork by gravity, and so would not push them up, or aside.

Fig. 11. The battleship is made of steel, yet it does not sink.

Why iron ships float. When people first talked about building iron ships,
others laughed at them. "Iron sinks," they said, "and your boats will go to the
bottom of the sea." If the boats were solid iron this would be true, for iron is
certainly much heavier than water. But if the iron is bent up at the edges,—as it
is in a dish pan,—it has to push much more water aside before it goes under
than it would if it were flattened out. The water displaced, or pushed aside,
would have to take up as much room as was taken up by the pan and all the

empty space inside of it, before the edge would go under. Naturally this
amount of water would weigh a great deal more than the empty pan.

But suppose you should fill the dish pan with water, or suppose it leaked full.
Then you would have the weight of all the water in it added to the weight of
the pan, and that would be heavy enough to push aside the water in which it
was floating and let the pan sink. This is why a ship sometimes sinks when it
springs a leak.

You may be able to see more clearly why an iron ship floats by this example:
Suppose your iron ship weighs 6000 tons and that the cargo and crew weigh
another 1000 tons. The whole thing, then, weighs 7000 tons. Now that ship is a
big, bulky affair and takes up more space than 7000 tons of water does. As it
settles into the water it pushes a great deal of water out of the way, and after it
sinks a certain distance it has pushed 7000 tons of water out of the way. Since
the ship weighs only 7000 tons, it evidently cannot push aside more than that
weight of water; so part of the ship stays above the water, and all there is left
for it to do is to float. If the ship should freeze solid in the water where it
floated and then could be lifted out of the ice by a huge derrick, you would
find that you could pour exactly 7000 tons of water into the hole where the
ship had been.

But if you built your ship with so little air space in it that it took less room than
7000 tons of water takes, it could go clear under the water without pushing
7000 tons of water aside. Therefore a ship of this kind would sink.

The earth's gravity is pulling on the ship and on the water. If the ship has
displaced (pushed aside) its own weight of water, gravity is pulling down on
the water as hard as it is on the ship; so the ship cannot push any more water
aside, and if there is enough air space in it, the ship floats.

Perhaps the easiest way to say it is like this: Anything that is lighter than the
same volume of water will float; since a cubic foot of wood weighs less than a
cubic foot of water, the wood will float; since a quart of oil is lighter than a
quart of water, the oil will float; since a pint of cream is lighter than a pint of
milk, the cream will rise. In the same way, anything that is lighter than the
same volume of air will be pushed up by the air. When a balloon with its
passengers weighs less than the amount of air that it takes the place of at any

one time, it will go up. Since a quart of warm air weighs less than a quart of
cold air, the warm air will rise.

You can see how a heavy substance like water pushes a lighter one, like oil, up
out of its way, in the following experiment:

Experiment 11. Fill one test tube to the brim with kerosene slightly
colored with a little iodine. Fill another test tube to the brim with water,
colored with a little blueing. Put a small square of cardboard over the test
tube of water, hold it in place, and turn the test tube upside down. You can
let go of the cardboard now, as the air pressure will hold it up. Put the
mouth of the test tube of water exactly over the mouth of the test tube of
kerosene. Pull the cardboard out from between the two tubes, or have
some one else do this while you hold the two tubes mouth to mouth. If
you are careful, you will not spill a drop. If nothing happens when the
cardboard is pulled away, gently rock the two tubes, holding their mouths
tightly together.

Fig. 12. The upper tube is filled with water and the lower with oil. What
will happen when she pulls the cardboard out?

Oil is lighter than water, as you know, because you have seen a film of oil
floating on water. When you have the two test tubes in such a position that the
oil and water can change, the water is pulled down under the kerosene because

gravity is pulling harder on the water than it is pulling on the kerosene. The
water, therefore, goes to the bottom and this forces the kerosene up.

Application 6. Three men were making a raft. For floats they meant to use
some air-tight galvanized iron cylinders. One of them wanted to fill the
cylinders with cork, "because," he said, "cork is what you put in life
preservers and it floats better than anything I know of." "They'd be better
with nothing in them at all," said a second. "Pump all the air out and leave
vacuums. They're air-tight and they are strong enough to resist the air
pressure." But the third man said, "Why, you've got to have some air in
them to buoy them up. Cork would be all right, but it isn't as light as air;
so air would be the best thing to fill them with."

Which way would the floats have worked best?

Application 7. A little girl was telling her class about icebergs. "They are
very dangerous," she said, "and ships are often wrecked by running into
them. You see, the sun melts the top off them so that all there is left is
under water. The sailors can't see the ice under water, and so their ships
run into it and are sunk." Another girl objected to this; she said, "That
couldn't be; the ice would bob up as fast as the top melted." "No, it
wouldn't," said a boy. "If that lower part wasn't heavier than water, it
never would have stayed under at all. And if it was heavier at the
beginning, it would still be heavier after the top melted off."

Who was right?

Inference Exercise

Explain the following:

11. When you wash dishes, a cup often floats on top of the water, while a
plate made of the same sort of china sinks to the bottom of the pan.

12. If you put the cup in sidewise, it sinks.

13. The water in the cup, when lying on its side, is exactly as high as the
water in the dish pan.

14. If you put a glass into the water, mouth first, the water cannot get up
into the glass; if you tip it a little, there are bubbles in the water and some
water enters the glass.

15. If you let a dish slip while you are wiping it, it crashes to the floor.

16. It is much harder to hold a large platter while you are wiping it than it
is to hold a small butter plate.

17. If you set a hot glass upside down on the oilcloth table cover, the
oilcloth bulges up into it when the hot air and steam shrink and leave a
partial vacuum within the glass.

18. If you spill any of the dishwater on the floor, it flattens out.

19. You may use a kind of soap that is full of invisible little air bubbles; if
you do, the soap will float on top of the water.

20. When you drop a dry dishcloth into water, it floats until all the pores
are filled with water; then it sinks.

Section 5. How things are kept from toppling over: Stability.

Why is it harder to keep your balance on stilts than on your feet?

Why does a rowboat tip over more easily if you stand up in it?

In Pisa, Italy, there is a beautiful marble bell tower which leans over as if it
were just about to fall to the ground. Yet it has stood in this position for
hundreds of years and has never given a sign of toppling. The foundations on
which it rested sank down into the ground on one side while the tower was
being built (it took over 200 years to build it), and this made it tip. But the men
who were building it evidently felt sure that it would not fall over in spite of its
tipping. They knew the law of stability.

All architects and engineers and builders have to take this law into
consideration or the structures they put up would topple over. And your body
learned the law when you were a little over a year old, or you never could have
walked. It is worth while for your brain to know it, too, because it is a very
practical law that you can use in your everyday life.

If you wish to understand why the Leaning Tower
of Pisa does not fall over, why it is hard to walk on
stilts, why a boat tips when a person stands up in it,
why blocks fall when you build too high with
them, and how to keep things from tipping over, do
the following experiment and read the explanation
that follows it:

Experiment 12.2 Unscrew the
bell from a doorbell or a
telephone. You will not harm it
at all, and you can put it back
after the experiment. Cut a
sheet of heavy wrapping paper
or light-weight cardboard about
5 × 9 inches. Roll this so as to
make a cylinder about 5 inches
high and as big around as the
Fig. 14. bell. Hold it in shape by pasting Fig. 13. The Leaning
it or putting a couple of rubber Tower of Pisa.
bands around it. Cut two strips
of paper about an inch wide and 8 inches long;
lay these crosswise; lay the bell, round side down, on the center of the
cross. Push a paper fastener through the hole in the bell (the kind shown
in Figure 14) and through the crossed pieces of paper, spreading the
fastener out so as to fasten the paper cross to the rounded side of the bell.
Bend the arms of the cross up around the bell and paste them to the sides
of the paper cylinder so that the bell makes a curved bottom to the
cylinder, as shown in Figure 15.
Footnote 2: To the Teacher. If you have a laboratory, it is well to have this
cylinder already made for the use of all classes.

Fig. 15. In this cylinder the center of weight is so high that it is not over
the bottom if the cylinder is tipped to any extent. So the cylinder falls over
easily and lies quietly on its side.

Fig. 16. But in this one the center of weight is so low that it is over the
base, no matter what position the cylinder is in.

Try to tip the cylinder over. Now stuff some crumpled paper loosely into the
cylinder, filling it to the top. Tip the cylinder again. Will it stay on its side
now? Force all the crumpled paper to the bottom of the cylinder. Now will it
stay on its side? Take out the crumpled paper and lay a flat stone in the bottom
of the bell, holding it in place by stuffing some crumpled paper in on top of it.

Will the cylinder tip over now? Take the
stone out, put the crumpled paper in the
bottom of the cylinder, put the stone on top
of the paper, and again try to tip the
cylinder over. Will it fall?

Fig. 17. So even if the
cylinder is laid on its side it
immediately comes to an
upright position again.

The center of the cylinder was always in one place, of course. But the center of
the weight in that cylinder was usually near the bottom, because the bell
weighed so much more than the paper. When you raised the center of weight
by putting the stone up high or filling the cylinder with crumpled paper, just a
little tipping moved the center of weight so that it was not directly over the bell
on which the cylinder was resting. Whenever the center of weight is not over
the base of support (the bottom on which the thing is standing), an object will
topple over. Moving the center of weight up (Figs. 15 and 16) makes an object
less stable.

The two main points to remember about stability are these: the wider the base
of an object, the harder it is to tip over; and the lower the center of the weight
is, the harder it is to tip over.

If you were out in a rowboat in a storm, would it be better to sit up straight in
the seat or to lie in the bottom of the boat?

Why is a flat-bottomed boat safer than a canoe?

Fig. 18. Which vase would be the hardest to upset?

Where do you suppose the center of weight of the Leaning Tower of Pisa is,—
near the bottom or near the top?

Application 8. If you had a large flower to put into a vase and you did not
want it to tip over easily, which of the three vases shown in Figure 18
would you choose?

Application 9. Some boys made themselves a little sail-boat and went
sailing in it. A storm came up. The boat rocked badly and was in danger
of tipping over. "Throw out all the heavy things, quick!" shouted one.
"No, no, don't for the life of you do it!" called another. "Chop down the
mast—here, give me the hatchet!" another one said. "Crouch way down—
lie on the bottom." "No, keep moving over to the side that is tipped up!"
"Hold the things in the bottom of the boat still, so they'll not keep rolling
from side to side." "Jump out and swim!" Every one was shouting at
once. Which parts of the advice should you have followed if you had been
on board?

Inference Exercise

Explain the following:

21. A ship when it goes to sea always carries ballast (weight) in its
bottom.

22. If the ship springs a leak below the water line, the water rushes in.

23. The ship's pumps suck the water up out of the bottom of the ship.

24. The water pours back into the sea from the mouths of the pumps.

25. As the sailors move back and forth on the ship during a storm, they
walk with their legs spread far apart.

26. Although the ship tips far from side to side, it rights itself.

27. However far the ship tips, the surface of the water in the bottom stays
almost horizontal.

28. While the ship is in danger, the people put on life preservers, which
are filled with cork.

29. When the ship rocks violently, people who are standing up are thrown
to the floor, but those who are sitting down do not fall over.

30. If the ship fills with water faster than the engines can pump it out, the
ship sinks.

CHAPTER TWO
MOLECULAR ATTRACTION

SECTION 6. How liquids are absorbed: Capillary attraction.

Why do blotters pull water into themselves when a flat piece of glass will
not?

How does a towel dry your face?

Suppose you could turn off nature's laws in the way that you can turn off
electric lights. And suppose you stood in front of a switchboard with each
switch labeled with the name of the law it would shut off. Of course, there is
no such switchboard, but we know pretty well what would happen if we could
shut off various laws. One of the least dangerous-looking switches would be
one labeled Capillary Attraction. And now, just for fun, suppose that you
have turned that switch off in order to see the effect.

At first you do not notice any change; but after a while you begin to feel
perspiration collecting all over your body as if your clothes were made of
rubber sheeting. Soon this becomes so uncomfortable that you decide to take a
bath. But when you put your wash cloth into the water you find that it will not
absorb any water at all; it gets a little wet on the outside, but remains stiff and
is not easy or pleasant to use. You reach for a sponge or a bath brush, but you
are no better off. Only the outside of the sponge and brush becomes wet, and
they remain for the most part harsh and dry.

Then perhaps you try to dry yourself with a towel. But that does not work; not
a drop of water will the towel absorb. You might as well try to dry yourself on
the glossy side of a piece of oilcloth.

By this time you are shivering; so you probably decide to light the oil stove
and get warm and dry over that. But the oil will not come up the wick! As a
last resort you throw a dressing gown around you (it does not get wet) and start
a fire in the fireplace. This at last warms and dries you; but as soon as you are

dressed the clammy feeling comes again—your clothes will not absorb any
perspiration. While the capillary attraction switch is turned off you will simply
have to get used to this.

Then suppose you start to write your experience. Your fountain pen will not
work. Even an ordinary pen does not work as well as it ought to. It makes a
blot on your paper. If you use the blotter you are dismayed to find that the blot
spreads out as flat as if you were pressing a piece of glass against it. You take
your eraser and try to remove the blot. To your delight you find that it rubs out
as easily as a pencil mark. The ink has not soaked into the paper at all. You
begin to see some of the advantages in shutting off capillary attraction.

Perhaps you are writing at the dining-room table, and you overturn the inkwell
on the tablecloth. Never mind, it is no trouble to brush the ink off. Not a sign
of stain is left behind.

By and by you look outdoors at the garden. Everything is withering. The
moisture does not move through the earth to where the roots of the plants can
reach it. Before everything withers completely, you rush to the switchboard
and turn on the capillary attraction again.

You can understand this force of capillary attraction better if you perform the
following experiments:

Experiment 13. Fill a glass with water and color it with a little blueing or
red ink. Into the glass put two or three glass tubes, open at both ends, and
with bores of different sizes. (One of these tubes should be so-called
thermometer tubing, with about 1 mm. bore.) Watch the colored water and
see in which of the tubes it is pulled highest.

Experiment 14. Put a clean washed lamp wick into the glass of colored
water and watch to see if the water is pulled up the wick. Now let the
upper end of the wick hang over the side of the glass all night. Put an
empty glass under the end that is hanging out. The next morning see what
has happened.

Fig. 19. Will the water be drawn up higher in the fine glass tube or in a
tube with a larger opening?

Fig. 20. The water rises through the lamp wick by capillary attraction.

The space between the threads of the wick, and especially the still finer spaces
between the fibers that make up the threads, act like fine tubes and the liquid
rises in them just as it did in the fine glass tube. Wherever there are fine spaces
between the particles of anything, as there are in a lump of sugar, a towel, a

blotter, a wick, and hundreds of other things, these spaces act like fine tubes
and the liquid goes into them. The force that causes the liquid to move along
fine tubes or openings is called capillary attraction.

Capillary attraction—this tendency of liquids to go into fine tubes—is caused
by the same force that makes things cling to each other (adhesion), and that
makes things hold together (cohesion). The next two sections tell about these
two forces; so you will understand the cause of capillary attraction more
thoroughly after reading them. But you should know capillary attraction when
you see it now, and know how to use it. The following questions will show
whether or not you do:

Application 10. Suppose you have spilled some milk on a carpet, and that
you have at hand wet tea leaves, dry corn meal, some torn bits of a glossy
magazine cover, and a piece of new cloth the pores of which are stopped
up with starch. Which would be the best to use in taking up the milk?

Application 11. A boy spattered some candle grease on his coat. His aunt
told him to lay a blotter on the candle grease and to press a hot iron on the
blotter, or to put the blotter under his coat and the iron on top of the
candle grease,—he was not quite sure which. While he was trying to
recall his aunt's directions, his sister said that he could use soap and water
to take the grease out; then his brother told him to scrape the spot with a
knife. Which would have been the right thing for him to do?

Inference Exercise

Explain the following:

31. A pen has a slit running down to the point.

32. When a man smokes, the smoke goes from the cigar into his mouth.

33. A blotter which has one end in water soon becomes wet all over.

34. Cream comes to the top of milk.

35. It is much harder to stand on stilts than on your feet.

36. Oiled shoes are almost waterproof.

37. City water reservoirs are located on the highest possible places in or
near cities.

38. You can fill a self-filling fountain pen by squeezing the bulb, then
letting go.

39. The oceans do not flow off the world.

40. When you turn a bottle of water upside down the water gurgles out
instead of coming out in a smooth, steady stream.

Section 7. How things stick to one another: Adhesion.

Why is it that when a thing is broken it will not stay together without
glue?

Why does chalk stay on the blackboard?

Now that you have found out something about capillary attraction, suppose
that you should go to the imaginary switchboard again and tamper with some
other law of nature. An innocent-looking switch, right above the capillary
attraction switch, would be labeled Adhesion. Suppose you have turned it off:

In an instant the wall paper slips down from the walls and crumples to a heap
on the floor. The paint and varnish drop from the woodwork like so much
sand. Every cobweb and speck of dust rolls off and falls in a little black heap
below.

When you try to wash, you cannot wet your hands. But they do not need
washing, as the dirt tumbles off, leaving them cleaner than they ever were
before. You can jump into a tank of water with all your clothes on and come
out as dry as you went in. You discover by the dryness of your clothes that
capillary attraction stopped when the adhesion was turned off, for capillary
attraction is just a part of adhesion. But you are not troubled now with the
clamminess of unabsorbed perspiration. The perspiration rolls off in little
drops, not wetting anything but running to the ground like so much quicksilver.

Your hair is fluffier than after the most vigorous shampoo. Your skin smarts
with dryness. Your eyes are almost blinded by their lack of tears. Even when
you cry, the tears roll from your eyeballs and eyelids like water from a duck's
back. Your mouth is too dry to talk; all the saliva rolls down your throat,
leaving your tongue and cheeks as dry as cornstarch.

I think you would soon turn on the adhesion switch again.

Experiment 15. Touch the surface of a glass of water, and then raise your
finger slightly. Notice whether the water tends to follow or to keep away
from your finger as you raise it. Now dip your whole finger into the water
and draw it out. Notice how the water clings, and watch the drops form
and fall off. Notice the film of water that stays on, wetting your finger,
after all dropping stops.

Fig. 21. As the finger is raised the water is drawn up after it.

Which do you think is the stronger, the pull of gravity which makes some of
the water drip off, or the pull of adhesion which makes some of the water cling
to your finger?

If the pull of gravity is stronger, would not all the water drop off, leaving your
finger dry? If the pull of adhesion is the stronger, would not all the water stay
on your finger, none dropping off?

The truth of the matter is that gravity is stronger than adhesion unless things
are very close together; then adhesion is stronger. The part of the water that is
very close to your finger clings to it in spite of gravity; the part that is farther
away forms drops and falls down because of the pull of gravity.

Adhesion, then, is the force that makes things cling to each other when they
are very close together.

Why it is easier to turn a page if you wet your finger. Water spreads out on
things so that it gets very close to them. The thin film of water on your finger
is close enough to your finger and to the page which you are turning to cling to
both; so when you move your finger, the page moves along with it.

Why dust clings to the ceiling and walls. The fine particles of dust are wafted
up against the ceiling and walls by the moving air in the room. They are so
small that they can fit into the small dents that are in plaster and paper and can
get very close to the wall. Once they get close enough, the force of adhesion
holds them with a pull stronger than that of gravity.

Oily and wet surfaces catch dust much more readily than clean, dry ones,
simply because the dust can get so much closer to the oil or water film and
because this film flows partly around each dust particle and holds it by the
force of adhesion. This is why your face gets much dirtier when it is perspiring
than when it is dry.

Application 12. Explain why cobwebs do not fall from the ceiling; why
dust clings to a wet broom; why a postage stamp does not fall off an
envelope.

Inference Exercise

Explain the following:

41. There are no springs on the tops of high mountains.

42. People used to shake sand over their letters after writing them in ink.

43. People used to make night lights for bedrooms by pouring some oil
into a cup of water and floating a piece of wick on the oil. The oil always

stayed on top of the water, and went up through the wick fast enough to
keep the light burning.

44. Your face becomes much dirtier when you are perspiring.

45. Ink bottles are usually made with wide bases.

46. When you spill water on the floor, you cannot wipe it up with
wrapping paper, but you can dry it easily with a cloth.

47. Oiled mops are used in taking up dust.

48. Cake will stick to a pan unless the pan is greased.

49. Although the earth turns completely over every day, we never fall off
it.

50. Signs are fastened sometimes to windows or to the wind shields of
automobiles by little rubber "suction caps."

Section 8. The force that makes a thing hold together: Cohesion.

What makes rain fall in drops?

Why are diamonds hard?

You have not yet touched any of the most dangerous switches on the imaginary
switchboard of universal laws. But if your experience in turning off the
capillary attraction and adhesion switches did not discourage you, you might
try turning off the one beside them labeled Cohesion:

Fig. 22. El Capitan, Yosemite Valley, California. If the force of cohesion
were suspended, a mountain like this would immediately become the fines
dust.

Things happen too swiftly for you to know much about them. The house you
are in falls to dust instantly. You fall through the place where the floor has
been; but you do not bump on the cement basement floor below, partly because
there is no such thing as a hard floor or even hard ground anywhere, and partly
because you disintegrate—fall to pieces—so completely that there is nothing
left of you but a grayish film of fine dust and a haze of warm water.

With a deafening roar, rocks, skyscrapers, and even mountains tumble down,
fall to pieces, and sink into an inconceivably fine dust. Nothing stands up in
the world—not a tree, not an animal, not an island. With a wild rush the oceans
flood in over the dust that has been nations and continents, and then this dust
turns to a fine muddy ooze in the bottom of a worldwide sea.

But it is an ocean utterly different from what we have in the real world. There
are no waves. Neither are there any reflections of clouds in its surface,—first
because the clouds would fly to pieces and turn to invisible vapor, and second,

because the ocean has no surface—it simply melts away into the air and no one
can tell where the water stops and where the air begins.

Then the earth grows larger and larger. The ocean turns to a heavy, dense,
transparent steam. The fine mud that used to be rocks and mountains and
living things turns to a heavy, dense gas.

Our once beautiful, solid, warm, living earth now whirls on through space, a
swollen, gaseous globe, utterly dead.

And the only thing that prevents all this from actually happening right now is
that there is a force called cohesion that holds things together. It is the pull
which one particle of anything has on another particle of the same material.
The paper in this book, the chair on which you are sitting, and you yourself are
all made of a vast number of unthinkably small particles called molecules,
each of which is pulling on its neighbor with such force that all stay in their
places. Substances in which they pull the hardest, like steel, are very hard to
break in two; that is, it is difficult to pull the molecules of these substances
apart. In liquids, such as water, the molecules do not pull nearly so hard on
each other. In a gas, such as air, they are so far apart that they have practically
no pull on each other at all. That is why everything would turn to a gas if the
force of cohesion stopped. Why things would turn cold will be explained in
Chapter 4.

Cohesion, adhesion, and capillary attraction, all are the result of the pull of
molecules on each other. The difference is that capillary attraction is the
pulling of particles of liquids up into fine spaces, as when a lamp wick draws
up oil; adhesion is the pull of the particles of one substance or thing on the
particles of another when they are very close together, as when water clings to
your hand or when dust sticks to the ceiling; while cohesion is the clinging
together of the particles of the same substance, like the holding together of the
particles of your chair or of this paper.

When you put your hand into water it gets wet because the adhesion of the
water to your hand is stronger than the cohesion of the water itself. The
particles of the water are drawn to your hand more powerfully than they are
drawn to each other. But in the following experiment, you have an example of
cases where cohesion is stronger than adhesion:

Experiment 16. Pour some mercury (quicksilver) into a small dish and
dip your finger into it. As you raise your finger, see if the mercury follows
it up as the water did in Experiment 14. When you pull your finger all the
way out, has the mercury wet it at all? Put a lamp wick or a part of your
handkerchief into the mercury. Does it draw the mercury up as it would
draw up water?

Fig. 23. The mercury does not wet the finger, and as the finger is lifted the
mercury does not follow it.

The reason for this peculiarity of mercury is that the pull between the particles
of mercury themselves is stronger than the pull between them and your finger
or handkerchief. In scientific language, the cohesion of the mercury is stronger
than its adhesion to your finger or handkerchief. Although this seems unusual
for a liquid, it is what we naturally expect of solid things; you would be
amazed if part of the wood of your school seat stuck to you when you got up,
for you expect the particles in solid things to cohere—to have cohesion—much
more strongly than they adhere to something else. It is because solids have
such strong cohesion that they are solids.

Application 13. Explain why mercury cannot wet your fingers; why rain
falls in drops; why it is harder to drive a nail into wood than into soap;
why steel is hard.

Inference Exercise

Explain the following:

51. Ink spilled on a plain board soaks in, but on a varnished desk it can be
easily wiped off.

52. When a window is soiled you can write on it with your finger; then
your finger becomes soiled.

53. A starched apron or shirt stays clean longer than an unstarched one.

54. When you hold a lump of sugar with one edge just touching the
surface of a cup of coffee, the coffee runs up the lump.

55. A drop of water on a dry plate is not flat but rounded.

56. It is hard to write on cloth because the ink spreads out and blurs.

57. If you roughen your finger nails by cleaning them with a knife, they
will get soiled much more quickly than if you keep them smooth by using
an orange stick.

58. When you dip your pen in the ink and then move it across the paper, it
makes ink marks on the paper.

59. If you suck the air out of a bottle, the bottle will stick to your tongue.

60. You cannot break a thick piece of iron with your hands.

Section 9. Friction.

What makes ice slippery?

How does a brake stop a car?

Why do things wear out?

It would not be such a calamity if we were to turn off friction from the world.
Still, I doubt whether we should want to leave it off much longer than was
necessary for us to see what would happen. Suppose we imagine the world
with all friction removed:

A man on a bicycle can coast forever along level ground. Ships at sea can shut
off steam and coast clear across the ocean. No machinery needs oiling. The
clothes on your body feel smoother and softer than the finest silk. Perpetual
motion is an established fact instead of an absolute impossibility; everything
that is not going against gravity will keep right on moving forever or until it
bumps into something else.

But, if there is no friction and you want to stop, you cannot. Suppose you are
in an automobile when all friction stops. You speed along helplessly in the
direction you are going. You cannot steer the machine—your hands would slip
right around on the steering wheel, and even if you turn it by grasping the
spoke, your machine still skids straight forward. If you start to go up a hill, you
slow down, stop, and then before you can get out of the machine you start
backward down the hill again and keep on going backward until you smash
into something.

A person on foot does not fare much better. If he is walking at the time friction
ceases, the ground is suddenly so slippery that he falls down and slides along
on his back or stomach in the same direction he was walking, until he bumps
into something big or starts to slip up a slope. If he reaches a slope, he, like the
automobile, stops an instant a little way up, then starts sliding helplessly
backward.

Another man is standing still when the friction is turned off. He cannot get
anywhere. As soon as he starts to walk forward, his feet slip out from under
him and he falls on his face. He lies in the same spot no matter how he
wriggles and squirms. If he tries to push with his hands, they slip over the
rough ground more easily than they now slip through air. He cannot push
sideways enough even to turn over. If there happens to be a rope within reach
and one end is tied to a tree, he might try to take hold of the rope to pull
himself along. But no matter how tightly he squeezes, the rope slips right
through his hands when he starts to pull. If, however, there is a loop in the
rope, he can slip his hand through the loop and try to pull. But the knots with
which the rope is tied immediately come untied and he is as helpless as ever.

Even if he takes hold of a board fence he is no more successful. The nails in
the board slip out of their holes and he is left with a perfectly slippery and
useless board on the ground beside him for a companion. As it grows cold
toward evening he may take some matches out of his pocket and try to start a

fire. Aside from the difficulty of his being unable to hold them except by the
most careful balancing or by shutting them up within his slippery hands, he is
entirely incapable of lighting them; they slip over the cement beneath him or
over the sole of his shoe without the least rubbing.

In the real world, however, it is fortunately as impossible to get away from
friction as it is to get away from the other laws we have tried to imagine as
being turned off. There is always some friction, or rubbing, whenever anything
moves. A bird rubs against the air, the point of a spinning top rubs against the
sidewalk on which it is spinning. Your shoes rub against the ground as you
walk and so make it possible for you to push yourself forward. The drive
wheels of machinery rub against the belts and pull them along. There is
friction between the wheels of a car and the track they are pushing against, or
the wheels would whirl around and around uselessly.

Fig. 24. Hockey is a fast game because there is little friction between the
skates and the ice.

But we can increase or decrease friction a great deal. If we make things rough,
there is more friction between them than if they are smooth. If we press things
tightly together, there is more friction than if they touch lightly. A nail in a
loose hole comes out easily, but in a tight hole it sticks; the pressure has
increased the friction. A motorman in starting a trolley car sometimes finds the
track so smooth that the wheels whirl around without pushing the car forward;

he pours some sand on the track to make it rougher, and the car starts. When
you put on new shoes, they are so smooth on the bottom that they slip over the
ground because of the lack of friction. If you scratch the soles, they are
rougher and you no longer slip. If you try to pull a stake out of the ground, you
have to squeeze it harder than the ground does or it will slip out of your hands
instead of slipping out of the ground. When you apply a brake to an
automobile, the brake must press tightly against the axle or wheel to cause
enough friction to stop the automobile.

There are always two results of friction: heat and wear. Sometimes these
effects of friction are helpful to us, and sometimes they are quite the opposite.
The heat from friction is helpful when it makes it possible for us to light a fire,
but it is far from helpful when it causes a hot box because of an ungreased
wheel on a train or wagon, or burns your hands when you slide down a rope.
The wear from friction is helpful when it makes it possible to sandpaper a
table, scour a pan, scrub a floor, or erase a pencil mark; but we don't like it
when it wears out automobile tires, all the parts of machinery, and our clothes.

Experiment 17. Hold a nail against a grindstone while you turn the stone.
Notice both the wear and heat. Let the nail rest lightly on the stone part of
the time and press hard part of the time. Which way does the nail get
hotter? Which way does it wear off more quickly? Run it over a pane of
glass and see if it gets as hot as it does on the grindstone; if it wears down
as quickly.

Why we oil machinery. We can decrease friction by keeping objects from
pressing tightly against each other, and by making their surfaces smooth. The
most common way of making surfaces smooth is by oiling or greasing them. A
film of oil or grease makes things so smooth and slippery that there is very
little friction. That is why all kinds of machinery will run so smoothly if they
are kept oiled. And since the oil decreases friction, it decreases the wear
caused by friction. So well-oiled machines last much longer than machines that
are not sufficiently oiled.

Fig. 25. The friction of the stone heats the nail and wears it away.

Why ball bearings are used. There is much less friction when a round object
rolls over a surface than when two surfaces slide over one another, unless the
sliding surfaces are very smooth; think how much easier it is to pull a wagon
forward than it would be to take hold of the wheels and pull the wagon
sidewise. So when you want the least possible friction in a machine you use
ball bearings. The bearings are located in the hub of a wheel. Then, instead of
the axle rubbing against the hub, the bearings roll inside of the hub. This
causes very little friction; and the friction is made still less by keeping the
bearings oiled.

Application 14. Suppose you were making a bicycle,—in which of the
following places would you want to increase the friction, and in which
would you want to decrease it? Handle grips, axles, pedals, tires, pedal
cranks, the sockets in which the handle bar turns, the nuts that hold the
parts together.

Application 15. A small boy decided to surprise his mother by oiling her
sewing-machine. He put oil in the following places:

On the treadle, on the large wheel over which the belt runs, on the axle of
the same wheel, on the groove in the little wheel up above where the belt
runs, on the joint where the needle runs up and down, on the little rough
place under the needle that pushes the cloth forward. Which of these did
he do well to oil and which should he have let alone?

Inference Exercise

Explain the following:

61. Rivers flow north as well as south, although we usually speak of north
as "up north."

62. Tartar and bits of food stick to your teeth.

63. Brushing your teeth with tooth powder cleans them.

64. When a chair has gliders (smooth metal caps) on its feet, it slides
easily across the floor.

65. When you wet your finger, you can turn a page more easily.

66. A lamp wick draws oil up from the lower part of a lamp to the burner.

67. The sidewalks on steep hills are made of rough cement.

68. Certain fish can rise in the water by expanding their air bladders,
although this does not make them weigh any less.

69. When your hands are cold, you rub them together to warm them.

70. It is dangerous to stand up in a rowboat or canoe.

CHAPTER THREE
CONSERVATION OF ENERGY

Section 10. Levers.

How a big weight can be lifted with a little force; how one thing moving slowly
a short distance can make another move swiftly a long distance.

Why can you go so much faster on a bicycle than on foot?

How can a man lift up a heavy automobile by using a jack?

Why can you crack a hard nut with a nutcracker when you cannot crack it
by squeezing it between two pieces of iron?

"Give me a lever, long enough and strong enough, and something to rest it on,
and I can lift the whole world," said an old Greek philosopher. And as a
philosopher he was right; theoretically it would be possible. But since he
needed a lever that would have been as long as from here to the farthest star
whose distance has ever been measured, and since he would have had to push
his end of the lever something like a quintillion (1,000,000,000,000,000,000)
miles to lift the earth one inch, his proposition was hardly a practical one.

But levers are practical. Without them there would be none of our modern
machines. No locomotives could speed across the continents; no derricks could
lift great weights; no automobiles or bicycles would quicken our travel; our
very bodies would be completely paralyzed. Yet the law back of all these
things is really simple.

You have often noticed on the see-saw that a small child at one end can be
balanced by a larger child at the other end, provided that the larger child sits
nearer the middle. Why should it matter where the larger child sits? He is
always heavier—why doesn't he overbalance the small child? It is because
when the small child moves up and down he goes a longer distance than the
large child does. In Figure 26 the large boy moves up and down only half as

far as the little girl does. She weighs only half as much as he, yet she balances
him.

Fig. 26. The little girl raises the big boy, but in doing it she moves twice as
far as he does.

You will begin to get a general understanding of levers and how they work by
doing the following experiment:

Experiment 18. For this experiment there will be needed a small pail
filled with something heavy (sand or stones will do), a yardstick with a
hole through the middle and another hole near one end and with notches
cut here and there along the edge, and a post or table corner with a heavy
nail driven into it to within an inch of the head. The holes in the yardstick
must be large enough to let the head of this nail through.

Put the middle hole of the yardstick over the nail, as is shown in Figure
27. The nail is the fulcrum of your lever. Now hang the pail on one of the
notches about halfway between the fulcrum and the end of the stick and
put your hand on the opposite side of the yardstick at about the same
distance as the pail is from the fulcrum. Raise and lower the pail several
times by moving the opposite end of the lever up and down. See how
much force it takes to move the pail.

Now slide your hand toward the fulcrum and lower and raise the pail
from that position. Is it harder or easier to lift the pail from here than from

the first position? Which moves farther up and down, your hand or the
pail?

Next, slide your hand all the way out to the end of the yardstick and raise
and lower the pail from there. Is the pail harder or easier to lift? Does the
pail move a longer or a shorter distance up and down than your hand?

If you wanted to move the pail a long way without moving your hand as
far, would you put your hand nearer to the fulcrum or farther from it than
the pail is?

Fig. 27. The yardstick is a lever by which he lifts the pail.

Fig. 28. A lever with the weight between the fulcrum and the force.

Suppose you wanted to lift the pail with the least possible effort, where
would you put your hand?

Notice another fact: when your hand is at the end of the yardstick, it takes
the same length of time to move a long way as the pail takes to move a
short way. Then which is moving faster, your hand or the pail?

Experiment 19. Put the end hole of the yardstick on the nail, as shown in
Figure 28. The nail is still the fulcrum of your lever. Put the pail about
halfway between the fulcrum and the other end of the stick, and hold the
end of the stick in your hands.

Raise and lower your hand to see how hard or how easy it is to lift the
pail from this position. Which is moving farther, your hand or the pail?
Which is moving faster?

Now put your hand about halfway between the fulcrum and the pail and
raise and lower it. Is it harder or easier to raise than before? Which moves
farther this time, your hand or the pail? Which moves faster?

If you wanted to make the pail move farther and faster than your hand,
would you put your hand nearer to the fulcrum than the pail is, or farther
from the fulcrum than the pail? If you wanted to move the pail with the
least effort, where would you put your hand?

Experiment 20. Use a pair of long-bladed shears and fold a piece of
cardboard once to lie astride your own or some one else's finger. Put the
finger, protected by the cardboard, between the two points of the shears.
Then squeeze the handles of the shears together. See if you can bring the
handles together hard enough to hurt the finger between the points.

Now watch the shears as you open and close the blades. Which move
farther, the points of the shears or the handles? Which move faster?

Next, put the finger, still protected by the cardboard, between the handles
of the shears and press the points together. Can you pinch the finger this
way harder or less hard than in the way you first tried?

Fig. 29. You cannot pinch hard enough this way to hurt.

Fig. 30. But this is quite different.

Do the points or handles move farther as you close the shears? Which part
closes with the greater force?

Experiment 21. Use a Dover egg beater. Fasten a small piece of string to
one of the blades, so that you can tell how many times it goes around.
Turn the handle of the beater around once slowly and count how many
times the blade goes around. Which moves faster, the handle or the blade?
Where would you expect to find more force, in the cogs or in the blades?
Test your conclusion this way: Put your finger between the blades and try
to pinch it by turning the handle; then place your finger so that the skin is
caught between the cogs and try to pinch the finger by turning the blades.
Where is there more force? Where is there more motion?

Fig. 31. When the handle is turned the blades of the egg beater move
much more rapidly than the hand. Will they pinch hard enough to
hurt?

Experiment 22. Put a spool over the nail
which was your fulcrum in the first two
experiments. (Take the stick off the nail
first, of course.) Use this spool as a pulley.
Put a string over it and fasten one end of
your string to the pail (Fig. 32). Lift the pail
by pulling down on the other end of the
string. Notice that it is not harder or easier
to move the pail when it is near the nail than
when it is near the floor. When your hand
moves down from the nail to the floor, how
far up does the pail move? Does the pail
move a greater or less distance than your
hand, or does it move the same distance?

Fig. 32. His hand goes
down as far as the pail Next fasten one end of the string to the nail.
goes up. Set the pail on the floor. Pass the string
through the handle of the pail and up over
the spool (Fig. 33). Pull down on the loose
end of the string. Is the pail easier to lift in this way or in the way you

first tried? As you pull down with your
hand, notice whether your hand moves
farther than the pail, not so far as the pail, or
the same distance. Is the greater amount of
motion in your hand or in the pail? Then
where would you expect the greater amount
of force?

Fig. 33. With this
arrangement the pail
travels more slowly
than the hand. Will it
seem heavier or lighter
than with the
arrangement shown in
Figure 32?

The whole idea of the lever can be summed up like this: one end of the
contrivance moves more than the other. But energy cannot be lost; so to make
up for this extra motion at one end more force is always exerted at the other.

This rule is true for all kinds of levers, blocks and tackles or pulley systems,
automobile and bicycle gears, belt systems, cog systems, derricks, crowbars,
and every kind of machine. In most machines you either put in more force than
you get out and gain motion, or you put in more motion than you get out and
gain force. In the following examples of the lever see if you can tell whether
you are applying more force and obtaining more motion, or whether you are
putting in more motion and obtaining more force:

Cracking nuts with a nut cracker.

Beating eggs with a Dover egg beater.

Going up a hill in an automobile on low gear.

Speeding on high gear.

Cutting cloth with the points of shears.

Cutting near the angle of the shears.

Turning a door knob.

Picking up sugar with sugar tongs.

Pinching your finger in the crack of a door on the hinge side.

Application 16. Suppose you wanted to lift a heavy frying pan off the
stove. You have a cloth to keep it from burning your hand. Would it be
easier to lift it by the end of the handle or by the part of the handle nearest
the pan?

Application 17. A boy was going to wheel his little sister in a
wheelbarrow. She wanted to sit in the middle of the wheelbarrow; her
brother thought she should sit as near the handles as possible so that she

would be nearer his hands. Another boy thought she should sit as near the
wheel as possible. Who was right?

Application 18. James McDougal lived in a hilly place. He was going to
buy a bicycle. "I want one that will take the hills easily," he said. The
dealer showed him two bicycles. On one the back wheel went around
three times while the pedals went around once; on the other the back
wheel went around four and a half times while the pedals went around
once. Which bicycle should James have chosen? If he had wanted the
bicycle for racing, which should he have chosen?

Application 19. A wagon stuck in the mud. The driver got out and tried to
help the horse by grasping the spokes and turning the wheel. Should he
have grasped the spokes near the hub, near the rim, or in the middle?

Inference Exercise

Explain the following:

71. When you turn on the faucet of a distilled-water bottle, bubbles go up
through the water as the water pours out.

72. A clothes wringer has a long handle. It wrings the clothes drier than
you can wring them by hand.

73. You use a crowbar when you want to raise a heavy object such as a
rock.

74. Sometimes it is almost impossible to get the top from a jar of canned
fruit unless you let a little air under the edge of the lid.

75. It is much easier to carry a carpet sweeper if you take hold near the
sweeper part than it is if you take hold at the end of the handle.

76. You can make marks on a paper by rubbing a pencil across it.

77. A motorman sands the track when he wishes to stop the car on a hill.

78. On a faucet there is a handle with which to turn it.

79. Before we pull candy we butter our fingers.

80. You can scratch glass with very hard steel but not with wood.

Section 11. Inertia.

Why is it that if you push a miniature auto rapidly, it will go straight?

Why does the earth never stop moving?

When you jerk a piece of paper from under an inkwell, why does the
inkwell stay still?

When you are riding in a car and the car stops suddenly, you are thrown
forward; your body tends to keep moving in the direction in which the car was
going. When a car starts suddenly, you are thrown backward; your body tends
to stay where it was before the car started.

When an automobile bumps into anything, the people in the front seat are often
thrown forward through the wind shield and are badly cut; their bodies keep on
going in the direction in which the automobile was going.

When you jump off a moving street car, you have to run along in the direction
the car was going or you fall down; your body tries to keep going in the same
direction it was moving, and if your feet do not keep up, you topple forward.

Generally we think that it takes force to start things to move, but that they will
stop of their own accord. This is not true. It takes just as much force to stop a
thing as it does to start it, and what usually does the stopping is friction.

When you shoot a stone in a sling shot, the contracting rubber pulls the stone
forward very rapidly. The stone has been started and it would go on and never
stop if nothing interfered with it. For instance, if you should go away off in
space—say halfway between here and a star—and shoot a stone from a sling
shot, that stone would keep on going as fast as it was going when it left your
sling shot, forever and ever, without stopping, unless it bumped into a star or
something. On earth the reason it stops after a while is that it is bumping into
something all the time—into the particles of air while it is in the air, and finally
against the earth when it is pulled to the ground by gravity.

If you threw a ball on the moon, the person who caught it would have to have a
very thick mitt to protect his hand, and it would never be safe to catch a batted
fly. For there is no air on the moon, and therefore nothing would slow the ball
down until it hit something; and it would be going as hard and fast when it
struck the hand of the one who caught it as when it left your hand or the bat.

Fig. 34. When the paper is jerked out, the glass of water does not move.

Try these experiments:

Experiment 23. Fill a glass almost to the brim with water. Lay a smooth
piece of writing paper 10 or 11 inches long on a smooth table, placing it
near the edge of the table. Set the glass of water on the paper near its
inner edge (Fig. 34).

Take hold of the edge of the paper that is near the edge of the table. Move
your hand a little toward the glass so that the paper is somewhat bent.
Then, keeping your hand near the level of the table, suddenly jerk the
paper out from under the glass. If you give a quick enough jerk and keep
your hand near the level of the table, not a drop of water will spill and the
glass will stay almost exactly where it was.

This is because the glass of water has inertia. It was standing still, and so it
tends to remain standing still. Your jerk was so sudden that there was not time

to overcome the inertia of the glass of water; so it stayed where it was.

Experiment 24. Have a boy on
roller skates skate down the hall
or sidewalk toward you and have
him begin to coast as he comes
near. When he reaches you, put
out your arm and try to stop him.
Notice how much force it takes
to stop him in spite of the fact
that he is no longer pushing
himself along.

Now let the boy skate toward
you again, coasting as before;
but this time have him swing
himself around a corner by
taking hold of you as he passes.
Notice how much force it takes
just to change the direction in
which he is moving.
Fig. 35. When a boy is moving
You see the boy's inertia makes him rapidly, it takes force to change the
tend to keep going straight ahead at direction of his motion.
the same speed; it resists any
change either in the speed or the
direction of his motion. So it takes a good deal of force either to stop him or to
turn him.

If, on the other hand, you had no inertia, you could neither have stopped him
nor turned him; he would have swept you right along with him. It was because
inertia made you tend to remain still, that you could overcome part of his
inertia. At the same time he overcame part of your inertia, for he made you
move a little.

Inertia is the tendency of a thing to keep on going forever in the same direction
if once it is started, or to stand still forever unless something starts it. If
moving things did not have inertia (if they did not tend to keep right on
moving in the same direction forever or until something changed their motion),

you could not throw a ball; the second you let go of it, it would stop and fall to
the ground. You could not shoot a bullet any distance; as soon as the gases of
the gunpowder had stopped pushing against it, it would stop dead and fall.
There would be no need of brakes on trains or automobiles; the instant the
steam or gasoline was shut off, the train or auto would come to a dead stop.
But you would not be jerked in the least by the stopping, because as soon as
the automobile or train stopped, your body too would stop moving forward.
Your automobile could even crash into a building without your being jarred.
For when the machine came to a sudden stop, you would not be thrown
forward at all, but would sit calmly in the undamaged automobile.

If you sat in a swing and some one ran under you, you would keep going up till
he let go, and then you would be pulled down by gravity just as you now are.
But just as soon as the swing was straight up and down you would stop; there
would be no inertia to make you keep on swinging back and up.

If the inertia of moving things stopped, the clocks would no longer run, the
pendulums would no longer swing, nor the balance wheels turn; nothing could
be thrown; it would be impossible to jump; there would cease to be waves on
the ocean; and the moon would come tumbling to the earth. The earth would
stop spinning; so there would be no change from day to night; and it would
stop swinging about in its orbit and start on a rush toward the sun.

But there is always inertia. And all things everywhere and all the time tend to
remain stock still if they are still, until some force makes them move; and all
things that are moving tend to keep on moving at the same speed and in the
same direction, until something stops them or turns them in another direction.

Application 20. Explain why you should face forward when alighting
from a street car; why a croquet ball keeps rolling after you hit it; why
you feel a jolt when you jump down from a high place.

Inference Exercise

Explain the following:

81. It is much easier to erase charcoal drawings than water-color
paintings.

82. When an elevator starts down suddenly you feel lighter for a moment,
while if it starts up quickly you feel heavier.

83. You can draw a nail with a claw hammer when you could not possibly
pull it with your hand even if you could get hold of it.

84. When an automobile bumps into anything, the people in the front seat
are often thrown forward through the wind shield.

85. Certain weighted dolls will rise and stand upright, no matter in what
position you lay them down.

86. Some automobile tires have little rubber cups all over them which are
supposed to make the tires cling to the pavement and thus prevent
skidding.

87. It is hard to move beds and bureaus which have no castors or gliders.

88. When you jump off a moving street car, you lean back.

89. All water flows toward the oceans sooner or later.

90. You can skate on ice, but not on a sidewalk, with ice skates.

Section 12. Centrifugal force.

Why does not the moon fall down to the earth?

Why will a lasso go so far after it is whirled?

Why does a top stand on its point while it is spinning?

If centrifugal force suddenly stopped acting, you would at first not notice any
change. But if you happened to get into an automobile and rode down a muddy
street, you would be delighted to find that the mud did not fly up from the
wheels as you sped along. And when you went around a slippery corner, your
automobile would not skid in the least.

If a dog came out of a pool of water and shook himself while centrifugal force
was not acting, the water, instead of flying off in every direction, would merely

drip down to the ground as if the dog were not shaking himself at all. A
cowboy would find that he could no longer throw his lasso by whirling it
around his head. A boy trying to spin his top would discover that the top would
not stand on its point while spinning, any better than when it was not spinning.

These are little things, however. Most people would be quite unconscious of
any change for some time. Then, as night came on and the full moon rose, it
would look as if it were growing larger and larger. It would seem slowly to
swell and swell until it filled the whole sky. Then with a stupendous crash the
moon would collide with the earth. Every one would be instantly killed. And it
would be lucky for them that they were; for if any people survived the shock
of the awful collision, they would be roasted to death by the heat produced by
the striking together of the earth and the moon. Moreover, the earth would be
whirled swiftly toward the sun, and a little later the charred earth would be
swept into the sun's vast, tempestuous flames.

When we were talking about inertia, we said that if there were no inertia, the
moon would tumble down to the earth and the earth, too, would fall into the
sun. That was because if there were no inertia there would be no centrifugal
force. For centrifugal force is not really a force at all, but it is one form of
inertia—the inertia of whirling things. Do this experiment:

Experiment 25. Hold a pail half full of water in one hand. Swing it back
and forth a couple of times; then swing it swiftly forward, up, and on
around, bringing it down back of you (Fig. 36). Swing it around this way
swiftly and evenly several times, finally stopping at the beginning of the
up swing.

Fig. 36. Why doesn't the water spill out?

It is centrifugal force that keeps the water in the pail. It depends entirely on
inertia. You see, while the pail is swinging upward rapidly, the water is moving
up and tends by its inertia to keep right on moving in the same upward
direction. Before you get it over your head, the tendency of the water to keep
on going up is so strong that it pulls on your arm and hand and presses against
the bottom of the pail above it. Its tendency to go on up is stronger than the
downward pull of gravity. As you swing the pail on backward, the water of
course has to move backward, too; so now it tends to keep on moving
backward; and when the pail is starting down behind you, the water is tending
to fly out in the backward direction in which it has just been going. Therefore
it still pushes against the bottom of the pail and pulls away from your shoulder,
which is in the center of the circle about which the pail is moving. By the time
you have swung the pail on down, the water in it tends to keep going down,
and it is still pulling away from your shoulder and pressing against the bottom
of the pail.

In this way, during every instant the water tends to keep going in the direction
in which it was going just the instant before. The result is that the water keeps
pulling away from your shoulder as long as you keep swinging it around.

All whirling things tend to fly away from the center about which they are
turning. This is the law of centrifugal force. The earth, for example, as it
swings around the sun, tends to fly away from the center of its orbit. This
tendency of the earth—its centrifugal force—keeps it from being drawn into
the sun by the powerful pull of the sun's gravitation. At the same time it is this
gravitation of the sun that keeps the earth from flying off into space, where we
should all be frozen to icicles and lost in everlasting night. For if the sun's pull
stopped, the earth would fly off as does a stone whirled from the end of a
string, when you let go of the string.

The moon, in like manner, would fly away from the earth and sun if
gravitation stopped pulling it, but it would crash into us if its centrifugal force
did not keep it at a safe distance.

Have you ever sat on a spinning platform, sometimes called "the social whirl,"
in an amusement park, and tried to stay on as it spun faster and faster? It is
centrifugal force that makes you slide away from the center and off at the edge.

Fig. 37. An automobile race. Notice how the track is banked to keep the
cars from overturning on the curves.

How cream is separated from milk by centrifugal force. The heavier things
are, the harder they are thrown out by centrifugal force. Milk is heavier than
cream, as you know from the fact that cream rises and floats on top of the
milk. So when milk is put into a centrifugal separator, a machine that whirls it
around very rapidly, the milk is thrown to the outside harder than the cream,
and the cream therefore stays nearer the middle. As the bowl of the machine
whirls faster, the milk is thrown so hard against the outside that it flattens out
and rises up the sides of the bowl. Thus you have a large hollow cylinder of
milk on the outside against the wall of the bowl, while the whirling cream
forms a smaller cylinder inside the cylinder of milk. By putting a spout on the
machine so that it reaches the inner cylinder, the cream can be drawn off, while
a spout not put in so far will draw off the milk.

Why a spinning top stands on its point. When a top spins, all the particles of
wood of which the top is made are thrown out and away from the center of the
top, or rather they tend to go out and away. And the pull of these particles out
from the center is stronger than the pull of gravitation on the edges of the top
to make it tip over; so it stands upright while it spins. Spin a top and see how
this is.

Application 21. Explain how a motor cyclist can ride on an almost
perpendicular wall in a circular race track. Explain how the earth keeps
away from the sun, which is always powerfully pulling the earth toward
it.

Inference Exercise

Explain the following:

91. As you tighten a screw it becomes harder to turn.

92. There is a process for partly drying food by whirling it rapidly in a
perforated cylinder.

93. It is easier to climb mountains in hobnailed shoes than in smooth-
soled ones.

94. When you bore a hole with a brace and bit, the hand that turns the
brace goes around a circle many times as large as the hole that is being

bored.

95. The hands of some persons become red and slightly swollen if they
swing them while taking a long walk.

96. A flywheel keeps an engine going between the strokes of the piston.

97. In dry parts of the country farmers break up the surface of the soil
frequently, as less water comes up to the surface through pulverized soil
than would come through the fine pores of caked earth.

98. After you have apparently cleaned a grease spot out of a suit it often
reappears when you have worn the suit a few days.

99. Mud flies up from the back wheel of a boy's bicycle when he rides
along a wet street.

100. A typewriter key goes down less than an inch, yet the type bar goes
up nearly 5 inches.

Section 13. Action and reaction.

How can a bird fly? What makes it stay up in the air?

What makes a gun kick?

Why do you sink when you stop swimming?

Whenever anything moves, it pushes something else in an opposite direction.
When you row a boat you can notice this; you see the oars pushing the water
backward to push the boat forward. Also, when you shoot a bullet forward you
can feel the gun kick backward; or when you pull down hard enough on a bar,
your body rises up and you chin yourself. But the law is just as true for things
which are not noticeable. When you walk, your feet push back against the
earth; and if the earth were not so enormous and you so small, and if no one
else were pushing in the opposite direction, you would see the earth spin back
a little for each step you took forward, just as the big ball that a performing
bear stands on turns backward as the bear tries to walk forward.

Fig. 38. The horse goes forward by pushing backward on the earth with
his feet.

The usual way of saying this is, "Action and reaction are equal and opposite."
If you climb a rope, the upward movement of your body is the action; but you
have to pull down on the rope to lift your body up. This is the reaction.

Without this law of action and reaction no fish could swim, no steamboat could
push its way across the water, no bird could fly, no train or machine of any
kind could move forward or backward, no man or animal could walk or crawl.
The whole world of living things would be utterly paralyzed.

Fig. 39. As he starts to toss the ball up, will he weigh more or less?

When anything starts to move, it does so by pushing on something else. When
your arms start to move up, they do so by pushing your body down a little.
When you swim, you push the water back and down with your arms and legs,
and this pushes your body forward and up. When a bird flies up into the air, it
pushes its body up by beating the air down with its wings. When an airplane
whirs along, its propeller fans the air backward all the time. Street-car tracks
are kept shiny by the wheels, which slip a little as they tend to shove the track
backward in making the car move forward. Automobile tires wear out in much
the same way,—they slip and are worn by friction as they move the earth back
in pushing the automobile forward. In fact, if there are loose pebbles or mud
on the road, you can see the pebbles or mud fly back, as the wheels of the
automobile begin to turn rapidly and give their backward push to the earth
beneath.

Here are a couple of experiments that will show you action and reaction more
clearly:

Experiment 26. Stand on a platform scale and weigh yourself. When the
beam is exactly balanced, move your hands upward and notice whether

you weigh more or less when they start up. Now move them downward;
when they start down, do you weigh more or less? Toss a ball into the air,
and watch your weight while you are tossing it. Does your body tend to
go up or down while you are making the ball go up?

Experiment 27. Go out into the yard
and sit in a rope swing. Stop the swing
entirely. Keep your feet off the ground
all through the experiment. Now try to
work yourself up in the swing; that is,
make it swing by moving your legs and
body and arms, but not by touching the
ground. (Try to make it swing forward
and backward only; when you try to
swing sidewise, the distance between
the ropes spoils the experiment.) See if
you can figure out why the swing will
not move back and forth. Notice your
bodily motions; notice that when half of
your body goes forward, half goes back;
when you pull back with your hands,
you push your body forward. If you
Fig. 40. Action and reaction watch yourself closely, you will see that
are equal; when he pushes every backward motion is exactly
forward on the ropes, he balanced by a forward motion of some
pushes backward with part of your body.
equal force on the seat.
Application 22. Explain why you push
forward against the table to shove your
chair back from it; why a bird beats down with its wings against the air to
force itself up; why you push back on the water with your oars to make a
rowboat go forward.

Inference Exercise

Explain the following:

101. Water comes up city pipes into your kitchen.

102. When you try to push a heavy trunk, your feet slip out from under
you and slide in the opposite direction.

103. When you turn a bottle of water upside down with a small piece of
cardboard laid over its mouth, the water stays in the bottle.

104. You can squeeze a thing very tightly in a vise.

105. There is a water game called "log rolling"; two men stand on a log
floating in the water and roll the log around with their feet, each one
trying to make the other lose his balance. Explain why the log rolls
backward when the man apparently runs forward.

106. The oil which fills up the spaces between the parts of a duck's
feathers keeps the duck from getting wet when a hen would be soaked.

107. Sleds run on snow more easily than wagons do.

108. In coasting down a hill, it is difficult to stop at the bottom.

109. When you light a pinwheel, the wheel whirls around as the powder
burns, and the sparks fly off in all directions.

110. You cannot lift yourself by your own boot straps.

Section 14. Elasticity.

What makes a ball bounce?

How does a springboard help you dive?

Why are automobile and bicycle tires filled with air?

Suppose there were a man who was perfectly elastic, and who made
everything he touched perfectly elastic. Fortunately there is no such person,
but suppose an elastic man did exist:

He walks with a spring and a bound; his feet bounce up like rubber balls each
time they strike the earth; his legs snap back into place after each step as if
pulled by a spring. If he stumbles and falls to the ground, he bounces back up

into the air without a scar. (You see, his skin springs back into shape even if it
is scratched, so that a scratch instantly heals.) And he bounces on and on
forever without stopping.

Suppose you, seeing his plight, try to stop him. Since we are pretending that he
makes everything he touches elastic, the instant you touch him you bounce
helplessly away in the opposite direction.

You may think your clothes will be wrinkled by all this bouncing about, but
since we are imagining that you have caught the elastic touch from the elastic
man, your clothes which touch you likewise become perfectly elastic. So no
matter how mussed they get, they promptly straighten out again to the
condition they were in when you touched the elastic man.

If you notice that your shoe lace was untied just before you became elastic,
and you now try to tie it and tuck it in, you find it most unmanageable. It
insists upon flying out of your shoe and springing untied again.

Perhaps your hair was mussed before you became elastic. Now it is impossible
to comb it straight; each hair springs back like a fine steel wire.

If you take a handkerchief from your pocket to wipe your perspiring brow, you
find that it does not stay unfolded. As soon as it is spread out on your hand, it
snaps back to the shape and the folds it had while in your pocket.

Suppose you bounce up into an automobile for a ride. The automobile, now
being made elastic by your magic touch, bounds up into the air at the first
bump it strikes, and thereafter it goes hopping down the street in a most
distressing manner, bouncing off the ground like a rubber ball each time it
comes down. And each time it bumps you are thrown off the seat into the air.

You find it hard to stay in any new position. Your body always tends to snap
back to the position you were in when you first became elastic. If you touch a
trotting horse and it becomes elastic, the poor animal finds that his legs always
straighten out to their trotting position, whether he wants to walk or stand still
or lie down.

Imagine the plight of a boy pitching a ball, or some one yawning and
stretching, or a clown turning a somersault, if you touch each of these just in
the act and make him elastic. Their bodies always tend to snap back to these

positions. Whenever the clown wants to rest, he has to get in the somersault
position. The boy pitcher sleeps in the position of "winding up" to throw the
ball. The one who was yawning and stretching has to be always on the alert,
because the instant he stops holding himself in some other position, his mouth
flies open, his arms fly out, and every one thinks he is bored to death.

You might touch the clay that a sculptor is molding and make it elastic. The
sculptor can mold all he pleases, but the clay is like rubber and always returns
at once to its original shape.

If you make a tree elastic when a man is chopping it down, his ax bounces
back from the tree with such force as nearly to knock him over, and no amount
of chopping makes so much as a lasting dent in the tree.

Suppose you step in some mud. The mud does not stick to your shoes. It bends
down under your weight, but springs back to form again as soon as your
weight is removed.

And if you try to spread some elastic butter on bread, nothing will make the
butter stay spread. The instant you remove your knife, the butter rolls up again
into the same kind of lump it was in before.

As for chewing your bread, you might as well try to chew a rubber band. You
force your jaws open, and they snap back on the bread all right; then they
spring open again, and snap back and keep this up automatically until you
make them stop. But for all this vigorous chewing your bread looks as if it had
never been touched by a tooth.

Sewing is about as difficult. The thread springs into a coil in the shape of the
spool. No hem stays turned; the cloth you try to sew springs into its original
folds in a most exasperating manner.

On the whole, a perfectly elastic world would be a hopeless one to live in.

Elasticity is the tendency of a thing to go back to its original shape or size
whenever it is forced into a different shape or size.

A thing does not have to be soft to be elastic. Steel is very elastic; that is why
good springs are almost always made of steel. Glass is elastic; you know how
you can bounce a glass marble. Rubber is elastic, too. Air is elastic in a

different way; it does not go back to its original shape, since it has no shape,
but if it has been compressed and the pressure is removed it immediately
expands again; so a football or any such thing filled with air is decidedly
elastic. That is why automobile and bicycle tires are filled with air; it makes
the best possible "springs."

Balls bounce because they are elastic. When a ball strikes the ground, it is
pushed out of shape. Since it is elastic it tries immediately to come back to its
former shape, and so pushes out against the ground. This gives it such a push
upward that it flies back to your hand.

Sometimes people confuse elasticity with action and reaction. But the
differences between them are very clear. Action and reaction happen at the
same time; your body goes up at the same time that you pull down on a bar to
chin yourself; while in elasticity a thing moves first one way, then the other;
you throw a ball down, then it comes back up to you. Another difference is that
in action and reaction one thing moves one way and another thing is pushed
the other way; while in elasticity the same thing moves first one way, then the
other. If you press down on a spring scale with your hand, you are lifting up
your body a little to do it; that is action and reaction. But after you take your
hand off the scale the pan springs back up: first it was pushed down, then it
springs back to its original position; it does this because of the elasticity of its
spring.

Application 23. Explain why basket balls are filled with air; why springs
are usually made of steel; why we use rubber bands to hold papers
together; why a toy balloon becomes small again when you let the air out.

Inference Exercise

Explain the following, being especially careful not to confuse action and
reaction with elasticity:

111. When you want to push your chair back from a table, you push
forward against the table.

112. The pans in which candy is cooled must be greased.

113. Good springs make a bed comfortable.

114. Paper clips are made of steel or spring brass.

115. A spring door latch acts by itself if you close the door tightly.

116. On a cold morning, you rub your hands together to warm them.

117. If an electric fan is not fastened in place and has not a heavy base, it
will move backward while it is going.

118. Doors with springs on them will close after you.

119. When you jump down on the end of a springboard, it throws you into
the air.

120. You move your hands backward to swim forward.

Note. There are really two kinds of elasticity, which have nothing to do
with each other. Elasticity of form is the tendency of a thing to go back to
its original shape, as rubber does. If you make a dent in rubber, it springs
right back to the shape it had before. Elasticity of volume is the tendency
of a substance to go back to its original size, as lead does. If you manage
to squeeze lead into a smaller space, it will spring right back to the same
size as soon as you stop pressing it on all sides. But a dent in lead will
stay there; it has little elasticity of form.

Air and water—all liquids, in fact—have a great deal of elasticity of
volume, but practically no elasticity of form. They do not tend to keep
their shape, but they do tend to fill the same amount of space. Putty and
clay likewise have very little elasticity of form; when you change their
shape, they stay changed.

Jelly and steel and glass have a great deal of elasticity of form. When you
dent them or twist them or in any way change their shape, they go right
back to their first shape as soon as they can.

When we imagined a man with an "elastic touch," we were imagining a
man who gave everything he touched perfect elasticity of form. It is
elasticity of form that most people mean when they talk about elasticity.

CHAPTER FOUR
HEAT

Section 15. Heat makes things expand.

How does a thermometer work? What makes the mercury rise in it?

Why does heat make things get larger?

When we look at objects through a microscope, they appear much larger and in
many cases we are able to see the smaller parts of which they are made. If we
had a microscope so powerful that it made a tiny speck of dust look as big as a
mountain (of course no such microscope exists), and if we looked through this
imaginary microscope at a piece of iron, we should find to our surprise that the
particles were not standing still. The iron would probably look as if it were
fairly alive with millions of tiny specks moving back and forth, back and forth,
faster than the flutter of an insect's wings.

These tiny moving things are molecules. Everything in the world is made of
them. It seems strange that we should know this, since there really are no
microscopes nearly powerful enough to show the molecules to us. Yet
scientists know a great deal about them. They have devised all sorts of
elaborate experiments—very accurate ones—and have tested the theories
about molecules in many ways. They have said, for instance, "Now, if this
thing is made of molecules, then it will grow larger when we make the
molecules move faster by heating it." Then they heated it—in your next
experiment you will see what happened. This is only one of thousands of
experiments they have performed, measuring over and over again, with the
greatest care, exactly how much an object expanded when it was heated a
certain amount; exactly how much heat was needed to change water to steam;
exactly how far a piece of steel of a certain size and shape could bend without
breaking; exactly how crystals form—and so on and so on. And they have
always found that everything acts as if it were made of moving molecules.
Their experiments have been so careful and scientists have found out so much
about what seem to be molecules,—how large they are, what they probably

weigh, how fast they move, and even what they are made of,—that almost no
one has any doubt left that fast-moving molecules make up everything in the
world.

Fig. 41. A thermometer.

To go back, then: if we looked at a piece of iron under a microscope that would
show us the molecules,—and remember, no such powerful microscope could
exist,—we should see these quivering particles, and nothing more. Then if
some one heated the iron while we watched the molecules, or if the sun shone
on it, we should see the molecules move faster and faster and separate farther
and farther. That is why heat expands things. When the molecules in an object
move farther apart, naturally the object expands.

Heat is the motion of the molecules. When the molecules move faster (that is,
when the iron grows hotter), they separate farther and the iron swells.

Fig. 42. A thermometer made of a flask of water. It does not show the exac
degree of heat of the water, but it does show whether the water is hot or
cold.

How we can tell the temperature by reading a thermometer. The mercury
(quicksilver) in the bulb of the thermometer like everything else expands
(swells) when it becomes warm. It is shut in tightly on all sides by the glass,
except for the little opening into the tube above. When it expands it must have
more room, and the only space into which it can move is up in the tube. So it
rises in the tube.

Fig. 43. Will the hot ball go through the ring?

Water will do the same thing. You can make a sort of thermometer, using water
instead of mercury, and watch the water expand when you heat it. Here are the
directions for doing this:

Fig. 44. When the wire is cold, it is fairly tight.

Experiment 28. Fill a flask to the top with water. Put a piece of glass
tubing through a stopper, letting the tube stick 8 or 10 inches above the
top of the stopper. Put the stopper into the flask, keeping out all air; the

water may rise 2 or 3 inches in the glass tube. Dry the flask on the outside
and put it on a screen on the stove or ring stand, and heat it. Watch the
water in the tube. What effect does heat have on the water?

Here are two interesting experiments that show how solid things expand when
they are heated:

Experiment 29. The brass ball and brass ring shown in Figure 43 are
called the expansion ball and ring. Try pushing the ball through the ring.
Now heat the ball over the flame for a minute or two—it should not be
red hot—and try again to pass it through the ring.

Heat both ball and ring for a short time. Does heating expand the ring?

Experiment 30. Go to the electric apparatus (described on page 379) and
turn on the switch that lets the electricity flow through the long resistance
wire. Watch the wire as it becomes hot.

Application 24. A woman brought me a glass-stoppered bottle of smelling
salts and asked me if I could open it. The stopper was in so tightly that I
could not pull it out. I might have done any of the following things: Tried
to pull the stopper out with a pair of pliers; plunged the bottle up to the
neck in hot water; plunged it in ice-cold water; tried to loosen the stopper
by tapping it all around. Which would have been the best way or ways?

Fig. 45. But notice how it sags when it is hot.

Application 25. I used to buy a quart of milk each evening from a farmer
just after he had milked. He cooled most of the milk as soon as it was
strained, to make it keep better. He asked me if I wanted my quart before

or after it was cooled. Either way he would fill his quart measure brim
full. Which way would I have received more milk for my money?

Inference Exercise

Explain the following:

121. Billiard balls will rebound from each other and from the edges of the
table again and again and finally stop.

122. In washing a tumbler in hot water it is necessary to lay it in sidewise
and wet it all over, inside and out, to keep it from cracking; if it is thick in
some parts and thin in others, like a cut-glass tumbler, it is not safe to
wash it in hot water at all.

123. The swinging of the moon around the earth keeps the moon from
falling to the earth.

124. A fire in a grate creates a draft up the chimney.

125. Telegraph wires and wire fences put up in the summer must not be
strung too tightly.

126. Candy usually draws in somewhat from the edge of the pan as it
hardens.

127. A meat chopper can be screwed to a table more tightly than you can
possibly push it on.

128. A floor covered with linoleum is more easily kept clean than a plain
wood floor.

129. Rough seams on the inside of clothes chafe your skin.

130. You can take the top off a bottle of soda pop with an opener that will
pry it up, but you cannot pull it off with your fingers.

Section 16. Cooling from expansion.

We get our heat from the sun; then why is it so cold up on the mountain
tops?

What is coldness?

Here is an interesting and rather strange thing about heat and expansion.
Although heat expands things, yet expansion does not heat them. On the
contrary, if a thing expands without being heated from an outside source, it
actually gets cold! You see, in order to expand, it has to push the air or
something else aside, and it actually uses up the energy of its own heat to do
this. You will understand this better after you do the next experiment.

Experiment 31. Wet the inside of a test tube. Hold the mouth of the test
tube against the opening of a carbon dioxid tank. Open the valve of the
tank with the wrench and let the compressed gas rush out into the test
tube until the mouth of the test tube is white. Shut off the valve. Feel your
test tube.

What has happened is this: The gas was tightly compressed in the tank. It was
not cold; that is, it had some heat in it, as everything has. When you let it
loose, it used up much of its heat in pushing the air in the test tube and all
around it out of the way. In this way it lost its heat, and then it became cold.
Cold means absence of heat, as dark means absence of light. So when the
compressed gas used up its heat in pushing the air out of its way, it became so
cold that it froze the water in your test tube.

Fig. 46. The expansion of the compressed gas freezes the moisture on the
tube.

One reason why it is always cold high up in the air. Even on hot summer
days aviators who fly high suffer from the cold. You might think that they
would get warmer as they went up nearer the sun; one reason that they get
colder instead is this:

As you saw in the last experiment, a gas that expands gets very cold. Air is a
kind of gas. And whenever air rises to where there is not so much air crowding
down on it from above, it expands. So the air that rises high and expands gets
very cold. Consequently mountains which reach up into this high, cold air are
snow covered all the year round; and aviators who fly high suffer keenly from
the cold. There are several reasons for this coldness of the high air. This is just
one of them.

Application 26. Explain why air usually cools when it rises; why high
mountain tops are always covered with snow.

Inference Exercise

Explain the following:

131. You should not fill a teakettle brim full of cold water when you are
going to put it on the stove.

132. It is harder to erase an ink mark than a pencil mark.

133. Bearings of good watches, where there is constant rubbing on the
parts, are made of very hard jewels.

134. You feel lighter for an instant when you are in an elevator which
starts down suddenly.

135. When men lay cement sidewalks, they almost always make cracks
across them every few feet.

136. To cool hot coffee one sometimes blows on it.

137. It is much easier to turn the latch of a door with the knob than with
the spindle when the knob is off.

138. Patent-leather shoes do not soil as easily as plain leather shoes.

139. We use rubber bands to hold things together tightly.

140. As air goes up it usually cools.

Section 17. Freezing and melting.

When water freezes in a pipe, why does the pipe burst?

What is liquid air?

Why does not the wire in an electric lamp melt when it is red hot?

Suppose we looked at a piece of ice through the imaginary microscope that
shows us the molecules. The ice molecules would be different from the iron
molecules in size, but they would be vibrating back and forth in exactly the
same way, only with less motion. It is because they have less motion that we
say the ice is colder than the iron. Then let us suppose that the sun was shining
on the ice while we watched the ice molecules.

First we should see movements of the ice molecules become gradually more
rapid, just as the iron molecules did when the iron was warmed. Then, as they
moved faster and faster, they would begin to bump into each other and go
around every which way, each molecule bumping first into one neighbor, then
into another, and bouncing back in a new direction after each collision. This is
what causes the ice to melt. When its molecules no longer go back and forth in
the same path all the time, the ice no longer keeps its shape, and we call it
water—a liquid.

Almost all solid substances will melt when they are heated. Or, to put it the
other way around, every liquid will freeze solid if it gets cold enough. Even
liquid air (which is ordinary air cooled and compressed until it runs like water)
can be frozen into a solid chunk. Some things will melt while they are still
very cold; solid air, for instance, melts at a temperature that would freeze you
into an icicle before you could count ten. Other things, such as stones, are
melted only by terrific heat.

When the little particles of water that make up the clouds become very cold,
they freeze as they gather and so make snowflakes. When the little particles of
water in the air, that usually make dew, freeze while they are gathering on a
blade of grass, we call it frost. When raindrops are carried up into colder,
higher air while they are forming, they freeze and turn to hail. When snow or
frost or hail or ice is heated, it melts and turns back to water.

Fig. 47. Why did the bottle break when the water in it turned to ice?

But here is a strange fact: although heat usually expands things, water expands
when it freezes. Like everything else, however, water also expands when it
becomes hot, as you found when you made a kind of thermometer, using a
flask of water and a glass tube. But if you should put that flask into a freezing
mixture of ice and salt, you would find that when the water became very cold
it would begin to expand a little immediately before it froze.

And it is very lucky for us that water does expand when it freezes, because if it
did not, ice would be heavier than water is. But since the water expands as it
freezes, ice weighs less than water and floats. And that is why lakes and oceans

and rivers freeze over the top and do not freeze at the bottom. If they froze
from the bottom up, as they would if the ice sank as it formed, every river and
lake would be solid ice in the winter. All the harbors outside the tropics would
probably be ice-bound all winter long. And the ice in the bottom of the lakes
and rivers and in the ocean would probably never melt.

So in the case of freezing water, and in the case of a couple of metals, there is a
point where coldness, not heat, makes things expand.

Experiment 32. Take a ketchup bottle with a screw cap and a cork that
fits tightly. Fill it to the top with water; put a long pin beside the cork
while you insert it, so that the water can be crowded out as the cork goes
down; then when you have pushed the cork in tightly, pull out the pin.
Screw the cap on the bottle so as to hold the cork fast. Put the bottle in a
pail or box, and pack ice and salt around it. Within an hour you should be
able to see what the freezing water does to the bottle.

Application 27. Explain why ice is lighter than water; why we have no
snow in summer.

Inference Exercise

Explain the following:

141. Sealing wax is held over a candle flame before it is applied to a
letter.

142. Automobile tires tighten upon a sudden change from cold weather to
hot.

143. When paper has been rolled, it tends to curl up again after being
unrolled.

144. Seats running across a car are much more comfortable when a car
starts and stops, than are seats running along the sides.

145. You cannot siphon water from a low place to a higher one.

146. Candles get soft in hot weather.

147. Meteorites fall to the earth from the sky.

148. When you preserve fruit and pour the hot fruit into the jars, you fill
the jars brim full and screw on the cap air-tight; yet a few hours later the
fruit does not fill the jars; there is some empty space between the top of
the fruit and the cover.

149. Water pipes burst in the winter when it is very cold.

150. When people want to make iron castings, they first melt the iron,
then pour it into molds. They leave it in the molds until cold. After that
the iron holds the shape of the molds. Explain why the iron changes from
a liquid to a solid.

Section 18. Evaporation.

Why is it that when ink is spilled it dries up, but when it is in the bottle it
does not dry up?

What put the salt into the ocean?

Why do you feel cold when you get out of the bathtub?

Wet clothes get dry when they are hung on the clothes-line. The water in them
evaporates. It turns to invisible vapor and disappears into the air. Water and all
liquids evaporate when they are long exposed to the air. If they didn't—well,
let us imagine what the world would be like if all evaporation should suddenly
stop:

You find that your face is perspiring and your hands as well. You wipe them on
your handkerchief, but soon they are moist again, no matter how cool the
weather. After wiping them a few more times your handkerchief becomes
soaking wet, and you hang it up to dry. There may be a good breeze stirring,
yet your handkerchief does not get dry. By this time the perspiration is running
off your face and hands, and your underclothes are getting drenched with
perspiration.

Fig. 48. An evaporating dish.

You hurry into the house, change your clothes, bathe and wipe yourself dry
with a towel. When you find that your wet things are not drying, and that your
dry ones are rapidly becoming moist, you hastily build a fire and hang your
clothes beside it. No use, your clothes remain as wet as ever. If you get them
very hot the moisture in them will boil and turn to steam, of course, but the
steam will all turn back to water as soon as it cools a little and the drops will
cling to your clothes and to everything around the room. You will have to get
used to living in wet clothes. You won't catch cold, though, since there is no
evaporation to use up your heat.

But the water problem outside is not one of mere inconvenience. It never rains.
How can it when the water from the oceans cannot evaporate to form clouds?
Little by little the rivers begin to run dry—there is no rain to feed them. No fog
blows in from the sea; no clouds cool the sun's glare; no dew moistens the
grass at night; no frost shows the coming of cold weather; no snow comes to
cover the mountains. In time there is no water left in the rivers; every lake with
an outlet runs dry. There are no springs, and, after a while, no wells. People
have to live on juicy plants. The crops fortunately require very little moisture,
since none evaporates from them or from the ground in which they grow. And
the people do not need nearly as much water to drink.

Little by little, however, the water all soaks too deep into the ground for the
plants to get it. Gradually the continents become great deserts, and all life
perishes from the land.

All these things would really happen, and many more changes besides, if water
did not evaporate. Yet the evaporation of water is a very simple occurrence. As
the molecules of any liquid bounce around, some get hit harder than others.
These are shot off from the rest up into the air, and get too far away to be
drawn back by the pull of the molecules behind. This shooting away of some
of the molecules is evaporation. And since it takes heat to send these molecules
flying off, the liquid that is left behind is colder because of the evaporation.
That is why you are always cold after you leave the bathtub until you are dry.
The water that evaporates from your body uses up a good deal of your heat.

Fig. 49. Diagram illustrating how in the evaporation of water some of the
molecules shoot off into the air.

Gasoline evaporates more quickly than water. That is why your hands become
so cold when you get them wet with gasoline.

Since heat is required to evaporate a liquid, the quickest way to dry anything is
to warm it. That is why you hang clothes in the sun or by the stove to dry.

Try these experiments:

Experiment 33. Read a thermometer that has been exposed to the room
air. Now dip it in water that is warmer than the air, taking it out again at
once. Watch the mercury. Does the thermometer register a higher or a
lower temperature than it did at the beginning? What is taking up the heat
from the mercury?

Experiment 34. Put a few drops of water in each of two evaporating
dishes. Leave one cold; warm the other over the burner, but do not heat it
to boiling. Which evaporates more quickly?

Why the sea is salt. You remember various fairy stories about why the sea is
salt. For a long time the saltness of the sea puzzled people. But the explanation
is simple. As the water from the rains seeps through the soil and rocks, it
dissolves the salt in them and continually carries some of it into the rivers. So
the waters of the rivers always carry a very little salt with them out to sea. The
water in the ocean evaporates and leaves the salt behind. For millions of years
this has been going on. So the rivers and lakes, which have only a little salt in
them, keep adding their small amounts to the sea, and once in the sea the salt
never can get out. The oceans never get any fuller of water, because water only
flows into the ocean as fast as it evaporates from the ocean. Yet more salt goes
into the ocean all the time, washed down by thousands of streams and rivers.
So little by little the ocean has been growing more and more salty since the
world began.

Fig. 50. A view of the Dead Sea.

Great Salt Lake and the Dead Sea, unlike most lakes, have no rivers flowing
out of them to carry the salt and water away, but rivers flow into them and
bring along small amounts of salt all the time. Then the water evaporates from
Great Salt Lake and the Dead Sea, leaving the salt behind; and that is why they
are so very salty.

When people want to get the salt out of sea water, they put the sea water in
shallow open tanks and let the water evaporate. The salt is left behind.

Experiment 35. Dissolve some salt in warm water until no more will
dissolve. Pour the clear liquid off into an evaporating dish, being careful
not to let any solid particles of the salt go over. Either set the dish aside
uncovered, for several days, or heat it almost to boiling and let it
evaporate to dryness. What is left in the dish?

Application 28. Some girls were heating water for tea, and were in a
hurry. They had only an open stew pan to heat the water in.

"Cover the pan with something; you'll let all the heat out!" Helen said.

"No, you want as much heat to go through the water as possible. Leave
the lid off so that the heat can flow through easily," said Rose.

"The water will evaporate too fast if the lid is off, and all the heat will be
used up in making it evaporate; it will take it much longer to get hot
without the lid," Louise argued.

"That's not right," Rose answered. "Boiling water evaporates fastest of all.
We want this to boil, so let it evaporate; leave the lid off."

What should they have done?

Application 29. Two men were about to cross a desert. They had their
supply of water in canvas water bags that leaked just enough to keep the
outside of the bags wet. Naturally they wanted to keep the water as cold
as possible.

"I'm going to wrap my rubber poncho around my water bag and keep the
hot desert air away from the water," said one.

"I'm not. I'm going to leave mine open to the air," the other said.

Which man was right? Why?

Inference Exercise

Explain the following:

151. When you go up high in an elevator, you feel the pressure of the air
in your ears.

152. Water is always flowing into Great Salt Lake; it has no outlet; yet it
is getting more nearly empty all the time.

153. A nail sinks while a cork floats in water.

154. Steep hillsides are paved with cobblestones instead of asphalt.

155. If you place one wet glass tumbler inside another you can pull them
apart only with difficulty, and frequently you break the outer one in the
attempt.

156. Sausages often break their skins when they are being cooked.

157. A drop of water splashed against a hot lamp chimney cracks it.

158. When you shoot an air gun, the air is compressed at first; then when
it is released it springs out to its original volume and throws the bullet
ahead of it.

159. Leather soles get wet through in rainy weather, while rubbers remain
perfectly dry on the inside.

160. When you want to clean a wooden floor, you scrub it with a brush.

Section 19. Boiling and condensing.

What makes a geyser spout?

How does a steam engine go?

Once more let us imagine we are looking at molecules of water through our
magical microscope. But this time suppose that the water has been made very
hot. If we could watch the molecules smash into each other and bound about
more and more madly, suddenly we should see large numbers of them go
shooting off from the rest like rifle bullets, and they would fly out through the
seemingly great spaces between the slower molecules of air. This would mean
that the water was boiling and turning to steam.

Here are a couple of experiments that will show you how much more room
water takes when it turns to steam than while it remains just water:

Experiment 36. Pour a half inch of water into the bottom of a test tube.
Put a cork in the test tube so tightly that it will not let any steam pass it,
but not too tightly. Hold the test tube with a test-tube clamp at arm's
length over a flame, pointing the cork away from you. Wait for results.

The reason the cork flew out of the test tube is this: Steam takes a great deal
more room than water does,—many times as much room; so when the water in
the test tube turned to steam, the steam had to get out and pushed the cork out
ahead of it.

Fig. 51. In a minute the cork will fly out.

Experiment 37. Pour about half an inch of water into the bottom of a
flask. Bring it to a vigorous boil over the burner and let it boil half a
minute. Now take the flask off the flame and quickly slip the mouth of a
toy balloon over the mouth of the flask. Watch what happens. If things go
too slowly, you can speed them up by stroking the outside of the flask
with a cold, wet cloth.

When the balloon has been drawn into the flask as far as it will go, you
can put the flask back on the burner and heat the water till it boils. When
the balloon has been forced out of the flask again and begins to grow
large, take the flask off the burner. Do this before the balloon explodes.

The reason the balloon was drawn into the flask was that the steam in the flask
turned back to water as it cooled, and took very much less space. This left a
vacuum or empty space in the flask. What pushed the balloon into the empty
space?

Fig. 52. A toy balloon has been slipped over the mouth of a flask that is
filled with steam.

Fig. 53. As the steam condenses and leaves a vacuum, the air pressure
forces the balloon into the flask.

How steam makes an engine go. The force of steam is entirely due to the fact
that steam takes so much more room than the water from which it is made. A
locomotive pulls trains across continents by using this force, and by the same
force a ship carries thousands of tons of freight across the ocean. The engines
of the locomotive and the ship are worked by the push of steam. A fire is built
under a boiler. The water is boiled; the steam is shut in; the only way the steam
can get out is by pushing the piston ahead of it; the piston is attached to
machinery that makes the locomotive or ship move.

One theory about the cause of volcanoes. The water that sinks deep down
into some of the hot parts of the earth turns to steam, takes up more room, and
forces the water above it out as a geyser. It is thought by some scientists that
volcanoes may be started by the water in the ocean seeping down through
cracks to hot interior parts of the world where even the stone is melted; then
the water, turning to steam, pushes its way up to the surface, forcing dust and
stone ahead of it, and making a passage up for the melted stone, or lava. The
persons who hold this view call attention to the fact that volcanoes are always
in or near the sea. If this is the true explanation of volcanoes, then we should
have no volcanoes if steam did not take more room than does the water from
which it comes.

Here is a very practical fact about boiling water that many people do not know;
and their gas bills would be much smaller if they knew it. Try this experiment:

Fig. 54. Will boiling water get hotter if you make it boil harder?

Experiment 38. Heat some water to boiling. Put the boiling-point
thermometer into the water (the thermometer graduated to 110°
Centigrade and 220° Fahrenheit), and note the temperature of the boiling
water. Turn up the gas and make the water boil as violently as possible.
Read the thermometer. Does the water become appreciably hotter over the
very hot fire than it does over the low fire, if it is boiling in both cases?
But in which case is more steam given off? Will a very hot fire make the
water boil away more rapidly than a low fire?

When you are cooking potatoes, are you trying to keep them very hot or are
you trying to boil the water away from them? Which are you trying to do in
making candy, to keep the sugar very hot or to boil the water away from it?

All the extra heat you put into boiling water goes toward changing the water
into steam; it cannot raise the water's temperature, because at the moment
when water gets above the boiling point it ceases to be water and becomes
steam. This steam takes up much more room than the water did, so it passes off
into the air. You can tell when a teakettle boils by watching the spout to see
when the steam3 pours forth from it in a strong, steady stream. If the steam
took no more room than the water, it could stay in the kettle as easily as the
water.
Footnote 3: What you see is really not the steam, but the vapor formed as the steam
condenses in the cool room. The steam itself is invisible, as you can tell by looking
at the mouth of the spout of a kettle of boiling water. You will see a clear space
before the white vapor begins. The clear space is steam.

Distilling. When liquids are mixed together and dissolved in each other, it
looks as if it would be impossible to take them apart. But it isn't. They can
usually be separated almost perfectly by simply boiling them and collecting
their vapor. For different substances boil at different temperatures just as they
melt at different temperatures. Liquid air will boil on a cake of ice; it takes the
intense heat of the electric furnace to boil melted iron. Alcohol boils at a lower
temperature than water; gasoline boils at a lower temperature than kerosene.
And people make a great deal of practical use of these facts when they wish to
separate substances which have different boiling temperatures. They call this
distilling. You can do some distilling yourself and separate a mixture of
alcohol and water in the following manner:

Experiment 39. First, pour a little alcohol into a cup—a few drops is
enough—and touch a lighted match to it. Will it burn? Now mix two
teaspoonfuls of alcohol with about half a cup of water and enough blueing
to color the mixture. Pour a few drops of this mixture into the cup and try
to light it. Will it burn?

Fig. 55. By distillation clear alcohol can be separated from the water
and red ink with which it was mixed.

Now pour this mixture into a flask. Pass the end of the long bent glass rod
(the "worm") through a one-hole rubber stopper that will fit the flask (Fig.
55). Put the flask on a ring stand and, holding it steady, fasten the neck of
the flask with a clamp that is attached to the stand. Put the stopper with
the worm attached into the flask, and support the worm with another
clamp. Put a dry cup or beaker under the lower end of the worm. Set a
lighted burner under the flask. When the mixture in the flask begins to
boil, turn the flame down so that the liquid will just barely boil; if it boils
violently, part of the liquid splashes up into the lower end of the worm.

As the vapor rises from the mixture and goes into the worm, it cools and
condenses. When several drops have gone down into the cup, try lighting
them. What is it that has boiled and then condensed: the water, the
alcohol, or the blueing? Or is it a mixture of them?

Alcohol is really made in this way, only it is already mixed in the water in
which the grains fermented and from which people then distil it. Gasoline and
kerosene are distilled from petroleum; there is a whole series of substances that
come from the crude oil, one after the other, according to their boiling points,
and what is left is the foundation for a number of products, including paraffine
and vaseline.

Experiment 40. Put some dry, fused calcium chlorid on a saucer and set
it on the plate of the air pump. This is to absorb the moisture when you do
the experiment. (This calcium chlorid is not the same as the chlorid of
lime which you buy for bleaching or disinfecting.) Fill a flask or beaker
half full of water and bring it to a boil over a Bunsen burner. Quickly set
the flask on the plate of the air pump. The water will stop boiling, of
course. Cover the flask and the saucer of calcium chlorid with the bell jar
immediately, and pump the air out of the jar. Watch the water.

The water begins to boil again because water will boil at a lower temperature
when there is less air pressure on its surface. So although the water is too cool
to boil in the open air, it is still hot enough to boil when the air pressure is
partially removed. It is because of this that milk is evaporated in a vacuum for
canning; it is not necessary to make it so hot that it will be greatly changed by
the heat, if the boiling is done in a vacuum. On a high mountain the slight air
pressure lets the water boil at so low a temperature that it never becomes hot
enough to cook food.

Application 30. Two college students were short of money and had to
economize greatly. They got an alcohol lamp to use in cooking their own
breakfasts. They planned to boil their eggs.

"Let's boil the water gently, using a low flame," one said; "we'll save
alcohol."

"It would be better to boil the eggs fast and get them done quickly, so that
we could put the stove out altogether," the other replied.

Which was right?

Application 31. Two girls were making candy. They put a little too much
water into it.

"Let us boil the candy hard so that it will candy more quickly," said one.

"Why, you wasteful girl," said the other. "It cannot get any hotter than the
boiling point anyhow, so you can't cook it any faster. Why waste gas?"

Which girl was right?

Inference Exercise

Explain the following:

161. Warm air rises.

162. The lid of a teakettle rattles.

163. Heating water makes a steam engine go.

164. When an automobile with good springs and without shock absorbers
goes over a rut, the passengers do not get a jolt, but immediately
afterward bounce up into the air.

165. Comets swing around close to the sun, then off again into space; how
do they get away from the sun?

166. When you wish to pour canned milk out, you need two holes in the
can to make it flow evenly.

167. Liquid air changes to ordinary air when it becomes even as warm as
a cake of ice.

168. Skid chains tend to keep automobiles from skidding on wet
pavement.

169. A warm iron and a blotter will take candle grease out of your clothes.

170. Candies like fudge and nougat become hard and dry when left
standing several days open to the air.

Section 20. Conduction of heat and convection.

Why does a feather comforter keep you so warm?

When you heat one end of a nail, how does the heat get through to the
other end?

How does a stove make the whole room warm?

Here is a way to make heat run a race. See whether the heat that goes through
an iron rod will beat the heat that goes through a glass rod, or the other way
round:

Fig. 56. The metal balls are fastened to the iron and glass rods with drops
of wax.

Experiment 41. Take a solid glass rod and a solid iron rod, each about a
quarter inch in diameter and about 6 inches long. With sealing wax or
candle grease stick three ball bearings or pieces of lead, all the same size,
to each rod, about an inch apart, beginning 2 inches from the end. Hold
the rods side by side with their ends in a flame, and watch the balls fall
off as the heat comes along through the rods. The heat that first melts off
the balls beats.

Fig. 57. Does the heat travel faster through the iron or through the glass?

What really happens down among the molecules when the heat travels along
the rods is that the molecules near the flame are made to move more quickly;
they joggle their neighbors and make them move faster; these joggle the ones
next to them, and so on down the line. Heat that travels through things in this
way is called conducted heat. Anything like iron, that lets the heat travel
through it quickly, is called a good conductor of heat. Anything like glass, that
allows the heat to travel through it only with difficulty, is called a poor
conductor of heat, or an insulator of heat.

A silver spoon used for stirring anything that is cooking gets so hot all the way
up the handle that you can hardly hold it, while the handle of a wooden spoon
never gets hot. Pancake turners usually have wooden handles. Metals are good
conductors of heat; wood is a poor conductor.

An even more obvious example of the conducting of heat is seen in a stove lid;
your fire is under it, yet the top gets so hot that you can cook on it.

When anything feels hot to the touch, it is because heat is being conducted to
and through your skin to the sensitive little nerve ends just inside. But when
anything feels cold, it is because heat is being conducted away from your skin
into the cold object.

Air carries heat by convection. One of the poorest conductors of heat is air;
that is, one particle of air can hardly give any of its heat to the next particle.
But particles of air move around very easily and carry their heat with them;
and they can give the heat they carry with them to any solid thing they bump
into. So when air can move around, the part that is next to the stove, for
instance, becomes hot; this hot air is pushed up and away by cold air, and
carries its heat with it. When it comes over to you in another part of the room,
some of its heat is conducted to your body. When air currents—or water
currents, which work the same way—carry heat from one place to another like
this, we say that the heat has traveled by convection.

Fig. 58. Convection currents carrying the heat of the stove about the
room.

Since heat is so often carried to us by convection,—by warm winds, warm air
from the stove, warm ocean currents, etc.,—it seems as if air must be a good
conductor of heat. But if you shut the air up into many tiny compartments, as a
bird's feathers do, or as the hair on an animal's back does, so that it cannot

circulate, the passage of heat is almost completely stopped. When you use a
towel or napkin to lift something hot, it is not so much the fibers of cotton
which keep the heat from your hand; it is principally the very small pockets of
air between the threads and even between the fibers of the threads.

Cold the absence of heat. Cold is merely the
absence of heat; so if you keep the heat from
escaping from anything warm, it cannot become
cold; while if you keep the heat from reaching a
cold thing it cannot become warm. A blanket is just
as good for keeping ice from melting, by shutting
the heat out, as it is for keeping you warm, by
holding heat in.

Application 32. Explain why ice is packed in
straw or sawdust; why a sweater keeps you
warm.

Select from the following list the good
conductors of heat from the poor conductors
(insulators): glass, silver, iron, wood, straw,
Fig. 59. Diagram of a excelsior, copper, asbestos, steel, nickel, cloth,
hot-water heater. leather.
What makes the
water circulate?
Inference Exercise

Explain the following:

171. If the axle of a wheel is not greased, it swells until it sticks fast in the
hub; this is a hot box.

172. When you have put liquid shoe polish on your shoes, your feet
become cold as it dries.

173. The part of an ice-cream freezer which holds the cream is usually
made of metal, while that which goes outside and contains the ice and salt
is usually made of wood.

174. The steam in a steam radiator rises from a boiler in the basement to
the upper floors.

175. When you throw a ball, it keeps going for a while after it leaves your
hand.

176. Clothes keep you warm, especially woolen clothes.

177. The Leaning Tower of Pisa does not fall over.

178. It is almost impossible to climb a greased pole.

179. Heat goes up a poker that is held in a fire.

180. A child can make a bicycle go rapidly without making his feet go
any faster than if he were walking.

CHAPTER FIVE
RADIANT HEAT AND LIGHT

Section 21. How heat gets here from the sun; why things glow when they
become very hot.

If we were to go back to our imaginary switchboard we should find a switch,
between the heat and the light switches, labeled Radiation. Suppose we turn it
off:

Instantly the whole world becomes pitch dark; so does the sky. We cannot see
the sun or a star; no electric lights shine; and although we can "light" a match,
it gives no light. The air above the burning match is hot, and we can burn our
fingers in the invisible flame, but we can see nothing whatever.

Yet the world does not get cold. If we leave the switch off for years, while the
earth remains in darkness and we all live like blind people, it never gets cold.
Winter and summer are alike, day and night are just the same. Gradually, after
many ages, the ice and snow in the north and in the far south begin to melt as
the warmth from the rest of the world is conducted to the polar regions. And
the heat from the interior of the earth makes all the parts of the earth's surface
warmer. Winds almost stop blowing. Ocean currents stop flowing. The land
receives less rainfall, until finally everything turns to a desert; almost the only
rain is on the ocean. Animals die even before the rivers dry up, for the flesh
eaters are not able to see their prey, and since, without light, all green things
die, the animals that live on plants soon starve. Men have to learn to live on
mushrooms, which grow in the dark. The world is plunged into an eternal
warm, pitch-black night.

Fig. 60. It is by radiation that we get all our heat and light from the sun.

Turning off the radiation would cause all these things to happen, because it is
by radiation that we get all our heat from the sun and all our light from any
source. And it is by radiation that the earth loses heat into space in the night
and loses still more heat into space during the winter.

We do not get our heat from the sun by conduction; we cannot, because there
is nothing between us and the sun to conduct it. The earth's air, in amounts
thick enough to count, goes up only a hundred miles or so. It is really just a
thin sort of blanket surrounding the earth. The sun is 93,000,000 miles away.
Between us and the sun there is empty space. There are no molecules to speak
of in that whole vast distance. So if heat traveled only by conduction,—that is,
if radiation stopped,—we should be so completely shut off from the sun that
we should not know there was such a thing.

But even if we filled the space between us and the sun with copper or silver,
which are about the best conductors of heat in the world, it would take the heat
from the sun years and years to be conducted down to us. Yet we know that the
sun's heat really gets to us in a few minutes. This is because heat can travel in a
very much quicker way than by conduction. It radiates through space, just as
light does. And it can come the whole 93,000,000 miles from the sun in about
8 minutes. This is so fast that if it were going around the world instead of
coming from the sun, it would go around 7-1/2 times before you could say

"Jack Robinson,"—really, because it takes you at least one second to say "Jack
Robinson."

We are not absolutely sure how heat gets here so fast. But what most scientists
think nowadays is that there is a sort of invisible rigid stuff, not made of
molecules or of anything but just itself, called ether. (This ether, if there really
is such a thing, is not related at all to the ether that doctors use in putting
people to sleep. It just happens to have the same name.) The ether is supposed
to fill all space, even the tiny spaces between molecules. The fast moving
particles of the sun joggle the ether up there, and make ripples that spread out
swiftly all through space. When those ripples strike our earth, they make the
molecules of earth joggle, and that is heat. The ripples that spread out from the
sun are called ether waves.

But the important and practical fact to know is that there is a kind of heat,
called radiant heat, that can pass through empty space with lightning-like
quickness. And when this radiant heat strikes things, it is partly absorbed and
changed to the usual kind of heat.

This radiant heat is closely related to light. As a matter of fact, light is only the
special kind of ether waves that affect our eyes. Radiant heat is invisible. The
ether waves that are visible we call light. In terms of ether waves, the only
difference between light and radiant heat is that the ripples in light are shorter.
So it is no wonder that when we get a piece of iron hot enough, it begins to
give off light; and we say it is red hot. What happens to the ether is this: As the
molecules of iron go faster and faster (that is, as the iron gets hotter and
hotter), they make the ripples in the ether move more frequently until they get
short enough to be light instead of radiant heat. Objects give off radiant heat
without showing it at all; the warmth that you feel just below a hot flatiron is
mainly radiant heat.

When anything becomes hot enough to glow, we say it is incandescent. That is
why electric lamps are called incandescent lamps. The fine wires—called the
filament—in the lamp get so hot when the electricity flows through them that
they glow or become incandescent, throwing off light and radiant heat.

It is the absorbing of the radiant heat by your hand that makes you feel the heat
the instant you turn an electric lamp on. Try this experiment:

Experiment 42. Turn on an incandescent lamp that is cold. Feel it with
your hand a second, then turn it off at once. Is the glass hot? (The lamp
you use should be an ordinary 25, 40, or 60 watt vacuum lamp.)

The radiant heat from the incandescent filament in the lamp passed right out
through the vacuum of the lamp, and much of it went on through the glass to
your hand. You already know what a poor conductor of heat glass is; yet it lets
a great deal of radiant heat pass through it, just as it does light. As soon as the
lamp stops glowing, the heat stops coming; the glass is not made hot and you
no longer feel any heat. In one way the electric filament shining through a
vacuum is exactly like the sun shining through empty space: the heat from
both comes to us by radiation.

If a lamp glows for a long time, however, the glass really does become hot.
That is partly because there is not a perfect vacuum within it (there is a little
gas inside that carries the heat to the glass by convection), and it is partly
because the glass does not let quite all of the radiant heat and light go through
it, but absorbs some and changes it to the regular conducted heat.

One practical use that is made of a knowledge of the difference between
radiant and conducted heat is in the manufacture of thermos bottles.

Experiment 43. Take a thermos bottle apart. Examine it carefully. If it is
the standard thermos bottle, with the name "thermos" on it, you will find
that it is made of two layers of glass with a vacuum between them. The
vacuum keeps any conducted heat from getting out of the bottle or into it.
But, as you know, radiant heat can flash right through a vacuum. So to
keep it from doing this the glass is silvered, making a mirror out of it. Just
as a mirror sends light back to where it comes from, it sends practically
all radiant heat back to where it comes from. Heat, therefore, cannot get
into the thermos bottle or out of it either by radiation or conduction. And
that is why thermos bottles will keep things very hot or ice-cold for such a
long time.

Fig. 61. How a thermos bottle is made. Notice the double layer of
glass in the broken one.

Fill the thermos bottle with boiling water, stopper it, and put it aside till
the next day. See whether the water is still hot.

If we could make the vacuum perfect, and surround all parts of the bottle, even
the mouth, with the perfect vacuum, and if the mirror were perfect, things put
into a thermos bottle would stay boiling hot or icy cold forever and ever.

Why it is cool at night and cold in winter. It is the radiation of heat from the
earth into space that makes the earth cooler at night and cold in winter. Much
of the heat that the earth absorbs from the sun in the daytime radiates away at
night. And since it keeps on radiating away until the sun brings us more heat
the next day, it is colder just before dawn than at midnight, more heat having
radiated into space.

For the same reason it is colder in January and February than in December. It
is in December that the days are shortest and the sun shines on us at the
greatest slant, so that we get the least heat from it; but we still have left some
of the heat that was absorbed in the summer. And we keep losing this heat by
radiation faster than we get heat from the sun, until almost spring.

Application 33. Distinguish between radiant and conducted heat in each
of the following examples:

(a) The sun warms a room through the window. (b) A room is cooler with
the shades down than up, when the sun shines on the window. (c) But
even with the shades down a room on the sunny side of the house is
warmer than a room on the shady side. (d) When a mirror is facing the
sun, the back gets hot. (e) If you put your hand in front of a mirror held in
the sun, the mirror reflects heat to your hand. (f) If you put a plate on a
steam radiator, the top of the plate gradually becomes hot. (g) If anything
very hot or cold touches a gold or amalgam filling of a sensitive tooth,
you feel it decidedly. (h) The handle of your soup spoon becomes hot
when the bowl of it is in the hot soup. (i) The moon is now very cold,
although it probably was once very hot.

Inference Exercise

Explain the following:

181. Trees bend in the wind, then straighten up again. Why do they
straighten up?

182. A cloth saturated with kerosene and placed in the bottom of a clock
will oil the clockworks above it.

183. In cold weather the doorknob inside the front door is cold.

184. It is cool in the shade.

185. Clothes get hot when you iron them.

186. Potatoes fried in deep fat cook more quickly than those boiled in
water.

187. If you hold your hand near a vacuum electric lamp globe that is
glowing, some of the heat will go out to your hand at once.

188. Rubbing silver with fine powder polishes it.

189. A mosquito can suck your blood.

190. A hot-water tank becomes hot at the top first, then gradually heats
downward. When you light the gas under an ordinary hot-water heater,
the hot water circulates to the top of the boiler, while the cold water from
the boiler pushes into the bottom part of the heater, as shown in Figure 59.
What causes this circulation?

Section 22. Reflection.

How is it that you can see yourself in a mirror?

What makes a ring around the moon?

Why can we see clouds and not the air?

Why is a pair of new shoes or anything smooth usually shiny?

If we turn off a switch labeled Reflection of Light on our imaginary
switchboard, we think at first that we have accidentally turned off Radiation
again, for once more everything instantly becomes dark around us. We cannot
see our hands in front of our faces. Although it is the middle of the day, the sky
is jet black. But this time we see bright stars shining in it. And among them is
the sun, shining as brightly as ever and dazzling our eyes when we look at it.
But its light does no good. When we look down from the sky toward the earth,
everything is so black that we should think we were blind if we had not just
seen the stars and sun.

Groping our way along to an electric lamp, we turn it on. It shines brightly, but
it does not make anything around it light; everything stays absolutely invisible.
It is as if all things in the world except the lights had put on some sort of magic
invisible caps.

We can strike a match and see its flame. We can see a fire on the hearth. We
may feel around for the invisible poker, and when we find it, we may put it in
the fire. When it becomes hot enough, it will glow red and become visible. We
can make a match head glow by rubbing it on a wet finger. We can even see a
firefly, if one comes around. But only those things which are glowing of
themselves, like flames, and red-hot pokers, and fireflies, will be visible.

The reason why practically everything would be invisible if there were no
reflection of light is this: When you look at anything, as a man, for instance,
what you really see is the light that hits him and bounces back (reflects) into
your eyes. Suppose you go into a dark room and turn on an electric light.
Instantly ripples of light flash out from the lamp in every direction. As soon as
they strike the object you are looking at, they reflect (bounce back) from it to
your eyes. When light strikes your eyes, you see.

Of course, when you look at an electric lamp, or a star, or the sun, or anything
that is incandescent (so hot that it shines by its own light), you can see it,
whether reflection exists or not. But most things you look at do not shine by
their own light. This book that you are reading simply reflects the light in the
room to your eyes; it would not give any light in a dark room. The paper
reflects a good deal of light that strikes it, so it looks very light; the print
reflects practically none of the light that strikes it, so it looks dark, or black,
just as a keyhole looks black because it does not reflect any light to your eyes.
But without reflection, the book would be entirely invisible. The only kind of
print you could read if there were no reflection would be the electric signs
made out of incandescent lamps arranged to form letters.

What the ring around the moon is; what sunbeams are. The reason you
sometimes see a ring around the moon is that some of the moonlight reflects
from tiny droplets of water in the air, making them visible. In the same way,
the dust in the air of a room becomes visible when the sun shines through it
and is reflected by each speck of dust; we call it a sunbeam. But we are not
really looking directly at the sunlight; we are seeing the part of the sunlight
that is reflected by the dust specks.

Have you ever noticed that when you stand a little to one side of a mirror
where you cannot see your own image in it, you can sometimes see that of
another person clearly, while he cannot see his own image but can see yours? It
is easy to understand this by comparing the reflection of the light from your
face to his eye and from his face to your eye, to the bouncing of a ball from
one person to another. Suppose you and a friend are standing a little way apart
on sandy ground where you cannot bounce a ball, but that between you there is
a plank. If each of you is standing well away from the plank, neither one of
you can possibly bounce the ball on it in such a way that he can catch it
himself. Yet you can easily bounce it to your friend and he can bounce it to
you.

Fig. 62. The ball bounces from one boy to the other, but it does not return
to the one who threw it.

The mirror is like that plank; it is something that will reflect (bounce) the light
directly. The light from your face goes into the mirror, just as you may throw
the ball against the plank, and the light is reflected to your friend just as the
ball is bounced to him; so he sees your image in the mirror. If he can see you,
you can see him, just as when you bounce the ball to him he can bounce it to
you. But you may be unable to see yourself, just as you may be unable to
bounce the ball on the plank so that you yourself can catch it.

In other words, when light strikes against something it bounces away, just as a
rubber ball bounces from a smooth surface. If you throw a ball straight down,
it comes straight up; if light shines straight down on a flat, smooth surface, it
reflects straight up. If you throw a ball down at a slant, it bounces up at the
same slant in the opposite direction; if light strikes a smooth surface at a slant,
it reflects at the same slant in the opposite direction.

Fig. 63. In the same way, the light bounces (reflects) from one boy to the
other. It does not return to the point from which it started and neither boy
can see himself.

But to reflect light directly and to give a clear image, the surface the light
strikes must be extremely smooth, just as a tennis court must be fairly smooth
to make a tennis ball rebound accurately. Any surface that is smooth enough
will act like a mirror, although naturally, if it lets most of the light go through,
it will not reflect as well as if it sends all the light back. A pane of glass is very
smooth, and you can see yourself in it, especially if there is not much light
coming through the glass from the other side to mix up with your reflection.
But if the pane of glass is silvered so that no light can get through, you have a
real mirror; most of the light that leaves your face is reflected to your eyes
again.

Why smooth or wet things are shiny. When a surface is very smooth, we say
it is shiny or glossy. Even black shoes, if they are polished, become smooth
enough to reflect much of the light that strikes them; of course the parts where
the light is being reflected do not look black but white, as any one who has
tried to paint or draw a picture of polished shoes knows. Anything wet is likely
to be shiny, because the surface of water is usually smooth enough to reflect
light rather directly.

If a surface is uneven, like a pool with ripples on it, the light reflects unevenly,
and you see a distorted image; your face seems to be rippling and moving in
the water.

Application 34. Some boys were playing
war and were in a ditch that they called a
trench. They wanted to make a simple
periscope so that they could look out of the
ditch at the "enemy" without being in
danger. They had an old stovepipe and a
mirror. Practically all of them agreed that if
the mirror were fixed in the top of the
stovepipe and if they looked up through the
bottom, they would be able to see over the
side of the ditch. But they had an argument
as to how the mirror should be placed. Each
drew a diagram to show how he thought the
mirror should be arranged, using dotted lines
to show how the light would come from the
enemy to their eyes. Three of the diagrams
are shown in Figure 64.

Fig. 64. How should the The boy who drew the first said: "If you
mirror be placed? want to see the enemy, the mirror's got to
face him. Then it will reflect the light down
to your eyes."

The boy who drew the second said: "No, the light would just go back to
him again. The mirror must slant so that the light that strikes it at a slant
will be reflected to your eye at the same slant."

"How could it get to your eye at all," the third boy said, "if the mirror
didn't face you? You've got to have the mirror reflect right down toward
your face. Then all the light that strikes it will come down to you."

Which arrangement would work?

Inference Exercise

Explain the following:

191. Your hands do not get wet when you put them into mercury.

192. When beating hot candy, we sometimes put it in a pan of water.

193. Electric stoves frequently have bright reflectors.

194. We put ice in the top of a refrigerator.

195. You can jack up the back part of an automobile when you could not
possibly lift it up.

196. The sun shines up into your face and sunburns you when you are on
the water.

197. People in the tropics dress largely in white.

198. Menthol rubbed into your skin makes it feel very cold afterward.

199. We feel the heat of the sun almost as soon as the sun rises.

200. You can shoot a stone far and hard with a sling shot.

Section 23. The bending of light: Refraction.

How do glasses help your eyes?

On a hot day, how is it that you see "heat waves" rising from the street?

What makes the stars twinkle?

Light usually travels in straight lines. If the light from an object comes from
straight in front of you, you know that the object is straight in front of you. But
you can bend light so that it seems to come from a different place, thus making
things seem to be where they are not.

Experiment 44. Hold a triangular glass prism vertically (straight up and
down) in front of one eye, closing the other eye. Look through the prism,
turning it or your head around until you see a chair through it. Watch only

the chair through the prism. When you are sure you know just where it is,
try to sit down in it.

Now look for a pencil or a piece of chalk through the prism, in the same
way. When you think you know where it is, try to pick it up.

The reason the chalk and chair seem to be where they are not is that the prism
bends the light that comes from them and makes the light seem to come from
somewhere else.

As you already know, when you look at a chair you see the light that reflects
from it. You judge where the chair is by the direction from which the light is
coming when it reaches your eye. But if the light is bent on its way, so that it
comes to your eye as it ordinarily comes from an object off to one side,
naturally you think the thing you are looking at is off to one side. Maybe the
diagram (Fig. 65) will make this clearer.

Fig. 65. In passing through the prism the light is bent so that an object at b
appears to be at c.

Here in a is an object the same height as the eye. The light comes straight to
the eye, and one knows that the object is level with the eye. In b the object is in
the same position as in a, but the prism bends the light so that it strikes the eye
with an upward slant. So the person thinks the object is below the eye at c.

Here is another experiment with bending light:

Experiment 45. Fill a china cup with water. Put a pencil in it, letting the
pencil rest at a slant from left to right. Lower your head until it is almost
level with the surface of the water. How does the pencil look?

Fig. 66. The pencil is not bent, but the light that comes from it is.

The reason the pencil looks bent is because the light from the part of it under
the water is bent when it passes from the water into the air on its way to your
eye; so the slant at which it comes to your eye is the same slant at which it
ordinarily would come from a bent pencil.

Experiment 46. Fill a glass with water. Put the pencil into it in the same
way you put it in the cup in the previous experiment, letting the pencil
slant from left to right. Lower your head this time until it is on a level
with the water in the glass, and look through the glass and water at the
pencil. Notice what happens where the pencil goes into the water.

What you see is explained in the same way as are the things that took place in
the other experiments in refraction, or bending of light. The light from the part
of the pencil above the water comes straight to your eye, of course; so you see
it just as it is. But the light from the part of the pencil in the water is bent when
it comes out of the water into the air on its way to your eye. This makes it

come to your eye from a different direction and makes the lower part of the
pencil seem to be in a place to one side of the place where it really is. The
pencil, therefore, looks broken.

Fig. 67. The bending of the light by the water in the glass causes the penci
to look broken.

Whenever light passes first through something dense like water or glass, and
then through something rare or thin like air, it is bent one way; whenever it
passes from a rare medium into a dense one, it is bent the other way. Light
passing from a fish to your eye is bent one way; light passing from you to the
fish's eye is bent the other way, but the main point is that it is bent. And when
light is bent before reaching your eyes it usually makes things seem to be
where they are not.

If light goes through a perfectly smooth, flat pane of glass, it is bent one way
when it goes into the glass and back the other way when it comes out; so it
seems to be perfectly straight and we see things practically as they are through
a good window. But if the window glass has flaws in it, so that some parts are
a little thicker than others, the uneven parts act like prisms and bend the light
to one side. This makes anything we look at through a poor window seem bent
out of shape. Of course the things are not bent any more than your pencil in the
water was bent, but they look misshapen because the light from them is bent;
the reflected light is all we see of things anyway.

The air itself is uneven in a way. The parts of
the air that are warm, as you already know, are
thinner and more expanded than are the cold
parts. So light going from cold air into warm or
from warm air into cold, will be bent. And this
is why you see what are called "heat waves"
above a stove or rising from a hot beach or
sidewalk. Really these are just waves of hot air
rising, and they bend the light that comes
through them so as to give everything behind
them a wavy appearance.

Stars twinkle for much the same reason. As the
starlight comes down through the cold air and
then through the warm air it is bent, and the
star seems to be to one side of where it really
is; but the air does not stand still,—sometimes
it bends the light more and sometimes less. So
Fig. 68. The light is bent
the star seems to move a little back and forth.
when it enters a window
And this is what we call "twinkling." Really it
pane and is bent again in
is the bending of light.
the opposite direction
when it leaves it. Application 35. Explain why an unevenness
in your eye will keep you from seeing
clearly; how glasses can help this; why good
mirrors are made from plate glass, which is very smooth, instead of from
the cheaper and more uneven window glass; why fishes in a glass tank
appear to be where they are not.

Inference Exercise

Explain the following:

201. The fire in the open fireplace ventilates a room well by making air
go up the chimney.

202. A drop of water glistens in the sun.

203. Dust goes up to the ceiling and clings there.

204. When you look at a person under moving water, his face seems
distorted.

205. You sit in the sun to dry your hair.

206. Paste becomes hard and unfit for use when left open to the air.

207. In laundries clothes are partly dried by whirling them in perforated
cylinders.

208. Circus balloons are filled by building a big fire under them.

209. Unevenness in a window pane makes telephone wires seen through it
look crooked and bent.

210. You can see the image of a star even in a shallow puddle.

Fig. 69. When the light from one point goes through the lens, it is bent and
comes together at another point called the focus.

Section 24. Focus.

How can you take pictures with a camera?

What causes the picture in the camera to be inverted?

Why is a magnifying glass able to set things on fire when you let the sun
shine through it?

In your eye, right back of the pupil, there is a flattened ball, as clear as glass,
called the lens. If the lens were left out of your eye, you never could see
anything except blurs of light and shadow. If you looked at the sun it would
dazzle you practically as much as it does now. However, you would not see a
round sun, but only a blaze of light. You could tell night from day as well as
any one, and you could tell when you stepped into the shade. If some one
stepped between you and the light, you would know that some one was
between you and the light or that a cloud had passed over the sun,—you could
not be quite sure which. In short, you could tell all degrees of light and dark
apart nearly as well as you can now, but you could not see the form of
anything.

In the front of a camera
there is a flattened glass
ball called the lens. If you
were to remove it, the
camera would not take any
pictures; it would take a
blur of light and shade and
nothing more.

In front of a moving-
picture machine there is a
large lens, a piece of glass
rounded out toward the
middle and thinner toward
the edges. If you were to
take that lens off while the
machine was throwing the
motion pictures on the Fig. 70. The light from each point of the
screen, you would have a candle flame goes out in all directions.
flicker of light and shade,
but no picture.

It is the lens that forms the pictures in your eye, on a photographic plate or
film, and on a moving-picture screen. And a lens is usually just a piece of glass
or something glassy, rounded out in such a way as to make all the spreading
light that reaches it from one point come together in another point, as shown in
Figure 69.

As you know, when light goes out from anything, as from a candle flame or an
incandescent lamp, or from the sun, it goes in all directions. If the light from
the point of a candle flame goes in all directions, and if the light from the base
of the flame also goes in all directions, the light from the point will get all
mixed up with the light from the base, as shown in Figure 70. Naturally, if the
light from the point of the candle flame is mixed up with the light from the
base and the beams are all crisscross, you will not get a clear picture of the
flame.

Fig. 71. The reading glass is a lens which focuses the light from the candle
flame and forms an image.

Experiment 47. Fasten a piece of paper against a wall and place a lighted
candle about 4 feet in front of it. Look at the paper. Is there any picture of
the candle flame on it? Now hold a magnifying glass (reading glass) near
the candle, between the candle and the paper, so that the light will shine
through the lens on to the paper. (The magnifying glass is a lens.) Move
the lens slowly toward the paper until you get a clear picture of the candle
flame. Is it right side up or upside down?

The lens has brought the light from the candle flame to a focus; all the light
that goes through the lens from one point of the flame has been brought
together at another point (Fig. 72). In the diagram you see all the light from the
point of the candle flame spreading out in every direction. But the part that

goes through the lens is brought together at one point, called the focus. Of
course the same thing happens to the light from the base of the candle flame
(Fig. 73). Just as before, all the light from the base of the flame is brought to a
focus. The light spreads out until it reaches the lens. Then the lens bends it
together again until it comes to a point.

Fig. 72. The light from the tip of the candle flame is focused at one point.

Fig. 73. And the light from the base of the flame is focused at another
point.

Fig. 74. The light from the tip and base (and from every other point) of th
flame is, of course, focused at the same time. In this way an image of the
flame is formed.

But of course the light from the base of the flame is focused at the same time
as the light from the point; so what really happens is that which is illustrated in
Figure 74. In this diagram, we have drawn unbroken lines to show the light
from the point of the candle flame and dotted lines to show the light from the
base of the flame. This is so that you can follow the light from each part and
see where it goes. Compare this diagram with the one where the light is shown
all crisscrossed (Fig. 70), and you will see why the lens makes an image, while
you have no image without it.

By looking at the last diagram (Fig. 74) you can also see how the image
happens to be upside down.

Experiment 48. Set up the candle and piece of paper as you did for the
last experiment, but move the magnifying glass back and forth between
the paper and the candle. Notice that there is one place where the image
of the candle is very clear. Does the image become clearer or less clear if
you move the lens closer to the candle? if you move it farther from the
candle?

The explanation is this: After the light comes together into a point, it spreads
out again beyond the point, as shown in Figure 75. So if you hold the lens in
such a way that the light comes to a focus before it reaches the paper, the paper

will catch the spreading light and you will get a blur instead of a sharp image.
It is as shown in Figure 76.

Fig. 75. The light spreads out again beyond the focus.

Fig. 76. So if the light comes to a focus before it reaches the paper, the
image will be blurred.

On the other hand, if you hold your lens in such a way that the light has not yet
come to a focus when it reaches the paper, naturally you again have a blur of
light instead of a point, and the image is not sharp and definite (Fig. 77).

Fig. 77. Or if the light reaches the paper before it comes to a focus, the
image will be blurred.

And that is why good cameras have the front part, in which the lens is set,
adjustable; you can move the lens back and forth until a sharp image is formed
on the plate. Motion-picture machines and stereopticons likewise have lenses
that can be moved forward and back until they form a sharp focus on the
screen. Even the lens in your eye has muscles that make it flatter and rounder,
so that it can make a clear image on the sensitive retina in the back of your
eye. The lens in the eyes of elderly people often becomes too hard to be
regulated in this way, and so they have to wear one kind of glasses to see
things near them clearly and another kind to see things far away.

The kind of lens we have been talking about is the convex lens. "Convex"
means bulging out in the middle. There are other kinds of lenses, some flat on
one side and bulging out on the other, some hollowed out toward the middle
instead of bulging, and so on. But the only lens that most people make much
use of (except opticians) is the convex lens that bulges out toward the center.
The convex lens makes a clear image and it is the only kind of lens that will do
this.

Fig. 78. Lenses of different kinds.

Why you can set fire to paper with a magnifying glass. A convex lens brings
light to a focus, and it also brings radiant heat to a focus. And that is why you
can set fire to things by holding a convex lens in the sunlight so that the light
and heat are focused on something that will burn. All the sun's radiant heat that
strikes the lens is brought practically to one point, and all the light which is
absorbed at this point is changed to heat. When so much heat is concentrated at
one point, that point becomes hot enough to catch fire.

Application 36. Explain why there is a lens in a moving-picture machine;
why a convex lens will burn your hand if you hold it between your hand
and the sun; why the front of a good camera is made so that it can be
moved closer to the plate or farther away from it, according to the
distance of the object you are photographing; why there is a lens in your
eye.

Inference Exercise

Explain the following:

211. Cut glass ware sparkles.

212. An unpainted floor becomes much dirtier and is harder to clean than
a painted one.

213. If you sprinkle wet tea leaves on a rug before sweeping it, not so
much dust will be raised.

214. Food leaves a spoon when the spoon is struck sharply upon the edge
of a stewpan.

215. An image is formed on the photographic plate of a camera.

216. Ripples in a pool distort the image seen in it.

217. Cream rises to the top of a bottle of milk.

218. Your eyes have to adjust themselves differently to see things near by
and to see things at a distance.

219. A vacuum cleaner does not wear out a carpet nearly as quickly as a
broom or a carpet sweeper does.

220. You can see a sunbeam in a dusty room.

Section 25. Magnification.

Why is it that things look bigger under a magnifying glass than under
other kinds of glass?

How does a telescope show you the moon, stars, and planets?

How does a microscope make things look larger?

Everybody knows, of course, that a convex lens in the right position makes
things look larger. People use convex lenses to make print look larger when
they read, and for that reason such lenses are often called reading glasses. For
practical purposes it is not necessary to understand how a convex lens
magnifies; the important thing is the fact that it does magnify. But you may be
curious to know just how a magnifying glass works.

First, you should realize that the image formed by a convex lens is not always
larger than the object. Repeat Experiment 41, but this time move the lens from
near the candle toward the paper, past the point where it makes its first clear
image. Keep moving the lens slowly toward the paper until a second image is
formed. Which image is larger than the flame? Which is smaller?

Fig. 79. A section of the eye.

The important point in this experiment is for you to see that if the lens is nearer
to the image on the paper than it is to the candle, the image is smaller than the
candle. That is why a photograph is usually smaller than the thing
photographed; it would be impossible to take a picture of a house or a
mountain if the lens in the camera gave a magnified image.
4
Your eye is a small camera. It has a lens in the front; it is lined with black;
and at the back there is a sensitive part on which the picture is formed. This
sensitive part of the eye is called the retina. It is in the back part of your

eyeball and is made of many very sensitive nerve endings. When the light
strikes these nerve endings, it sends an impulse through the nerves to the back
part of the brain; then you know that the image is formed. And, of course,
since your eyeball is small and many of the things you see are large, the image
on the retina must be much smaller than the object itself, and this is because
the lens is so much nearer to the retina than it is to the object.
Footnote 4: The following explanation may be omitted by any children who are not
interested in it. Let such children skip to the foot of page 156.

Fig. 80. How an image is formed on the retina of the eye.

Fig. 81. A simpler diagram showing how an image is formed in the eye.

Fig. 82. A diagram showing how a reading glass causes things to look
larger by making the image on the retina larger.

Fig. 83. Diagram showing how a reading glass enlarges the image on the
retina. More lines are drawn in than in Figure 82.

You can understand magnification best by looking at Figures 80, 81, 82, and
83.

In Figure 80 there are a candle flame, the lens of an eye, and the retina on
which the image is being formed.

Figure 81 is the same as Figure 80, with all the lines left out except the outside
ones that go to the lens. It is shown in this way merely for the sake of
simplicity. All the lines really belong in this diagram as in the first. In both
diagrams the size of the image on the retina is the distance between the point
where the top line touches it and the point where the bottom line touches it.

In order to make anything look larger, we must make the image on the retina
larger. A magnifying glass, or convex lens, if put in the right place, will do this.
In the next diagram, Figure 82, we shall include the magnifying glass, leaving
out all lines except the two outside ones shown in Figure 81.

You will notice that the magnifying glass starts to bend the lines together, and
that the lens in the eye bends them farther together; so they cross sooner, and
the image is larger. Figure 83 shows more of the lines drawn in.

Fig. 84. Diagram of a microscope.

The two important points to notice are these: First, the magnifying glass is too
close to the eye for the light to be brought to a focus before it reaches the eye;
the light is bent toward a focus, but it reaches the eye before the focus is
formed. The focus is formed for the first time on the retina itself. Second, the
magnifying glass bends the light on its way to your eye so that the light crosses
sooner in your eye and spreads out farther before it comes to a focus. This
forms the larger image, as you see in the simple diagram, Figure 82.

Fig. 85. This is the way a concave mirror forms a magnified image.

Fig. 86. The concave mirror forms an image of the burning candle.

How the microscope works. But the microscope is different. It works like
this: The first lens is put very near the object which you are examining. This
lens brings the light from the object to a focus and forms an image, much
larger than the object itself, high up in the tube. If you held a piece of paper
there you would see the image. But since there is nothing there to stop the
light, it goes on up the tube, spreading as it goes. Then there is another lens
which catches this light and bends it inward on its way to your eye, just as any
magnifying glass does. Next the lens in the eye forms an image on the retina.
The diagram (Fig. 84) will make this clearer. (A real microscope is not so
simple, of course, and usually has two lenses wherever the diagram shows
one.) What actually happens is that the first lens makes an image many times
as big as the object; then you look at this image through a magnifying glass, so
that the object is made to look very much larger than it really is. That is why
you can see blood corpuscles and germs and cells through a microscope, when
you cannot see them at all with your naked eye.

Fig. 87. The great telescope of the Yerkes Observatory at Lake Geneva,
Wisconsin.

A mirror that magnifies. A convex lens is not the only thing that can magnify.
A concave mirror, which is one that is hollowed out toward the middle, does
the same thing. When light is reflected by such a mirror, it acts exactly as if it
had gone through a convex lens (Fig. 85).

Experiment 49. Place the lighted candle and the paper about 4 feet apart,
as you did in Experiment 47. Hold a concave mirror back of the candle
(so that the candle is between the mirror and the paper); then move the
mirror back, the mirror casting the reflection of the candle light on the
paper, until a clear image of the candle is formed.

Look at your image in the concave mirror. Does it look larger or smaller
than you?

How telescopes are made. Astronomers use convex lenses in some of their
telescopes; in others, called reflecting telescopes, they use concave mirrors.

Both do the same work, making the moon, the planets, and the sun look much
larger than they otherwise would.

Application 37. Explain how a reading glass makes print look larger; how
you can see germs through a microscope; what kind of mirror will
magnify; what kind of lens will magnify.

Inference Exercise

Explain the following:

221. The water that forms rain comes from the ocean, yet the rain is not
salty.

222. Iron glows when it is very hot.

223. You can start a fire with sunlight by holding a reading glass at the
right distance above the fuel.

224. Big telescopes make it possible for us to see in detail the surface
structure of the moon.

225. A room is lighter if it has white walls than if it has dark walls.

226. Iron is heated by a blacksmith before he shapes it.

227. A dentist's mirror is concave; he sees your teeth enlarged in it.

228. Good penholders usually have cork or rubber tips.

229. A man's suit becomes shiny when it gets old.

230. When you look at a window from the sidewalk, you frequently see
images of the houses across the street.

Section 26. Scattering of light: Diffusion.

Why is it that on a dark day the sun cannot be seen through light clouds?

Why do not the stars come out in the daytime?

If you were on the moon, you could see the stars in the daytime. The sun
would be shining even more brightly than it does here, but the sky around the
sun would be pitch black, except for the stars shining out of its blackness. The
reason is that there is no air on the moon to scatter the light.

Why we cannot see the stars in the daytime. Most of the sun's light that
comes to the earth reaches us rather directly; that is why we can see the image
of the sun. But part of the sunlight is scattered by particles of air, and that is
why the whole sky is bright in the daytime. You know, of course, that the blue
sky is only the air that surrounds the earth. Enough of the light is scattered
around to make the sky as bright as the stars look from here; so we cannot see
the stars through the sky in the daytime.

How a cloud can hide the sun without cutting off all its light. When a cloud
drifts between us and the sun, we no longer see the sun; yet the earth does not
become dark. The sun's light is evidently still reaching us. The cloud is made
of millions of very tiny droplets of water. When the sunlight strikes the curved
sides of these droplets, it is reflected at all angles according to the way it
strikes, as shown in Figure 89.

Fig. 88. The sunlight is scattered (diffused) by the clouds. The photograph
shows in the foreground the Parliament Buildings, London, England.

Some of the light is reflected back into the sky; that is why everything
becomes darker when the sun goes behind a cloud; but much of the light
comes through to us, at all sorts of slants. When it comes all higgledy-piggledy
and crisscross like this, no lens can put it together again; it is as hopelessly
broken up as Humpty-Dumpty was. But much of the light gets here just the
same; so we see it without seeing the form of the sun. Light that cannot be
brought to a focus is called scattered or diffused light.

When you look through a ground-glass electric lamp, you cannot see the
filament; the light passing through all the rough parts of the glass gets so
scattered that you cannot bring it to a focus. Therefore, no image of the
filament in the incandescent lamp can be formed on the retina of your eye.

Fig. 89. How the droplets in a cloud scatter the rays of light.

A piece of white paper reflects practically all the light that strikes it. Yet you
cannot see yourself in a piece of ordinary white paper. The trouble is that the
paper is too rough; there are too many little uneven places that reflect the light

at all sorts of angles; the light is scattered and the lens in your eye cannot bring
it to a focus.

Application 38. Explain why a scrim curtain will keep people from seeing
into a room, but will not shut the light out; why curtains soften the light of
a room; why indirect lighting (i.e. light thrown up against the ceiling and
then reflected down into the room by the rough ceiling) is better for your
eyes than is the old-time direct lighting.

Inference Exercise

Explain the following:

231. The alcohol formed by the yeast in making bread light is practically
all gone by the time the bread is baked.

232. The oceans do not flow off the earth at the south pole.

233. Lamp globes often have frosted bottoms.

234. A damp dust cloth will take up the dust, without making it fly.

235. The stars twinkle when their light passes through the moving air
currents that surround the earth.

236. Shears for cutting tin and metal have long handles and short blades.

237. A coin at the bottom of a glass of water seems raised when you look
at it a little from one side.

238. You have to brace your feet to row well.

239. Light from the northern part of the sky, where the sun is not, does
not make sharp shadows.

240. Pokers and lifters for stove lids often have open spiral handles.

Section 27. Color.

What makes the ocean look green in some places and blue in others?

What makes the sky blue?

What causes material to be colored?

What makes a rainbow?

What is color?

Fig. 90. Making a rainbow on the wall.

Color is merely a kind of light. We say that a sweater is red; really the sweater
is not red, but the light that it reflects to our eyes is red. We speak of a piece of
red glass, but the glass is not red; it is the light that it lets pass through it that is
red.

White is not really a color; all colors put together make white. Experiments 50
and 51 will prove this.

Fig. 91. The prism separates the white light into the rainbow colors.

Experiment 50. Hold a prism in the sunlight by the window and make a
"rainbow" on the wall. The diagram here shown illustrates how the prism
breaks up the single beam of white light into different-colored beams of
light.

Fig. 92. When the wheel is rapidly whirled the colors blend to make
white.

Experiment 51. Rotate the color disk on the rotator and watch it. Make it
go faster and faster until all the colors are perfectly merged. What color
do you get by combining all the colors of the rainbow? If the colors on
the disk were perfectly clear rainbow colors, in exactly the same

proportion as in the rainbow, the whirling would give a white of dazzling
purity.

Since you can break up pure white light into all the colors, and since you can
combine all the colors and get pure white light, it is clear that white light is
made up of all the colors.

As we have already said, light is probably vibrations or waves of ether. Light
made of the longest waves that we can see is red. If the waves are a little
shorter, the light is orange; if they are shorter yet, it is yellow; still shorter,
green; shorter still, blue; while the shortest waves that we can see are those of
violet light. Black is not a color at all; it is the absence of light. We say the
night is black when we cannot see anything. A deep hole looks black because
practically no light is reflected up from its depths. When you "see" anything
black, you really see the things around it and the parts of it that are not
perfectly black. A pair of shoes, for instance, has particles of gray dust on
them; or if they are very shiny they reflect part of the light that strikes them as
a white high-light. But the really black part of your shoes would be invisible
against an equally black background.

A black thing absorbs the light that strikes it and turns it to heat. Here is an
experiment that will prove this to you:

Experiment 52. (a) On a sunny day, take three bottles, all of the same
size and shape, and pour water out of a pitcher or pan into each bottle. Do
not run the water directly from the faucet into the bottle, because
sometimes that which comes out of the faucet first is warmer or colder
than that which follows; in the pitcher or pan it will all be mixed together,
and so you can be sure that the water in all three bottles is of the same
temperature to begin with. Wrap a piece of white cotton cloth twice
around one bottle; a piece of red or green cotton cloth of the same weight
twice around the second bottle, and a piece of black cotton cloth of the
same weight twice around the third bottle, fastening each with a rubber
band. Set all three bottles side by side in the sunlight, with 2 or 3 inches
of space between them. Leave them for about an hour. Now put a
thermometer into each to see which is warmest and which is least warm.

Fig. 93. Which color is warmest in the sunlight?

From which bottle has most of the light been reflected back into the air by
the cloth around it? Which cloth absorbed most of the light and changed it
into heat? Does the colored cloth absorb more or less light than the white
one? than the black one?

(b) On a sunny day when there is snow on the ground, spread three pieces
of cotton cloth, all of the same size and thickness, one white, one red or
green, and one black, on top of the snow, where the sun shines on them.
Watch them for a time. Under which does the snow melt first?

The white cloth is white because it reflects all colors back at once. It
therefore absorbs practically no light. But the reason the black cloth looks
black is that it reflects almost none of the colors—it absorbs them all and
changes them to heat. The colored cloth reflects just the red or the green
light and absorbs the rest.

Maybe you will understand color better if it is explained in another way.
Suppose I throw balls of all colors to you, having trained you to keep all the
balls except the red ones. I throw you a blue ball; you keep it. I throw a red

ball; you throw it back. I throw a green ball; you keep it. I throw a yellow ball;
you keep it. I throw two balls at once, yellow and red; you keep the yellow and
throw back the red. I throw a blue and yellow ball at the same time; you keep
both balls.

Now suppose I change this a little. Instead of throwing balls, I shall throw
lights to you. You are trained always to throw red light back to me and always
to keep (absorb) all other kinds of light. I throw a blue light; you keep it, and I
get no light back. I throw a red light; you throw it back to me. I throw a green
light; you keep it, and I get no light back. I throw a yellow light; you keep it,
and I get no light back. I throw two lights at the same time, yellow and red;
you keep the yellow and throw back only the red. But yellow and red together
make orange; so when I throw an orange light, you throw back the red part of
it and keep the yellow.

Now if we suppose that instead of throwing lights to you I throw them to
molecules of dye which are "trained" to throw back the red lights and keep all
the other kinds (absorb them and change them to heat), we can understand
what the dye in a red sweater does. The dye is not really trained, of course, but
for a reason which we do not entirely understand, some kinds of dye always
throw back (reflect) any red that is in the light that shines on them, but they
keep all other kinds of light, changing them to heat. Other dyes or coloring
matter always throw back any green that is in the light that shines on them,
keeping the other colors. Blue coloring matter throws back only the blue part
of the light, and so on through all the colors.

So if you throw a white light, which contains all the colors, on a "red" sweater,
the dye in the sweater picks out the red part of the white light and throws that
back to your eyes (reflects it to you) but it keeps the rest of the colors of the
white light, changing them to heat; and since only the red part of the light is
reflected to your eyes, that is the only part of it that you can see; so the sweater
looks red. The "green" substance (chlorophyll) in grass acts in the same way;
only it throws the green part of the sunlight back to your eyes, keeping the rest;
so the part of the light that reaches you from the grass is the green light, and
the grass looks green.

Anything white, like a piece of paper, reflects all the light that strikes it; so if
all the colors (white light) strike it, all are reflected to your eyes and the object
looks white.

You have looked at people under the mercury-vapor lights in photo-postal
studios, have you not? The lights are long, inclined tubes which glow with a
greenish-violet light. No matter how good the color of a person is in ordinary
light, in that light it is ghastly.

Fig. 94. A mercury-vapor lamp.

Go into the kitchen tonight, light a burner of the gas stove, turn out the light
and sprinkle salt on the blue gas flame. The flame will leap up, yellow. Look at
your hands, at some one's lips, at a piece of red cloth, in this light. Does
anything look red?

The reason why nothing looks pink or red in these two kinds of light is this:
The light given by glowing salt vapor or mercury vapor has no red in it; if you
tried to make a "rainbow" from it with a prism, you would find no red or
orange color in it. A thing looks red when it absorbs all the parts of the light
that are not red and reflects the red light to your eyes. If there is no red in the
light to reflect, obviously a thing cannot look red in that light.

When you look through a piece of colored glass, the case is somewhat
different. A piece of blue glass, for instance, acts as a sort of strainer. The
coloring matter in it lets the blue light through it, but it holds back (absorbs)
the other kinds of light. So if you look through a piece of blue glass you see
everything blue; that is, only the blue part of the light from different objects

can reach your eyes through this kind of glass. Anything that is transparent and
colored acts in a similar way.

Why the sky is blue. And that is why the sky looks blue. Air holds back all
colors of light except blue; that is, it holds them back a little. A room full of air
holds the colors back hardly at all. A few miles of air hold them back more;
mountains in the distance look bluish because only the blue light from them
can reach you through the air. The hundred or more miles of air above you
hold back a considerable amount of the other colors of light, letting through
much more of blue than of any other color. So the sky looks blue; that is, when
the air scatters the sunlight above you, it is chiefly the blue parts of the
sunlight that it allows to reach your eyes.

Why bodies of water look green in some places and blue in others. Water
acts in a similar way, but it lets the green light through instead of the blue. A
little water holds back (absorbs) the other colors so slightly that you cannot
notice the effect in a glass of water. But in a swimming tank full of water, or in
a lake or an ocean, you can notice it decidedly when you look straight down
into the water itself.

When you look at a smooth body of water at a slant on a clear day, the blue sky
is reflected to you and the water looks blue instead of green. And it may even
look blue when you look straight down in it if it is too deep for you to see the
bottom and the sky is reflected from the surface.

Why the sky is often red at sunset. Dust lets more of red and yellow light
through than of any other color, and for this reason there is much red and
yellow in the sunset. Just before the sun sets, it shines through the low, dusty
air. The dust filters out most of the light except the red and yellow. The red
light and yellow light are reflected by the clouds (for the clouds are themselves
"white"; that is, they reflect all the colors that strike them), and you have the
beautiful sunset clouds. Sometimes there is a purple in the sunset, and even
green. But since the air itself is blue (that is, it lets mostly blue light go
through), it is easy to see how this blue can combine with the red or yellow
that the dust lets through, to form purple or green.

But we could not have sunset colors or all the colors we see on earth, if it were
not that the sunlight is mostly white—that it contains all colors. And that, too,
is why we can have a rainbow.

How rainbows are formed. You already know fairly well how a rainbow is
formed, since you made an imitation of one with a prism. A rainbow appears in
the sky when the sun shines through the rain; the plain white light of the sun is
divided up into red, orange, yellow, green, blue, indigo, and violet. As the
white light of the sun passes through the raindrops, the violet part of the light
is bent more than any of the rest, the indigo part is bent not quite so much, and
so on to the red, which is bent least of all. So all the colors fan out from the
single beam of white light and form a band of color, which we call the
rainbow.

Fig. 95. Explain why the breakers are white and the sea green or blue.

How we can tell what the sun and stars are made of. When a gas or vapor
becomes hot enough to give off light (when it is incandescent), it does not give
off white light but light of different colors. An experiment will let you see this
for yourself.

Experiment 53. Sprinkle a little copper sulfate (bluestone) in the flame of
a Bunsen burner. What color does it make the flame?

Copper vapor always gives this greenish-blue light when it is heated. The
photographer's mercury-vapor light gave a greenish-violet glow. When you
burn salt or soda in a gas flame, you remember that you get a clear yellow
light. By breaking up these lights, somewhat as you broke up the sunlight with

the prism, chemists and astronomers can tell what kind of gas is glowing. The
instrument they use to break up the light into its different colors is called a
spectroscope, and the band of colors formed is called the spectrum. With the
spectroscope they examine the light that comes from the sun and stars and by
the colors of the spectra they can tell what these far-distant bodies are made of.

Application 39. If you were going to the tropics, would it be better to
wear outside clothes that were white or black?

Application 40. A dancer was to dance in a spotlight on the stage. The
light was to change colors constantly. She wanted her robe to reflect each
color that was thrown on it. Should she have worn a robe of red, yellow,
white, green, or blue?

Application 41. If you looked through a red glass at a purple flower
(purple is red mixed with blue), would the flower look red, blue, purple,
black, or white?

Inference Exercise

Explain the following:

241. Mercury is separated from its ore by heating the ore so strongly that
the mercury rises from it as a vapor.

242. Hothouses are built of glass.

243. A "rainbow" is sometimes seen in the spray of a garden hose.

244. Your feet become hot when your shoes are being polished.

245. Doors into offices usually have windows of ground glass or frosted
glass.

246. Opera glasses are of value to those sitting at a distance from the
stage.

247. In order to see clearly through opera glasses, you have to adjust
them.

248. It is warm inside an Eskimo's hut although it is built of ice and snow.

249. It is usually cooler on a lawn than on dry ground.

250. Black clothes are warmer in the sunlight than clothes of any other
color.

CHAPTER SIX
SOUND

Section 28. What sound is.

What makes a dictaphone or a phonograph repeat your words?

What makes the wind howl when it blows through the branches of trees?

Why can you hear an approaching train better if you put your ear to the
rail?

If you were to land on the moon tonight, and had with you a tank containing a
supply of air which you could breathe (for there is no air to speak of on the
moon), you might try to speak. But you would find that you had lost your
voice completely. You could not say a word. You would open and close your
mouth and not a sound would come.

Then you might try to make a noise by clapping your hands; but that would not
work. You could not make a sound. "Am I deaf and dumb?" you might wonder.

The whole trouble would lie in the fact that the moon has practically no air.
And sound is usually a kind of motion of the air,—hundreds of quick, sharp
puffs in a second, so close together that we cannot feel them with anything less
sensitive than the tiny nerves in our ears.

If you can once realize the fact that sound is a series of quick, sharp puffs of
air, or to use a more scientific term, vibrations of air, it will be easy for you to
understand most of the laws of sound.

A phonograph seems almost miraculous. Yet it works on an exceedingly
simple principle. When you talk, the breath passing out of your throat makes
the vocal cords vibrate. These and your tongue and lips make the air in front of
you vibrate.

When you talk into a dictaphone horn, the vibrating air causes the needle at the
small end of the horn to vibrate so that it traces a wavy line in the soft wax of
the cylinder as the cylinder turns. Then when you run the needle over the line
again it follows the identical track made when you talked into the horn, and it
vibrates back and forth just as at first; this makes the air in the horn vibrate
exactly as when you talked into the horn, and you have the same sound.

All this goes back to the fundamental principle that sound is vibrations of air;
different kinds of sounds are simply different kinds of vibrations. The next
experiments will prove this.

Experiment 54. Turn the rotator rapidly, holding the corner of a piece of
stiff paper against the holes in the disk. As you turn faster, does the sound
become higher or lower? Keep turning at a steady rate and move your
paper from the inner row of holes to the outer row and back again. Which
row has the most holes in it? Which makes the highest sound? Hold your
paper against the teeth at the edge of the disk. Is the pitch higher or lower
than before? Blow through a blowpipe against the different rows of holes
while the disk is being whirled. As the holes make the air vibrate do you
get any sound?

This experiment shows that by making the air vibrate you get a sound.

The next experiment will show that when you have sound you are getting
vibrations.

Experiment 55. Tap a tuning fork against the desk, then hold the prongs
lightly against your lips. Can you feel them vibrate? Tap it again, and hold
the fork close to your ear. Can you hear the sound?

Fig. 96. An interesting experiment in sound.

The experiment which follows will show that we usually must have air to do
the vibrating to carry the sound.

Experiment 56. Make a pad of not less than a dozen thicknesses of soft
cloth so that you can stand an alarm clock on it on the plate of the air
pump. The pad is to keep the vibrations of the alarm from making the
plate vibrate. A still better way would be to set a tripod on the plate of the
air pump and to suspend the alarm clock from the tripod by a rubber
band. Set the alarm so that it will ring in 3 or 4 minutes, put it under the
bell jar, and pump out the air. Before the alarm goes off, be sure that the
air is almost completely pumped out of the jar. Can you hear the bell
ring? Distinguish between a dull trilling sound caused by the jarring of
the air pump when the alarm is on, and the actual ringing sound of the
bell.

Fig. 97. When the air is pumped out of the jar, you cannot hear the bell
ring.

The experiment just completed shows how we know there would be no sound
on the moon, since there is practically no air around it. The next experiment
will show you more about the way in which phonographs work.

Experiment 57. Put a blank cylinder on the dictaphone, adjust the
recording (cutting) needle and diaphragm at the end of the tube, start the
motor, and talk into the dictaphone. Shut off the motor, remove the
cutting needle, and put on the reproducing needle (the cutting needle,
being sharp, would spoil the cylinder). Start the reproducing needle where
the recording needle started, turn on the motor, and listen to your own
voice.

Notice that in the dictaphone the air waves of your voice are all
concentrated into a small space as they go down the tube. At the end of
the tube is a diaphragm, a flat disk which is elastic and vibrates back and
forth very easily. The air waves from your voice would not vibrate the
needle itself enough to make any record; but they vibrate the diaphragm,
and the needle, being fastened rigidly to it, vibrates with it.

In the same way, when the
reproducing needle vibrates as it
goes over the track made by the
cutting needle, it would make air
vibrations too slight for you to
hear if it were not fastened to the
diaphragm. When the diaphragm
vibrates with the needle, it
makes a much larger surface of
air vibrate than the needle alone
could. Then the tube, like an ear
trumpet, throws all the air
vibrations in one direction, so
that you hear the sound easily.

Experiment 58. Put a clean
white sheet of paper around the
recording drum, pasting the two
Fig. 98. Making a phonograph ends together to hold it in place.
record on an old-fashioned Put a small piece of gum
phonograph. camphor on a dish just under the
paper, light it, and turn the drum
so that all parts will be evenly
smoked. Be sure to turn it rapidly enough to keep the paper from being
burned.

Fig. 99. A modern dictaphone.

Melt a piece of glass over a burner and draw it out into a thread. Break off
about 8 inches of this glass thread and tie it firmly with cotton thread to
the edge of one prong of a tuning fork. Clamp the top of the tuning fork
firmly above the smoked drum, adjusting it so that the point of the glass
thread rests on the smoked paper. Turn the handle slightly to see if the
glass is making a mark. If it is not, adjust it so that it will. Now let some
one turn the cylinder quickly and steadily. While it is turning, tap the
tuning fork on the prong which has not the glass thread fastened to it. The
glass point should trace a white, wavy line through the smoke on the
paper. If it does not, keep on trying, adjusting the apparatus until the point
makes a wavy line.

Making a record in this way is, on a large scale, almost exactly like the making
of a phonograph record. The smoked paper on which a tracing can easily be
made as it turns is like the soft wax cylinder. The glass needle is like the
recording needle of a phonograph. The chief difference is that you have struck
the tuning fork to make it and the needle vibrate, instead of making it vibrate
by air waves set in motion by your talking. It is because these vibrations of the
tuning fork are more powerful and larger than are those of the recording needle
of a phonograph that you can see the record on the recording drum, while you
cannot see it clearly on the phonograph cylinder.

Fig. 100. How the apparatus is set up.

In all ordinary circumstances, sound is the vibration of air. But in swimming
we can hear with our ears under water, and fishes hear without any air. So, to
be accurate, we should say that sound is vibrations of any kind of matter. And
the vibrations travel better in most other kinds of matter than they do in air.
Vibrations move rather slowly in air, compared with the speed at which they
travel in other substances. It takes sound about 5 seconds to go a mile in air; in
other words, it would go 12 miles while an express train went one. But it
travels faster in water and still faster in anything hard like steel. That is why
you can hear the noise of an approaching train better if you put your ear to the
rail.

Fig. 101. When the tuning fork vibrates, the glass needle makes a wavy
line on the smoked paper on the drum.

Why we see steam rise before we hear a whistle blow. But even through
steel, sound does not travel with anything like the speed of light. In the time
that it takes sound to go a mile, light goes hundreds of thousands of miles,
easily coming all the way from the moon to the earth. That is why we see the
steam rise from the whistle of a train or a boat before we hear the sound. The
sound and the light start together; but the light that shows us the steam is in our
eyes almost at the instant when the steam leaves the whistle; the sound lags
behind, and we hear it later.

Application 42. Explain why a bell rung in a vacuum makes no noise;
why the clicking of two stones under water sounds louder if your head is
under water, than the clicking of the two stones in the air sounds if your
head is in the air; why you hear a buzzing sound when a bee or a fly
comes near you; how a phonograph can reproduce sounds.

Inference Exercise

Explain the following:

251. The paint on woodwork blisters when hot.

252. You can screw a nut on a bolt very much tighter with a wrench than
with your fingers.

253. When a pipe is being repaired in the basement of a house, you can
hear a scraping noise in the faucets upstairs.

254. Sunsets are unusually red after volcanic eruptions.

255. Thunder shakes a house.

256. Shooting stars are really stones flying through space. When they
come near the earth, it pulls them swiftly down through the air. Explain
why they glow.

257. At night it is difficult to see out through a closed window of a room
in which a lamp is lighted.

258. When there is a breeze you cannot see clear reflections in a lake.

259. Rubbing with coarse sandpaper makes rough wood smooth.

260. A bow is bent backward to make the arrow go forward.

Section 29. Echoes.

When you put a sea shell to your ear, how is it that you hear a roar in the
shell?

Why can you sometimes hear an echo and sometimes not?

If it were not for the fact that sound travels rather slowly, we should have no
echoes, for the sound would get back to us practically at the instant we made
it. An echo is merely a sound, a series of air vibrations, bounced back to us by
something at a distance. It takes time for the vibration which we start to reach
the wall or cliff that bounces it back, and it takes as much more time for the
returning vibration to reach our ears. So you have plenty of time to finish your
shout before the sound bounces back again. The next experiment shows pretty
well how the waves, or vibrations, of sound are reflected; only in the

experiment we use waves of water because they go more slowly and we can
watch them.

Experiment 59. Fill the long laboratory sink (or the bathtub at home) half
full of water and start a wave from one end. Watch it move along the side
of the sink. Notice what happens when it reaches the other end.

Air waves do the same thing; when they strike against a flat surface, they
bounce back like a rubber ball. If you are far enough away from a flat wall or
cliff, and shout, the sound (the air vibrations you start) is reflected back to you
and you hear the echo. But if you are close to the walls, as in an empty room,
the sound reverberates; it bounces back and forth from one wall to the other so
rapidly that no distinct echo is heard, and there is merely a confusion of sound.

Fig. 102. When the wave reaches the end of the sink, it is reflected back.
Sound waves are reflected in the same way.

When you drop a pebble in water, the ripples spread in all directions. In the
same way, when you make a sound in the open air, the air waves spread in all
directions. But when you shout through a megaphone the air waves are all
concentrated in one direction and consequently they are much stronger in that
direction. However, while the megaphone intensifies sound, the echoing from
the sides of the megaphone makes the sound lose some of its distinctness.

Why it is hard to understand a speaker in an empty hall. A speaker can be
heard much more easily in a room full of people than in an empty hall. The
sound does not reflect well from the soft clothes of the audience and the
uneven surfaces of their bodies, just as a rubber ball does not bounce well in
sand. So the sound does not reverberate as in an empty hall.

Application 43. Explain why a carpeted room is quieter than one with a
bare floor; why you shout through your hands when you want to be heard
at a distance.

Inference Exercise

Explain the following:

261. It is harder to walk when you shuffle your feet.

262. The air over a lamp chimney, or over a register in a furnace-heated
house, is moving upward rapidly.

263. The shooting of a gun sounds much louder within a room than it
does outdoors.

264. A drum makes a loud, clear sound when the tightened head is struck.

265. The pull of the moon causes the ocean tides.

266. Sand is sometimes put in the bottom of vases to keep them from
falling over.

267. It is difficult to understand clearly the words of one who is speaking
in an almost empty hall.

268. The ridges in a washboard help to clean the clothes that are rubbed
over them.

269. One kind of mechanical toy has a heavy lead wheel inside. When
you start this to whirling, the toy runs for a long time.

270. If you raise your finger slightly after touching the surface of water,
the water comes up with your finger.

Section 30. Pitch.

What makes the keys of a piano give different sounds?

Why does the moving of your fingers up and down on a violin string
make it play different notes?

Why is the whistle of a peanut roaster so shrill, and why is the whistle of
a boat so deep?

Did you ever notice how tiresome the whistle on a peanut roaster gets? Well,
suppose that whenever you spoke you had to utter your words in exactly that
pitch; that every time a car came down the street its noise was like the whistle
of the peanut roaster, only louder; that every step you took sounded like hitting
a bell of the same pitch; that when you went to the moving-picture theater the
orchestra played only the one note; that when any one sang, his voice did not
rise and fall; in short, that all the sounds in the world were in one pitch. That is
the way it would be if different kinds of air vibrations did not make different
kinds of notes,—if there were no differences in pitch.

Pitch due to rapidity of vibration. When air vibrations are slow,—far apart,
—the sound is low; when they are faster, the sound is higher; when they are
very quick indeed, the sound is very shrill and high. In various ways, as by
people talking and walking and by the running of street cars and automobiles,
all sorts of different vibrations are started, giving us a pleasant variety of high
and low and medium pitches in the sounds of the world around us.

An experiment will show how pitch varies and how it is regulated:

Experiment 60. Move the slide of an adjustable tuning fork well up from
the end of the prongs, tap one prong lightly on the desk, and listen. Move
the slide somewhat toward the end of the prongs, and repeat. Is a higher
or a lower sound produced as the slide shortens the length of the prongs?

Whistle a low note, then a high one. Notice what you do with your lips;
when is the opening the smaller? Sing a low note, then a high one. When
are the cords in your throat looser? Fill a drinking glass half full of water,
and strike it. Now pour half the water out, and strike the glass again. Do
you get the higher sound when the column of water is shorter or when it
is longer? Stretch a rubber band across your thumb and forefinger. Pick

the band as you make it tighter, not
making it longer, but pulling it tighter
with your other fingers. Does it make a
higher or a lower sound as you increase
the tightness? Stretch the band from your
thumb to your little finger and pick it;
now put your middle finger under the
band so as to divide it in halves, and pick
it again. Does a short strand give a
higher or lower pitch than a long strand?

A violinist tunes his violin by tightening
the strings; the tighter they are and the
thinner they are, the higher the note they
give. Some of the strings are naturally
higher than others; the highest is a smaller,
finer string than the lowest. When the
violinist plays, he shortens the strings by
holding them down with his fingers, and Fig. 103. When the prongs of
the shorter he makes them the higher the the tuning fork are made
note. A bass drum is much larger than a longer or shorter, the pitch of
high-pitched kettledrum. The pipes of an the sound is changed.
organ are long and large for the low notes,
shorter and smaller for the high ones.

In general, the longer or larger the object is that vibrates, the slower the rate of
vibration and consequently the lower the pitch. But the shorter or finer the
object is that vibrates, the higher the rate of vibration and the higher the pitch.

All musical instruments contain devices which can be made to vibrate,—either
strings or columns of air, or other things that swing to and fro and start waves
in the air. And by tightening them, or making them smaller or shorter, the pitch
can be made higher; that is, the number of vibrations to each second can be
increased.

Application 44. Explain why a steamboat whistle is usually of much
lower pitch than is a toy whistle; why a banjo player moves his fingers
toward the drum end of the banjo when he plays high notes; why the

sound made by a mosquito is higher in pitch than that made by a
bumblebee.

Application 45. A boy had a banjo given him for Christmas. He wanted to
tune it. To make a string give a higher note, should he have tightened or
loosened it? Or could he have secured the same result by moving his
finger up and down the string to lengthen or shorten it?

Application 46. A man was tuning a piano for a concert. The hall was
cold, yet he knew it would be warm at the time of the concert. Should he
have tuned the piano to a higher pitch than he wanted it to have on the
concert night, to the exact pitch, or to a lower pitch?

Inference Exercise

Explain the following:

271. A cowboy whirls his lasso around and around his head before he
throws it.

272. Furnaces are always placed in the basements of buildings, never on
top floors.

273. A rather slight contraction of a muscle lifts your arm a considerable
distance.

274. A player on a slide trombone changes the pitch of the notes by
lengthening and shortening the tube while he blows through it.

275. Rain runs off a tar roof in droplets, while on shingles it soaks in
somewhat and spreads.

276. There is a sighing sound as the wind blows through the branches of
trees, or through stretched wires or ropes.

277. Sometimes a very violent noise breaks the membrane in the drum of
a person's ear.

278. As a street car goes faster and faster, the hum of its motor is higher
and higher.

279. If a street is partly dry, the wet spots shine more than the dry spots
do.

280. Molten type metal, when poured into a mold, becomes hard, solid
type when it cools.

CHAPTER SEVEN
MAGNETISM AND ELECTRICITY

Section 31. Magnets; the compass.

What makes the needle of a compass point north?

What causes the Northern Lights?

For many hundreds of years sailors have used the compass to determine
directions. During all this time men have known that one point of the needle
always swings toward the north if there is no iron near to pull it some other
way, but until within the past century they did not know why. Now we have
found the explanation in the fact that the earth is a great big magnet. The
experiment which follows will help you to understand why the earth's being a
magnet should make the compass needle point north and south.

Experiment 61. Lay a magnetic compass flat on the table. Notice which
point swings to the north. Now hold a horseshoe magnet, points down,
over the compass. Turn the magnet around and watch the compass needle;
see which end of the magnet attracts the north point; hold that end of it
toward the south point and note the effect. Hold the magnet, ends up,
under the table directly below the compass and turn the magnet, watching
the compass needle.

The earth is a magnet, and it acts just as your magnet does: one end attracts
one point of the compass, and the other end attracts the other point. That ought
to make it clear why the compass points north. But how is the compass made?
The next experiment will show this plainly.

Experiment 62. Take a long shoestring and make a loop in one end of it.
Slip the magnet through the loop and suspend it, ends down. Fasten the
shoestring to the top of a doorway so that the magnet can swing easily.
Steady the magnet and let it turn until it comes to a rest. Mark the end that
swings to the north. Turn this end around to the south; let go and watch it.

Place the magnet the other way around in the loop so that you can be sure
that it is not twisting of the shoestring that makes the magnet turn in this
direction.

Fig. 104. The compass needle follows the magnet.

Now stroke a needle several times along one arm of the magnet, always in
the same direction, as shown in Figure 105. Hold the needle over some
iron filings or touch any bit of iron or steel with it. What has the needle
become? Lay it on a cardboard milk-bottle top of the flat kind, and on that
float it in the middle of a glass or earthenware dish of water. Notice which
end turns north. Turn this end to the south and see what happens. Hold
your magnet, ends up, under the dish, and turn the magnet. What does the
needle do?

Now it should be easy to understand why the compass points north. One end of
any magnet pulls on one end of another magnet and drives the other end away.
The earth is a big magnet. So if you make a magnet and balance it in such a
way that it is free to swing, the north end of the big earth magnet pulls one end
of the little magnet toward it and pushes the other end away. Therefore one end
of your compass always points north.

Other effects of the earth's magnetism. Another interesting effect of the
earth's being a big magnet is to be seen if you lay a piece of steel so that it
points north and south, and then pound it on one end. It becomes magnetized

just as your needle became magnetized when it was rubbed on the small
magnet.

Fig. 105. Magnetizing a needle.

Fig. 106. A compass made of a needle and a piece of cardboard.

And still another effect of the earth's magnetism is this: Tiny particles of
electricity, called electrons, are probably shooting through space from the sun.
It is believed that as they come near the earth, the magnetism of the north and
south polar regions attracts them toward the poles, and that as they rush
through the thin, dry upper air, they make it glow. And this is probably what
causes the Northern Lights or Aurora Borealis.

What happens when a needle is magnetized. The reason that a needle
becomes magnetic if it is rubbed over a magnet is probably this: Every
molecule of iron may be an extremely tiny magnet; if it is, each molecule has a
north and south pole like the needle of a compass. In an ordinary needle (or in
any unmagnetized piece of iron or steel) these molecules would be facing
every way, as shown in Figure 107.

Fig. 107. Diagram of molecules in Fig. 108. Diagram of magnetized
unmagnetized iron. The north iron. The north and south poles
and south poles of the molecules of the molecules are all supposed
are supposed to be pointing in all to point in the same direction.
directions.
But when a piece of steel or iron that is
already magnetized is brought near the
unmagnetized needle, all the north poles of the molecules of the needle are
pulled in the same direction—it is almost like combing tangled hair to stroke a
needle over a magnet. Then the molecules are arranged more as shown in
Figure 108. When all the molecules, each of which is a tiny magnet, pull in the
same direction, they make a strong magnet, and they magnetize any iron that
comes near them just as they were magnetized.

Steel will stay magnetized a long time; but ordinary soft iron loses magnetism
almost as soon as another magnet is taken away from it,—the molecules
become all disarranged again.

In a later section you will find that whenever electricity flows through a wire
that is coiled around a piece of iron, the iron becomes magnetized just as when
it is rubbed with a magnet.

Application 47. An explorer lost his compass. In clear weather he could
tell the directions by the sun and stars, but in cloudy weather he was
badly handicapped. He had with him a gun, plenty of ammunition, a
sewing kit, a hunting knife, and some provisions. How could he have
made a compass?

Inference Exercise

Explain the following:

281. Snow turns to water in the first warm weather.

282. A person's face looks ghastly by the greenish light of a mercury-
vapor lamp.

283. If a red-hot coal is touched with a cold poker, the coal turns black at
the place touched.

284. Stereopticon slides are put in upside down, yet the picture on the
screen is right side up.

285. If the vocal cords of your throat did not vibrate, you could not talk
out loud.

286. A watch is sometimes put out of order if it is held near a magnet.

287. The water will be no higher on the inside of a leaky boat than it is on
the outside.

288. A bass viol is considerably larger than a violin.

289. Ships that are used by men testing the earth's magnetism carry very
sensitive compasses. Explain why such ships are made entirely of wood
and brass.

290. Thunder rolls; that is, after the first peal there is a reverberating
sound that becomes less and less distinct.

Section 32. Static electricity.

What is electricity?

What makes thunder and lightning?

Why does the barrel or cap of a fountain pen pick up small bits of paper
after it has been rubbed on your coat sleeve?

Why do sparks fly from the fur of a cat when you stroke it in the dark?

The Greeks, 2000 years ago, knew that there was such a thing as electricity,
and they used to get it by rubbing amber with silk. In the past century men
have learned how to make electricity do all sorts of useful work: making boats
and cars and automobiles go, ringing bells, furnishing light, and, in the
telephone and telegraph, carrying messages. But no one knew what electricity
really was until, within the last 25 years, scientists found out.

Atoms and electrons. When we talked about molecules, we said that they
were as much smaller than a germ as a germ is smaller than a mountain. Well,
a molecule is made up, probably, of some things that are much smaller still,—
so small that people thought that nothing could be smaller. Those unthinkably
tiny things are called atoms; you will hear more about them when you come to
the parts of this book that tell about chemistry.

But if you took the smallest atom in the world and divided it into 1700 pieces,
each one of these would be about the size of a piece of electricity.

Electricity is made up of the tiniest things known to man—things so small that
nobody really can think of their smallness. These little pieces of electricity are
called electrons, and for all their smallness, scientists have been able to find
out a good deal about them. They have managed to get one electron all by
itself on a droplet of oil and they have seen how it made the oil behave. Of
course they could not see the electron, but they could tell from various
experiments that they had just one. Scientists know how many trillions of
electrons flow through an incandescent electric lamp in a second and how
many quadrillions of them it would take to weigh as much as a feather. They
know what the electrons do when they move, how fast they can move, and
what substances let electrons move through them easily and what substances
hold them back; and they know perfectly well how to set them in motion. How
the scientists came to know all these things you will learn in the study of

physics; it is a long story. But you can find out some things about electrons
yourself. The first experiment is a simple one such as the Greeks used to do
with amber.

Experiment 63. Rub a hard
rubber comb on a piece of
woolen cloth. The sleeve of a
woolen coat or sweater will do.
Rub the comb quickly in the
same direction several times.
Now hold it over some small bits
of paper or sawdust. What does
it do to them? Hold it over some
one's hair. The rest of this
experiment will work well only
on cool, clear days. Rub the
comb again, a dozen or more
times in quick succession. Now
touch it gently to the lobe of
your ear. Do you hear the snap as
the small spark jumps from the
comb to your ear?
Fig. 109. When the comb is rubbed Pull a dry hair out of your head
on the coat, it becomes charged with and hold it by one end. Charge
electricity. your comb by rubbing it again,
and bring it near the loose end of
the hair. If the end of the hair
clings to the comb at first, leave it clinging until it flies off. Now try to
touch the hair with the comb. Next, pinch the end of the hair between
your thumb and finger and again bring the charged comb near it. Is the
hair attracted or repelled? After touching the comb what does it do?

You can get the same effects by rubbing glass or amber on silk.

Objects negatively and positively charged with electricity. There are
probably electrons in everything. But when there is just the usual number of
electrons in an object, it acts in an ordinary way and we say that it is not
charged with electricity. If there are more than the usual number of electrons

on an object, however, we say that it is negatively charged, or that it has a
negative charge of electricity on it. But if there are fewer electrons than usual
in an object, we say that it has a positive charge of electricity on it, or that it is
positively charged.

You might expect a "negative charge" to
indicate fewer electrons than usual, not
more. But people called the charge
"negative" long before they knew anything
about electrons; and it is easier to keep the
old name than to change all the books that
have been written about electricity. So we
still call a charge "negative" when there are
unusually many electrons, and we call it
"positive" when there are unusually few. A
negative charge means that more electrons
are present than usual. A positive charge
means that fewer electrons are present than
usual.

Before you rubbed your comb on wool,
neither the comb nor the wool was charged;
both had just the usual number of electrons.
But when you rubbed them together, you Fig. 110. The charged comb
rubbed some of the electrons off the wool picks up pieces of paper.
on to the comb. Then the comb had a
negative charge; that is, it had too many
electrons—too many little particles of electricity.

When you brought the comb near the hair, the hair had fewer electrons than the
comb. Whenever one object has more electrons on it than another, the two
objects are pulled toward each other; so there was an attraction between the
comb and the hair, and the hair came over to the comb. As soon as it touched
the comb, some of the extra electrons jumped from the comb to the hair. The
electrons could not get off the hair easily, so they stayed there. Electrons repel
each other—drive each other away. So when you had a number of electrons on
the end of the comb and a number on the end of the hair, they pushed each
other away, and the hair flew from the comb. But when you pinched the hair,
the electrons could get off it to your moist hand, which lets electricity through

it fairly easily. Then the comb had extra electrons on it and the hair did not; so
the comb pulled the hair over toward it again.

When you brought the charged comb near your ear, some of the electrons on
the comb pushed the others off to your ear, and you heard them snap as they
rushed through the air, making it vibrate.

How lightning and thunder are caused. In thunderstorms the strong currents
of rising air blow some of the forming raindrops in the clouds into bits of
spray. The tinier droplets get more than their share of electrons when this
happens and are carried on up to higher clouds. In this way clouds become
charged with electricity. One cloud has on it many more electrons than another
cloud that is made, perhaps, of lower, larger droplets. The electricity leaps
from the cloud that has the greater number of electrons to the cloud that has
the less number, or it leaps from the heavily charged cloud down to a tree or
house or the ground. You see the electricity leap and call it lightning. Much
more leaps, however, than leaped from the comb to your ear, and so it makes a
very much louder snap. The snap is caused in this way: As the electric spark
leaps through the air, it leaves an empty space or vacuum immediately behind
it. The air from all sides rushes into the vacuum and collides there; then it
bounces back. This again leaves a partial vacuum; so the air rushes in once
more, coming from all sides at once, and again bounces back. This starts the
air vibrations which we call sound. Then the sound is echoed from cloud to
cloud and from the clouds to the earth and back again, and we call it thunder.

The electricity you have been reading about and experimenting with in this
section is called static electricity. "Static" means standing still. The electricity
you rubbed up to the surface of the comb or glass stayed still until it jumped to
the bit of paper or hair; then it stayed still on that. This was the only kind of
electricity most people knew anything about until the nineteenth century; and
it is not of any great use. Electricity must be flowing through things to do
work. That is why people could not invent electric cars and electric lights and
telephones before they knew how to make electricity flow steadily rather than
just to stand still on one thing until it jumped across to another and stood there.
In the next chapter we shall take up the ways in which electrons are made to
flow and to do work.

Application 48. Explain why the stroking of a cat's back will sometimes
cause sparks and make the cat's hairs stand apart; why combing

sometimes makes your hairs fly apart. Both of these effects are best
secured on a dry day, because on a damp day the water particles in the air
will let the electrons pass to them as fast as they are rubbed up to the
surface of the hair.

Inference Exercise

Explain the following:

291. If you shuffle your feet on a carpet in clear, cold weather and then
touch a person's nose or ear, a slight spark passes from your finger and
stings him.

292. If you stay out in the cold long, you get chilled through.

293. The air and earth in a greenhouse are warmed by the sun through the
glass even when it is cold outside and when the glass itself remains cold.

294. When you hold a blade of grass taut between your thumbs and blow
on it, you get a noise.

295. Shadows are usually black.

296. Some women keep magnets with which to find lost needles.

297. You can grasp objects much more firmly with pliers than with your
fingers.

298. If the glass in a mirror is uneven, the image of your face is unnatural.

299. A sweater clings close to your body.

300. Kitchens, bathrooms, and hospitals should have painted walls.

CHAPTER EIGHT
ELECTRICITY

Section 33. Making electricity flow.

What causes a battery to produce electricity?

What makes electricity come into our houses?

The kind of electricity you get from rubbing (friction) is not of much practical
use, you remember. Men had to find a way to get a steady current of electricity
before they could make electricity do any work for them. The difference
between static electricity—when it leaps from one thing to another—and
flowing electricity is a good deal like the difference between a short shower of
rain and a river. Both rain and river are water, and the water of each is moving
from one place to another; but you cannot get the raindrops to make any really
practical machine go, while the rivers can do real work by turning the wheels
in factories and mills.

Within the past century two devices for making electricity flow and do work
have been perfected: One of these is the electric battery; the other is the
dynamo.

The electric battery. A battery consists of two pieces of different kinds of
metal, or a metal and some carbon, in a chemical solution. If you hang a piece
of zinc and a carbon, such as comes from an arc light, in some water, and then
dissolve sal ammoniac in the water, you will have a battery. Some of the
molecules of the sal ammoniac divide into two parts when the sal ammoniac
gets into the water, and the molecules continue to divide as long as the battery
is in use or until it "wears out." One part of each molecule has an unusually
large number of electrons; the other part has unusually few. The parts with
unusually large numbers of electrons gather around the zinc; so the zinc is
negatively charged,—it has more than the ordinary number of electrons. The
part of the sal ammoniac with unusually few electrons goes over to the carbon;

so the carbon is positively charged,—it has fewer than the ordinary number of
electrons.

Making the current flow. Now if we can make some kind of bridge between
the carbon and the zinc, the electrons will flow from the place where there are
many to the place where there are few. Electrons can flow through copper wire
very easily. So if we fasten one end of the copper wire to the carbon and the
other end to the zinc, the electrons will flow from the zinc to the carbon as
long as there are more electrons on the zinc; that is, until the battery wears out.
Therefore we have a steady flow of electricity through the wire. While the
electricity is flowing from one pole to the other, we can make it do work.

Experiment 64. Set up two or three Samson cells. They consist of a glass
jar, an open zinc cylinder, and a smaller carbon cylinder. Dissolve a little
over half a cup of sal ammoniac in water and put it into the glass jar; then
fill the jar with water up to the line that is marked on it. Put the carbon
and zinc which are attached to the black jar cover into the jar. Be careful
not to let the carbon touch the zinc. One of these cells will probably not
be strong enough to ring a doorbell for you; so connect two or three
together in series as follows:

Fasten a piece of copper wire from the carbon of the first cell to the zinc
of the second. If you have three cells, fasten another piece of wire from
the carbon of the second cell to the zinc of the third, as shown in Figure
111.

Fig. 111. A wet battery of three cells connected to ring a bell.

Fasten one end of a copper wire to the zinc of the first cell and the other
end of this wire to one binding post of an electric bell. Fasten one end of
another piece of copper wire to the carbon of the third cell, if you have
three, and touch the other end of this wire to the free binding post of the
electric bell. If you have everything connected rightly, the bell should
ring.

Different kinds of batteries. There are many different kinds of batteries. The
one you have just made is a simple one frequently used for doorbells. Other
batteries are more complicated. Some are made with copper and zinc in a
solution of copper sulfate; some, even, are made by letting electricity from a
dynamo run through a solution from one lead plate to another until a chemical
substance is stored on one of them; then, when the two lead plates are
connected by a wire, the electrons run from one to the other. This kind of
battery is called a storage battery, and it is much used in submarines and
automobiles.

Fig. 112. A battery of three dry cells.

But all the different batteries work on the same general principle: A chemical
solution divides into two parts, one with many electrons and the other with a
less number. One part of the solution gathers on one pole (piece of metal in the

solution) and charges it positively; the other part gathers on the other pole and
charges it negatively. Then the electricity flows from one pole to the other.

Positive and negative poles.
Before people knew anything about
electrons, they knew that electricity
flowed from one pole of a battery
to the other. But they always said
that it flowed from the carbon to
the zinc; and they called the carbon
the positive pole and the zinc the
negative. Although we now know
that the electrons flow from the
zinc to the carbon, it is much more
convenient to use the old way of
speaking, as was explained on page
199. Practically, it makes no
difference which way the electrons
are going as long as a current of
electricity is flowing through the
wire from one pole of the battery to Fig. 113. A storage battery.
the other pole. So every one speaks
of electricity as flowing from the
positive pole of a battery (usually the carbon or copper) to the negative pole
(usually the zinc), although the electrons actually move in the other direction.

Batteries make enough electricity flow to do a good deal of work. But they are
rather expensive, and it takes a great many to give a flow of electricity
sufficient for really heavy work, such as running street cars or lighting a city.
Fortunately there is another way of getting large amounts of electricity to flow.
This is by means of dynamos.

How a dynamo makes a current flow. To understand a dynamo, you must
first realize that there are countless electrons in the world—perhaps all things
are made entirely of them. But you remember that when we want to get these
electrons to do work we must make them flow. This can be done by spinning a
loop of wire between the poles of a magnet. Whenever a loop of wire is turned
between the two poles of a magnet, the magnetism pushes the electrons that

are already in the wire around and around the loop. As long as we keep the
loop spinning, a current of electricity flows.

If only one loop of wire is spun between
the poles of a magnet, the current is very
feeble. If you loop the wire around twice,
as shown in Figure 114, the magnet acts on
twice as much of the wire at the same time;
so the current is stronger. If a very long
piece of wire is used and is looped around
many times, and the whole coil is spun
Fig. 114. Spinning loops of rapidly between the poles of a powerful
wire between the poles of a magnet, myriads of the electrons in the
magnet causes a current of wire rush around and around the loops—a
electricity to flow through the powerful current of electricity flows
wire. through the wire.

Fig. 115. The more loops there are, the stronger the current.

Now suppose you bring one loop of the long wire out, as shown in Figure 115,
and suppose you spin the rest of the loops between the poles of the magnet.
Then, to flow through the loops by the magnet the electricity will have to go
clear out through the long loop and back again. While it is flowing through this
long loop, we can make it work. We can cut the long loop and attach one
broken end to one part of an electric lamp and the other end to the other part,
so that the electricity has to flow through the lamp in order to get back to the
spinning coil of wire, as shown in Figure 116. Such an arrangement as this is
really an extremely simple dynamo.

Fig. 116. If the electricity passes through a lamp on its way around the
circuit the filament of the lamp glows.

Fig. 117. A dynamo in an electric light plant.

You could make a dynamo that would actually work, by arranging such an
apparatus so that the coil would spin between the poles of the magnet. But of
course the big commercial dynamos are very much more complicated in their
construction. Figure 116 shows only the general principle on which they work.
The main point to note is that by spinning a coil of wire between the poles of a
magnet, you can make electricity flow rapidly through the wire. And it is in
this way that most of the electricity we use is made.

The power spinning the coil of wire is sometimes steam, and sometimes
gasoline or distillate; and water power is very often used. A large amount of
our electricity comes from places where there are waterfalls. Niagara, for

instance, turns great dynamos and generates an enormous amount of
electricity.

Why many automobiles have to be cranked. In an automobile, the magneto
is a little dynamo that makes the sparks which explode the gasoline. While the
automobile is going the engine spins the coil of wire between the magnets, but
at starting you have to spin the coil yourself; and doing that is called
"cranking" the automobile. "Self-starters" have a battery and motor to spin the
coil for you until the engine begins to go; then the engine turns the coil of the
magneto.

How old-fashioned telephones are rung. The old-fashioned telephones, still
often used in the country, have little cranks that you turn to ring for central.
The crank turns a coil of wire between the poles of the magnet and generates
the electricity for ringing the bell. These little dynamos, like those in
automobiles, are usually called magnetos.

Fig. 118. The magneto in an automobile is a small dynamo.

Alternating current. For the sake of simplicity and convenience we speak of
electricity as always flowing in through one wire and out through the other.
With batteries this is actually the case. It is also the case where people have
what is called direct-current (d. c.) electricity. But it is easier to raise and lower
the voltage (pressure) of the current if instead of being direct it is alternating;
that is, if for one instant the electricity flows in through one wire and out
through the other, the next instant flowing the opposite way, then the first way
again, and so on. This kind of current is called alternating current (a. c.),
because the current alternates, coming in the upper wire and out of the lower
for a fraction of a second; then coming in the lower and out of the upper for
the next fraction of a second; then coming in the upper again and out of the
lower for a fraction of a second; and so on, back and forth, all the time. For
heating and lighting, this alternating current is just as good as the direct
current, and it is probably what you have in your own home. For charging
storage batteries and making electromagnets, separating water into two gases,
and for running certain kinds of motors, however, the direct current is
necessary. Find out whether the current in your laboratory is direct or
alternating.

Application 49. Explain why we need fuel or water to generate large
currents of electricity; how we can get small amounts of electricity to
flow without using dynamos; why automobiles must be cranked unless
they have batteries to start them.

Inference Exercise

Explain the following:

301. Mexican water jars are made of porous clay; the water that seeps
through keeps the water inside cool.

302. When you crank an automobile, electricity is generated.

303. Potatoes will not cook any more quickly in water that is boiling
violently than in water that is boiling gently.

304. When you brush your hair on a winter morning, it sometimes stands
up and flies apart more and more as you continue to brush it.

305. You cannot see a person clearly through a ground-glass window,
although it lets most of the light through.

306. There is a layer of coarse, light-colored gravel over the tar on roofs,
to keep the tar from melting.

307. It is very easy to slip on a well-waxed hardwood floor.

308. If you have a silver filling in one of your teeth and you touch the
filling with a fork or spoon, you get a slight shock.

309. You can shake a thing down into a bottle when it will not slip down
by itself.

310. If you rub a needle across one pole of a magnet three or four times in
the same direction, then float it on a cork in water one end of the needle
will point north.

Section 34. Conduction of electricity.

How does electricity travel?

Why do you get a shock if your hands are wet when you touch a live
wire?

If you were to use a piece of string instead of a copper wire to go from one
pole of a battery to another or to spin between the poles of the magnet of the
dynamo, you could get no flow of electricity to speak of. Electrons do not flow
through string easily, but they flow through a copper wire very easily.
Anything that carries, or conducts, electricity well is called a good conductor.
Anything that carries it poorly is called a poor conductor. Anything that allows
practically no electricity to pass through it is called an insulator.

Experiment 65.5 Turn on an electric lamp. Turn it off by opening the
knife switch. Cover the blade of the knife switch with a fold of paper and
close it. Will the lamp glow? Try a fold of dry cloth; a fold of the same
cloth wet. Connect the blade to the slot with a piece of iron; with a piece
of glass; with porcelain; with rubber; with dry wood; with wood that is
soaking wet; with a coin. Which of these are good conductors of
electricity? Which could be used as insulators?

Footnote 5: Read footnote, page 226, before doing this experiment.

Fig. 119. Electricity flows through the coin.

How you can get an electrical shock. A person's body is not a very good
conductor of electricity, but will conduct it somewhat. When electricity goes
through your body, you get a shock. The shock from the ordinary current of
electricity, 110 volts, is not enough to injure you at all; in fact, if you were
standing on dry wood, it would be safe, although you would get a slight shock,
to connect the blade of a knife switch to the slot of the switch, through your
hand or body. Your body would not allow enough current to pass through it to
light the lamp. Stronger currents, like those of power lines and even trolley
wires, are extremely dangerous.

All the electric wires entering your house are made of copper. They are all
covered with cloth and rubber and are fastened with glass or porcelain knobs.
The reason is simple: Copper and practically all other metals are very good
conductors of electricity; that is, they allow electricity to pass through them
very easily. Cloth, rubber, glass, and porcelain are very poor conductors, and
they are therefore used as insulators,—to keep the electricity from going where
you do not want it to go.

Fig. 120. Will electricity go through the glass?

Experiment 66. To each binding post of an electric bell fasten a piece of
insulated copper wire with bare ends and at least 4 feet long. Connect the
free end of one of these wires with one pole of a battery, using a regular
laboratory battery or one you made yourself. Attach one end of another
piece of wire a foot or so long, with bare ends, to the other pole of the
battery. Touch the free end of this short wire to the free end of the long
wire, as shown in Figure 120. Does the bell ring? If it does not, something
is wrong with the connection or with the battery; fix them so that the bell
will ring. Now leave a gap of about an inch between the free end of the
long wire and the free end of the short wire. Try making the electricity
flow from the short wire into the long one through a number of different
things, such as string, a key, a knife, a piece of glass tubing, wet cloth, dry
cloth, rubber, paper, a nail, a dish of mercury (dip the ends of the wire
into the dish so that they both touch the mercury at the same time), a dish
of water, a stone, a pail, a pin, and anything else that you may like to try.

Fig. 121. Electrical apparatus: A, plug fuse; B, cartridge fuse; C, knife
switch; D, snap switch; E, socket with nail plug in it; F, fuse gap; G, flush

switch; H, lamp socket; I, J, K, resistance wire.

Each thing that makes the bell ring is a good conductor. Each one that will not
make it ring is a poor conductor or an insulator. Make a list of the things you
have tried; in one column note the good conductors, and in another column
note the insulators and poor conductors.

The water and wet cloth did not ring the bell, but this is because the pressure
or voltage of the electricity in the batteries is not very high. In dealing with
high-power wires it is much safer to consider water, or anything wet, as a
pretty good conductor of electricity. Absolutely pure, distilled water is an
extremely poor conductor; but most water has enough minerals dissolved in it
to make it conduct electricity fairly well. In your list you had better put water
and wet things in the column with the good conductors.

Fig. 122. Which should he choose to connect the broken wires?

Application 50. Robbers had cut the telegraph line between two railroad
stations (Fig. 122). The broken ends of the wire fell to the ground, a
number of feet apart. A farmer caught sight of the men speeding away in
an automobile and he saw the cut wires on the ground. He guessed that
they had some evil purpose and decided to repair the damage. He could

not bring the two ends of the wire together. He ran to his barn and found
the following things there:

A ball of cord, a pickax, a crowbar, some harness, a wooden wagon
tongue, a whip, a piece of iron wire around a bale of hay (the wire was
not long enough to stretch the whole distance between the two ends of the
telegraph wire, even if you think he might have used it to patch the gap),
a barrel with four iron hoops, and a rope.

Which of these things could he have made use of in connecting the
broken ends of the telegraph wire?

Application 51. A man was about to put in a new socket for an electric
lamp in his home. He did not want to turn off the current for the whole
house, as it was night and there was no gas to furnish light while he
worked.

"I've heard that if you keep your hands wet while you work, the film of
water on them will keep you from getting a shock," his wife said.

"Don't you wet your hands, Father," said his 12-year-old boy; "keep them
dry, and handle the wires with your pliers, so that you won't have to touch
it."

"I advise you to put on your canvas gloves while you work; then you can't
get a shock," added another member of the family.

"That's a good idea," said the wife, "but wet the gloves, then you will
have the double protection of the water and the cloth."

The man laughed and went to work on the socket. He did not get a shock.
Which advice, if any, do you think he followed?

Inference Exercise

Explain the following:

311. A red postage stamp looks greenish gray in the green light of a
mercury-vapor lamp.

312. Cracks are left between sections of the roadbed in cement auto
highways.

313. Electricity goes up a mountain through a wire.

314. It is impossible to stand sidewise against a wall on one foot, when
that foot touches the wall.

315. A charged storage battery will run an electric automobile.

316. An empty house is noisier to walk in and talk in than is a furnished
one.

317. Lightning rods are made of metal.

318. It is harder to hold a frying pan by the end of the handle than by part
of the handle close to the pan.

319. Diamonds flash many colors.

320. In swimming, if you have hold of a fastened rope and try to pull it
toward you, you go toward it.

Section 35. Complete circuits.

Why does a doorbell ring when you push a button?

Why is it that when you touch one electric wire you feel no shock, while
if you touch two wires you sometimes get a shock?

When a wire is broken in an electric light, why does it not light?

Suppose you have some water in an open circular trough like the one shown in
Figure 123. Then suppose you have a paddle and keep pushing the water to
your right from one point. The water you push pushes the water next to it, that
pushes the water next to it, and so on all around the trough until the water just
behind your paddle pushes in toward the paddle; the water goes around and
around the trough in a complete circuit. There never is too much water in one
place; you never run out of water. But then suppose a partition is put across the

trough somewhere along the circuit. When the water reaches that, it cannot
pass; it has no place to flow to, and the current of water stops.

The electric circuit. The flow of electricity in an electric circuit may be
compared to the flow of the water in the tank we have been imagining. The
long loop of wire extending out from the dynamo to your house and back again
corresponds to the tank. The electricity corresponds to the water. Your dynamo
pushes the electricity around and around the circuit, as the paddle pushes the
water. But let some one break the circuit by putting a partition between two
parts of it, and the electricity immediately stops flowing. One of the most
effective partitions we can put into an electric circuit is a gap of air. It is very
difficult for any electricity to flow through the air; so if we simply cut the wire
in two, electricity can no longer flow from one part to the other, and the
current is broken.

Fig. 123. Electricity flows around a completed circuit somewhat as water
might be made to flow around this trough.

Breaking and making the circuit. The most convenient way to put an air
partition into an electric circuit and so to break it, or to close the circuit again
so it will be complete, is to use a switch.

Experiment 67. In the laboratory, examine the three different kinds of
switches where the electricity flows into the lamp and resistance wire and
then out again. Trace the path the electricity must take from the wire
coming into the building down to the first switch that it meets; then from
one end of the wire through the brass or copper to which the wire is
screwed, through the switch and on out into the end of the next piece of

wire. Turn the first switch off and see how a partition of air is made
between the place where the electricity comes in and the place where it
would get out if it could. Turn the switch on and notice how this gives the
electricity a complete path through to the next piece of wire. In this way
follow the circuit on through all the switches to the electric lamp.

If you examine the socket into which the lamp screws and examine the lamp
itself, you will see that electricity which goes to the outer part of the socket
passes into the rim of the lamp; from here it goes into one end of the filament.
It passes through the filament to the other end, which is connected to the little
brass disk at the end of the lamp. From this you can see that it goes into the
center point of the socket, and then on into the second wire that connects to the
socket. Trace the current on back through this other wire until you see where
this wire leads toward the dynamo. You should understand that the electric
lamp, the switches, the fuses, all things along the circuit, are simply parts of
the long loop from the dynamo, as shown in Figure 124.

Connecting in parallel. The trouble with Figure 124 is that it is a little too
simple. From looking at it you might think that the loop entered only one
building. And it might seem that turning off one switch would shut off the
electricity all along the line. It would, too, if the circuit were arranged exactly
as shown above. To avoid this, and for other reasons, the main loop from the
dynamo has branches so that the electricity can go through any or all of them
at the same time and so that shutting off one branch will not affect the others.
Electricians call this connecting in parallel; there are many parallel circuits
from one power house.

Fig. 124. Diagram of the complete circuit through the laboratory switches

Figure 125 illustrates the principle just explained. As there diagrammed, the
electricity passes out from the dynamo along the lower wire and goes down the
left-hand wire of circuit A through one of the electric lamps that is turned on,

and then it goes back through the right-hand wire of the A circuit to the upper
wire of the main circuit and then on back to the dynamo. But only a part of the
electricity goes through the A circuit; part goes on to the B circuit, and there it
passes partly through the electric iron. Then it goes back through the other
wire to the dynamo. No electricity can get through the electric lamp on the B
circuit, because the switch to the lamp is open. The switch on the C circuit is
open; so no electricity can pass through it.

The purpose of the diagram is to show that electricity from the dynamo may go
through several branch circuits and then get back to the dynamo, and that
shutting off the electricity from one branch circuit does not shut it off from the
others. And the purpose of this section is to make it clear that electricity can
flow only through a complete circuit; it must have an unbroken path from the
dynamo back to the dynamo again or from one pole of the battery back to the
other pole. If the electricity does not have a complete circuit, it will not flow.

Application 52. A small boy disconnected the doorbell batteries from the
wires that ran to them, and when he wanted to put the wires back, he
could not remember how they had been connected. He tried fastening
both wires to the carbon part of the battery, connecting one wire to the
carbon and one to the zinc, and connecting both to the zinc. Then he
decided that one wire was all that had to be connected anyway, that the
second was simply to make it stronger. Which of the ways he tried, if any,
would have been right?

Fig. 125. Parallel circuits.

Fig. 126. How should he connect them?

Application 53. Dorothy was moving. "When they took out our
telephone," she said to her chum, Helen, "the electrician just cut the wires
right off."

"He must have turned off the electricity first," Helen answered, "or else it
would all have run out of the cut ends of the wire and gone to waste."

"Why, it couldn't," Dorothy said. "Electricity won't flow off into the air."

"Of course it can if there is nothing to hold it in," Helen argued.

Which was right?

Inference Exercise

Explain the following:

321. It is very easy to get chilled when one is perspiring.

322. Ice cream becomes liquid if you leave it in your dish too long.

323. You should face forward when alighting from a street car.

324. There are always at least two electric wires going into a building that
is wired.

325. Woolen sweaters keep you warm.

326. Steel rails are not riveted to railroad ties but the spikes are driven
close to each rail so that the heads hook over the edge and hold the rail
down without absolutely preventing its movement forward and backward.
Why should rails be laid in this way?

327. The earth keeps whirling around the sun without falling into it,
although the pull from the sun is very great.

328. Electricity is brought down from power houses in the mountains by
means of cables.

329. White clothes are cooler than black when the person wearing them is
out in the sun.

330. All the street cars along one line are stopped when a trolley wire
breaks.

Section 36. Grounded circuits.

Why can a bird sit on a live wire without getting a shock, while a man
would get a shock if he reached up and took hold of the same wire?

We have just been laying emphasis on the fact that for electricity to flow out of
a dynamo or battery, it must have a complete circuit back to the battery or
dynamo. Yet only one wire is needed in order to telegraph between two
stations. Likewise, a single wire could be made to carry into a building the
current for electric lights. This is because the ground can carry electricity.

If you make all connections from a battery or dynamo just as for any complete
circuit, but use the earth for one wire, the electricity will flow perfectly well
(Fig. 127). To connect an electric wire with the earth, the wire must go down
deep into the ground and be well packed with earth; but since water pipes go
down deep and the earth is already packed around them, the most convenient

way to ground a circuit is to connect the wire that should go into the ground
with the water pipe. The next experiment, the grounding of a circuit, should be
done by the class with the help of the teacher.

Fig. 127. The ground can be used in place of a wire to complete the circuit

Experiment 68. Caution: Keep the switches turned off throughout this
experiment.6
Footnote 6: All through this chapter it is assumed that the electrical apparatus
described in the appendix is being used. In this apparatus all the switches are on one
wire, the other wire being alive even when the switches are turned off.

(a) Put a piece of fuse wire across the fuse gap. Screw the plug with nails
in it into the lamp socket. Connect the bare end of a piece of insulated
wire to the water faucet and touch the other end to one nail of the plug. If
nothing happens, touch it to the other nail instead. The electricity has
gone down into the ground through the water pipe, instead of into the
other wire. The ground carries the electricity back to the dynamo just as a
wire would.

(b) Put a new piece of fuse wire across the gap. Keep switches turned off.
Touch the brass disk at the bottom of an electric lamp to the nail which
worked, and touch the wire from the faucet to the other brass part of the
lamp (Fig. 129). What happens?

Caution: Under no circumstances allow the switch to be turned on while
you are doing any part of this experiment. Under no circumstances touch
the wire from the faucet to the binding posts of the fuse gap. Do only as
directed. Explain what would happen if you disobeyed these rules.

Fig. 128. Grounding the circuit. The faucet and water pipe lead the
electricity to the ground.

Why a bird is not electrocuted when it sits on a live wire. If a man
accidentally touches a live wire that carries a strong current of electricity he is
electrocuted; yet birds perch on such a wire in perfect safety. If a man should
leap into the air and grasp a live wire, hanging from it without touching the
ground, he would be no more hurt by it than a bird is. A person who is
electrocuted by touching such a wire must at the same time be standing on the
ground or on something connected with it. The ground completes the electric
circuit which passes through the body. An electric circuit can always be
completed through the ground, and when this is done, it is called grounding a
circuit.

Fig. 129. How the lamp and wire are held to ground the circuit.

Application 54. Explain why only one wire is needed to telegraph
between two stations; why you should not turn an electric light on or off
while standing in a tub of water.

Application 55. In a house in the country, the electric wires passed
through a double wall. They were separated from each other and well
covered with insulation, but they were not within an iron pipe, as is now
required in many cities. The current was alternating. One night when the
lights were out a rat in the wall gnawed through the insulation of the wire
and also gnawed clear through one of the wires. Did he get a shock? The
next morning, the woman of the house wanted to use the electric iron in
the kitchen and it would not work. The kitchen had in it a gas stove, a
sink with running water, a table, a couple of chairs, and the usual kitchen
utensils. There was also a piece of wire about long enough to reach across
the kitchen. The electrician could not come out for several hours, and the
woman wanted very much to do her ironing. Figure 130 is a diagram of
the wires and the kitchen. Show what the woman might have done in
order to use her iron until the electrician arrived.

Fig. 130. How can the electric iron be used after one wire has been cut?

Application 56. A man wanted to change the location of the wiring in his
cement-floored garage. While he was working, would it have been best
for him to stand on the bare cement floor, on a wire mat, on an old
automobile tire, on a wet rug, or on some skid chains that were there?

Inference Exercise

Explain the following:

331. An ungreased wheel squeaks.

332. Lightning rods extend into the earth.

333. A banjo player moves his fingers toward the drum end of the banjo
when he plays high notes.

334. When the filament breaks, an electric lamp will no longer glow.

335. An inverted image is formed by the lens of a camera.

336. A blown-out fuse may be replaced temporarily with a hairpin or with
a copper cent.

337. Sparks fly when a horse's shoe hits a stone.

338. A room requires less artificial light if the wall paper is light than if it
is dark.

339. Phonographs usually have horns, either inside or outside.

340. An electric car needs only one wire to make it go.

Section 37. Resistance.

What makes an electric heater hot?

Why does lightning kill people when it strikes them?

What makes an electric light glow?

We have talked about making electricity work when it flows in a steady
stream, and everybody knows that it makes lights glow, makes toasters and
electric stoves hot, and heats electric irons. But did it ever strike you as
remarkable that the same electricity that flows harmlessly through the wires in
your house without heating them, suddenly makes the wire in your toaster or

the filament in your incandescent lamp glowing hot? The insulation is not what
keeps the wire cool, as you can see by the next experiment.

Experiment 69. Between two of the laboratory switches you will find
one piece of wire which has no insulation. Turn on the electricity and
make the lamp glow; see that you are standing on dry wood and are not
touching any pipes or anything connected to the ground. Feel the bare
piece of wire with your fingers. Why does this not give you a shock?
What would happen if you touched your other hand to the gas pipe or
water pipe? Do not try it! But what would happen if you did?

The reason that the filament of the electric lamp gets white hot while the
copper wire stays cool is this: All substances that conduct electricity resist the
flow somewhat; there is something like friction between the wire and the
electricity passing through it. The smaller around a wire is, the greater
resistance it offers to the passing of an electric current. The filament of an
electric lamp is very fine and therefore offers considerable resistance.
However, if the filament were made of copper, even as fine as it is, it would
take a much greater flow of electricity to make it white hot, and it would be
very expensive to use. So filaments are not made of copper but of substances
which do not conduct electricity nearly as well and which therefore have much
higher resistance. Carbon was once used, but now a metal called tungsten is
used for most incandescent lamps. Both carbon and tungsten resist an electric
current so much that they are easily heated white hot by it. On the other hand,
they let so little current through that what does pass flows through the larger
copper wires very easily and does not heat them noticeably.

Fig. 131. Feeling one live wire does not give her a shock, but what would
happen if she touched the gas pipe with her other hand?

Experiment 70. Turn on the switch that lets the electricity flow through
the long resistance wire that passes around the porcelain posts. Watch the
wire.

The resistance wire you are using is an alloy, a mixture of metals that will
resist electricity much more than ordinary metals will. This is the same kind of
wire that is used in electric irons and toasters and heaters. It has so great a
resistance to the electricity that it is heated red hot, or almost white hot, by the
electricity passing through it.

Application 57. A power company wanted to send large quantities of
electricity down from a mountain. Should the company have obtained
resistance wire or copper wire to carry it? Should the wire have been large
or fine?

Application 58. A firm was making electric toasters. In the experimental
laboratory they tried various weights of resistance wire for the toasters.
They tried a very fine wire, No. 30; a medium weight wire, No. 24; and a
heavy wire, No. 18. One of these wires did not get hot enough, and it took
so much electricity that it would have been too expensive to run; another
got so hot that it soon burned out. One worked satisfactorily. Which of the
three sizes burned out? Which was satisfactory?

Inference Exercise

Explain the following:

341. If you attach one end of a wire to a water faucet and connect the
other end to an electric lamp in place of one of the regular lighting wires,
the lamp will light.

342. The needle of a sewing machine goes up and down many times to
each stroke of the treadle.

343. Trolley wires are bare.

344. If you had rubbers on your feet, you could take hold of one live wire
with perfect safety, provided you touched nothing else.

345. If you were on the moon, you would look up at the earth.

346. Toy balloons burst when they go high up where the air is thin.

347. You have to put on the brakes to stop a car quickly.

348. Telephone wires are strung on glass supporters.

349. If you pour boiling water into a drinking glass, frequently the glass
will crack.

350. An asbestos mat tends to keep food from burning.

Fig. 132. Pencils ready for making an arc light.

Section 38. The electric arc.

How can electricity set a house on fire?

This is one of the most important sections in the book.

Do you know that you can make an arc light with two ordinary pencils? The
next experiment, which should be done by the class with the help of the
teacher, shows how to do it.

Experiment 71. Sharpen two pencils. About halfway between the point
and the other end of each pencil cut a notch all the way around and down
to the "lead," or burn a notch down by means of the glowing resistance
wire. What you call the "lead" of the pencil is really graphite, a form of
carbon. The leads of your two pencils are almost exactly like the carbons
used in arc lights, except, of course, that they are much smaller. Turn off
the electricity both at the snap switch and at the knife switch. Fasten the
bare end of a 2-foot piece of fine insulated wire (about No. 24) around the
center of the lead in each pencil so that you get a good contact, as shown
in Figure 132. Fasten the other bare end of each wire to either side of the
open knife switch so that when this switch is open the electricity will
have to pass down one wire to the lead of one pencil, from that to the lead
of the other pencil, and from that back through the second wire to the
other side of the knife switch and on around the circuit, as shown in
Figure 133. Keep the two pencils apart and off the desk, while some one
turns on the snap switch and the "flush" switch that lets the electricity
through the resistance wire. Now bring the pencil points together for an
instant, immediately drawing them apart about half an inch. You should
get a brilliant white arc light.

Fig. 133. The pencil points are touched together and immediately drawn
apart.

Caution: Do not look at this brilliant arc for more than a fraction of a
second unless you look through a piece of smoked or colored glass.

Blow out the flame when the wood catches fire. After you have done this
two or three times, the inside of the wood below the notches will be
burned out so completely that you can pull it off with your fingers,
leaving the lead bare all the way up to the wires.

Let the class stand well back and watch the teacher do the next part of the
experiment.

Connect two heavy insulated copper wires, about No. 12, to the sides of
the knife switch just as you connected the fine wires. But this time bring
the ends of the copper wires themselves together for an instant, then draw
them apart. Hold the ends of the wires over the zinc of the table while you
do this, as melted copper will drop from them.

Fig. 134. A brilliant arc light is the result.

What happens when an arc is formed. What happens when you form an
electric arc is this: As you draw the two ends of the pencils apart, only a speck
of the lead in each touches the other. The electricity passing for an instant
through the last speck at the end of the pencil makes it so hot that it turns to
vapor. The vapor will let electricity go through it, and makes a bridge from one
pencil point to the other. But the vapor gets very hot, because it has a rather
high resistance. This heat vaporizes more carbon and makes more vapor for the
electricity to pass through, and so on. The electricity passing through the
carbon vapor makes it white hot, and that is what causes the brilliant glow.
Regular arc lights are made exactly like this experimental one, except that the
carbons used are much bigger and are made to stand the heat better than the
small carbons in your pencil.

Carbon is one of those substances that turn directly from a solid to a gas
without first melting. That is one reason why it is used for arc lights. But
copper melts when it becomes very hot, as you saw when you made an arc
light with the copper wires. So copper cannot be used for practical arc lights.

Fires caused by arcs. There is one extremely important point about this
experiment with arcs: most fires that result from defective wiring are caused
by the forming of arcs. You see, if two wires touch each other while the current
is passing and then move apart a little, an arc is formed. And you have seen

how intensely hot such an arc is. Two wires rubbing against each other, or a
wire not screwed tightly to its connection, can arc. A wire broken, but with its
ends close enough together to touch and then go apart, can cause an arc. And
an arc is very dangerous in a house if there is anything burnable near it.

Wires should never be just twisted together and then bound with tape to form a
joint. Twisted wires sometimes break and sometimes come loose; then an arc
forms, and the house catches fire. Good wiring always means soldering every
joint and screwing the ends of the wires tightly into the switches or sockets to
which they lead.

Fig. 135. An arc lamp. The carbons are much larger than the carbons in
the pencils, and the arc gives an intense light.

Keeping arcs from forming. Well-wired houses have the wires brought in
through iron pipes, called conduits, and the conduits are always grounded; so if
an arc should form anywhere along the line, the house would be protected by
an iron conduit and if one of the loose ends of wire came in contact with the
conduit, the current would rush to the ground through it, blowing out a fuse.
The next section tells about the purpose of fuses.

The directions that usually come with electric irons, toasters, and stoves say
that the connection should be broken by pulling out the plug rather than by
turning off the switch. This is because the switch in the electric-light socket
sometimes loses its spring and instead of snapping all the way around and
quickly leaving a big gap, it moves only a little way around and an arc is
formed in the socket; if you hear a sizzling sound in a socket, you may be
pretty sure that an arc has been formed. But when you pull the plug entirely
out of the iron or stove, the gap is too big for an arc to form and you are
perfectly safe.

Fire commissions usually condemn extension lights, because if the insulation
wears out on a lamp cord so that the two wires can come in contact, a
dangerous arc may easily form. And the insulation might suddenly be scraped
off by something heavy moving across the cord. This can happen whether the
light at the end of the cord is turned on or off. So it is best if you have an
extension light always to turn it off at the socket from which the cord leads,
not at the lamp itself. Many people do not do this, and go for years without
having a fire. But so might you live for years with a stick of dynamite in your
bureau drawer and never have an explosion. Still, it is not wise to keep
dynamite in your bureau.

Arc lights themselves, of course, are no more dangerous than is a fire in a
kitchen stove. For an arc light is placed in such a way that nothing can well
come near it to catch fire. The danger from the electric arc is like the danger
from gasoline spilled and matches dropped where you are not expecting them,
so that you are not protected against them.

Fortunately ordinary batteries have not enough voltage to cause dangerous
arcs. So you do not have to be as careful in wiring for electric bells and
telegraph instruments. It requires the high voltage of a city power line to make
a dangerous electric arc.

So many fires are caused by electric arcs forming in buildings, that you had
better go back to the beginning of this section and read it all through again
carefully. It may save your home and even your life.

After you have reread this section, test your understanding of it by answering
the following questions:

1. How can you make an electric arc?

2. Why should wires not be twisted together to make electric connections?

3. Why should wires be brought into houses and through walls in iron
conduits?

4. Why should you pull out the plug of an electric iron, percolator, toaster,
heater, or stove?

5. Why do fire commissions condemn extension lights?

6. If you use an extension light, where should it be turned off?

7. If you hear a sizzling and sputtering in your electric-light socket, what does
it mean? What should you do?

8. Is there any danger in defective sockets with switches that do not snap off
completely? What is the danger?

9. In Application 55, page 228, if the rat had gnawed the wire in two while the
electric iron was being used, would anything have happened to the rat? Would
there have been any danger to the house?

10. Where a wire is screwed into an electric-light socket, what harm, if any,
might result from not screwing it in tightly?

11. How can a wire be safely spliced?

12. Why is an electric arc in a circuit dangerous?

Inference Exercise

Explain the following:

351. White objects look blue when seen through a blue glass.

352. When you pull the plug out of an electric iron, the iron cools.

353. People who do not hear well sometimes use speaking trumpets.

354. The sounding board of a piano is roughly triangular; the longest
strings are the extreme left, and those to the right get shorter and shorter.

355. Birds can sit on live wires without getting a shock.

356. Deaf people can sometimes identify musical selections by holding
their hands on the piano.

357. An electric toaster gets hot when a current passes through it.

358. The cord of an electric iron sometimes catches fire while the iron is
in use, especially if the cord is old.

359. If a live wire touches the earth or anything connected with it, the
current rushes into the earth.

360. When you stub your toe, you have to run forward to keep from
falling.

Section 39. Short circuits and fuses.

Why does a fuse blow out?

Sometimes during the evening when the lights are all on in your home, some
one tinkers with a part of the electric circuit or turns on an electric heater or
iron, and suddenly all the lights in that part of the house go out. A fuse has
blown out. If you have no extra fuses on hand, it may be necessary to wait till
the next day to replace the one that is blown out. It is always a good idea to
keep a couple of extra fuses; they cost only 10 cents each. And if you do not
happen to know how fuses work or how to replace them when they blow out, it
will cost a dollar or so to get an electrician to put in a new fuse. The next three
experiments will help you to understand fuses.

Fig. 136. A, the "fuse gap" and B, the "nail plug."

Experiment 72. On the lower wire leading to the electric lamp in the
laboratory you will find a "gap," a place where the wire ends in a piece of
a knife switch, and then begins again about an inch away in another piece
of the switch, as shown in Figure 136. There must be some kind of wire
or metal that will conduct electricity across this gap. But the gap is there
to prevent as much electricity from flowing through as might flow
through copper wire. So never put copper wire across this gap. If you do,
you will have to pay for the other fuses which may blow out. Always
keep a piece of fuse wire stretched across the gap. Fuse wire is a soft
leadlike wire, which melts as soon as too much electricity passes through
it.

Unscrew the lamp, and into the socket where it was, screw the plug with
the two nails sticking out of it. Turn the electricity on. Does anything
happen? Turn the electricity off. Now touch the heads of the two nails
together, or connect them with a piece of any metal, and turn on the
electricity. What happens? Examine the pieces of the fuse wire that are
left.

It was so easy for the electricity to pass through the nails and wire, that it
gushed through at a tremendous rate. This melted the fuse wire, or blew out the
fuse. If the fuse across the gap by the socket had not been the more easily
burned out, one or perhaps both of the more expensive fuses up above, where
the wire comes in, would have blown out. These cost about 10 cents each to

replace, while the fuse wire you burned out costs only a fraction of a cent. If
there were no fuses in the laboratory wirings and you had "short circuited" the
electricity (given it an easy enough path), it would have blown out the much
more expensive fuses where the electricity enters the building. If there were no
big fuses where the electricity enters the building, the rush of electricity would
make all the copper wires through which it flowed inside the building so hot
that they would melt and set fire to the building. As long as you keep a piece of
fuse wire across the gap, there is no danger from short circuits.

Why fuse wire melts. For two reasons, the fuse wire melts when ordinary wire
would not. First, it has enough resistance to electricity so that if many amperes
(much current) flow through, it gets heated. It has not nearly as much
resistance, however, as the filament in an electric lamp or even as has the long
resistance wire. It does not become white hot as they do.

Second, it has a low melting point. It melts immediately if you hold a match to
it; try this and see. Consequently, long before the fuse wire becomes red hot, it
melts in two. It has enough resistance to make it hot as soon as too many
amperes flow through; and it has such a low melting point that as soon as it
gets hot it melts in two, or blows out. This breaks the circuit, of course, so that
no more electricity can flow. In this way the fuse protects houses from
catching fire through short circuits.

Fig. 137. What will happen when the pin is thrust through the cords and
the electricity turned on?

Unfortunately, however, the fuse is almost no protection against an electric arc.
The copper vapor through which the electricity passes in an arc has enough
resistance to keep the amperage (current) low; so the arc may not blow out the
fuse at all. But if it were not for fuses, there would be about as much danger of
houses being set on fire by short circuits as by arcs. Perhaps there would be
more danger, because short circuits are the more common.

Experiment 73. Put a new piece of fuse wire across the fuse gap. Leave
the "nail plug" screwed in the socket. Use a piece of flexible lamp cord—
the kind that is made of two strands of wire twisted together (see Fig.
137). Fasten one bared end of each wire around each nail of the "nail
plug." See that the other ends of the lamp cord are not touching each
other. Turn on the electricity. Does anything happen? Turn off the
electricity. Now put a pin straight through the middle of the two wires.
Turn on the electricity again. What happens?

There is not much resistance in the pin, and so it allows the electricity to rush
through it. People sometimes cause fuses to blow out by pinning pictures to
electric lamp wires or by pinning the wires up out of the way.

A short circuit an "easy circuit." You always get a short circuit when you
give electricity an easy way to get from one wire to the other. But you get no
current unless you give it some way to pass from one wire to the other, thus
completing the circuit. Therefore you should always complete the circuit
through something which resists the flow of electricity, like an electric lamp, a
heater, or an iron. Remember this and you will have the key to an
understanding of the practical use of electricity.

The term "short circuit" is a little confusing, in that electricity may have to go
a longer way to be short circuited than to pass through some resistance, such as
a lamp. Really a short circuit should be called an "easy circuit" or something
like that, to indicate that it is the path of least resistance. Wherever the
electricity has a chance to complete its circuit without going through any
considerable resistance, no matter how far it goes, we have a short circuit. And
since everything resists electricity a little, a large enough flow of electricity
would even heat a copper wire red hot; that is why a short circuit would be
dangerous if you had no fuses.

Application 59. To test your knowledge of short circuits and fuses, trace
the current carefully from the upper wire as it enters the laboratory,
through the plug fuse. Show where it comes from to enter the plug fuse,
exactly how it goes through the fuse, where it comes out, and where it
goes from there. Trace it on through the cartridge fuse in the same way,
through all the switches into the lamp socket, through the lamp, out of the
lamp socket to the fuse gap, across this to the other wire, and on out of the
room.

It goes on from there through more fuses and back to the dynamo from
which the other wire comes.

Test yourself further with the following questions:

1. Where in this circuit is the resistance supposed to be?

2. What happens when you put a good conductor in place of this resistance if
the electricity can get from one wire to the other without passing through this
resistance?

3. Why do we use fuses?

4. What is a short circuit?

5. What makes an electric toaster get hot?

6. Why should you not stick pins through electric cords?

Experiment 74. Take the fuse wire out of the fuse gap and put a single
strand of zinc shaving in its place. Instead of the nail plug, screw the lamp
into the socket. Do not turn on the switch that lets the electricity flow
through the resistance wire, but turn on the electricity so that the lamp
will glow. Does the zinc shaving work satisfactorily as a fuse wire? Now
turn the electricity on through the resistance wire. What happens?

When are the greater number of amperes of electricity flowing through
the zinc shaving? (Note. "Amperes" means the amount of current
flowing.) Can the zinc shaving stand as many amperes as the fuse wire
you ordinarily use? Which lets more electricity pass through it, the lamp
or the resistance wire? Why do electric irons and toasters often blow out
fuses? If this happens at your home, examine the fuse and see how many
amperes (how much current) it will allow to flow through it. It will say
6A if it allows 6 amperes to pass through it; 25A if it allows 25 amperes to
pass through it, etc. The fuse wire across the fuse gap allows about 8
amperes to pass through before it melts. The zinc shaving allows only
about 2. Read the marks on the cartridge and plug fuses. How many
amperes will they stand?

Application 60. A family had just secured an electric heater. The first
night it was used, the fuse blew out.

The boy said: "Let's put a piece of copper wire across the fuse socket;
then there can't be any more trouble."

The father said that they had better get a new fuse to replace the old one.
The old fuse was marked 10A.

Was the boy or was the father right? If the father was right, should they
have got a fuse marked 6A, one marked 10A, or one marked 15A?

Application 61. The family were putting up an extension light. They
wanted the cord held firmly up out of the way. One suggested that they
drive a nail through both parts of the cord and into the wall. Another
thought it would be better to put a loop of string around the cord and
fasten the loop to the wall. A third suggested the use of a double-pointed
carpet tack that would go across the wires, but not through them, and if
driven tightly into the wall would hold the wire more firmly than would
the loop.

Which way was best?

Inference Exercise

Explain the following:

361. If the insulation wears off both wires of a lamp cord, the fuse will
blow out.

362. Street cars are heated by electricity.

363. The handles of pancake turners are often made of wood.

364. Glue soaks into the pores of pieces of wood and gradually hardens.

365. The glue then holds the pieces tightly together.

366. You need a fuse of higher amperage, as a 10-ampere fuse, instead of
a 6-ampere one, where you use electricity for an iron, and one of still
higher amperage for an electric stove.

367. You should be careful about turning on electric lights or doing
anything with electric wires when you are on a cement, iron, or earthen
floor, or if you are standing in water.

368. The keys and buttons with which you turn on electric lights are
usually made of a rubber composition.

369. Defective wiring, because of which bare wires may touch, has
caused many fires.

370. A person wearing glasses can sometimes see in them the image of a
person behind him.

Section 40. Electromagnets.

How is a telegram sent?

What carries your voice when you telephone?

So far we have talked about electricity only making heat and light by being
forced through something that resists it. But everybody knows that electricity
can be made to do another kind of work. It can be made to move things,—to
run street cars, to click telegraph instruments, to vibrate the thin metal disk in a
telephone receiver, and so on. The following experiments will show you how
electricity moves things:

Fig. 138. The magnetized bolt picks up the iron filings.

Experiment 75. Bare an inch of each end of a piece of insulated wire
about 10 feet long. Fasten one end to the zinc of your battery or to one
wire from the storage battery; wrap the wire around and around an iron

machine bolt, leaving the bolt a foot or so from the battery, until you have
only about a foot of wire left. Hold your bolt over some iron filings. Is it a
magnet? Now touch the free end of your wire to the carbon of your
battery or to the other wire from the storage battery, and hold the bolt over
the iron filings. Is it a magnet now?

You have completed the circuit by touching the free end of the wire to the
free pole of your battery; so the electricity flows through the wire, around
the bolt, and back to the battery.

Disconnect one end of the wire from the battery. You have now broken
the circuit, and the electricity can no longer flow around the bolt to
magnetize it. See if the bolt will pick up the iron filings any more; it may
keep a little of its magnetism even when no electricity is flowing, but the
magnetism will be noticeably less. When you disconnect the wire so that
the electricity can no longer flow through a complete circuit from its
source back to its source again, you are said to break the circuit.

Fig. 139. Sending a message with a cigar-box telegraph.

Experiment 76. Examine the cigar-box telegraph (see Appendix B) and
notice that it is made on the same principle as was the magnetized bolt in
Experiment 75. Complete the circuit through the electromagnet (the bolt
wound with wire) by connecting the two ends of the wire that is wrapped

around the bolt, with wires from the two poles of the battery. By making
and breaking the circuit (connecting and disconnecting one of the wires)
you should be able to make the lower bolt jump up and down and give the
characteristic click of the telegraph instrument.

Fig. 140. Connecting up a real telegraph instrument.

In this experiment it does not matter how long the wires are if the
batteries are strong enough. Of course it makes no difference where you
break the circuit. So you could have the batteries in the laboratory and the
cigar box a hundred miles away, with the wire going from the batteries to
the bolt and back again. Then if you made and broke the circuit at the
laboratory, the instrument would click a hundred miles away. If you want
to, you may take the cigar-box telegraph out into the yard, leaving the
batteries in the laboratory, while you try to telegraph this short distance.

Examine a regular telegraph instrument. Trace the wire from one binding
post, around the coil and through the key, back to the other binding post,
and notice how pushing down the key completes the circuit and how
raising it up breaks the circuit.

Experiment 77. Connect two regular telegraph instruments, leaving one
at each end of the long laboratory table. Make the connections as follows:

Take a wire long enough to go from one instrument to the other. Fasten
the bare ends of this wire into the right-hand binding post of the
instrument at your left, and into the left-hand binding post of the
instrument at your right; that is, connect the binding posts that are nearest
together, as in Figure 141.

Now connect one wire from the laboratory battery to the free post of the
right-hand instrument. Connect the other wire from the laboratory battery
to the ground through a faucet, radiator, or gas pipe, making the
connection firm and being sure that there is a good, clear contact between
the bare end of the wire and the metal to which the wire is attached.

Fig. 141. Diagram showing how to connect up two telegraph instruments.
The circles on the tables represent the binding posts of the instruments.

Make another ground connection near the left-hand instrument; that is,
take a wire long enough to reach from some pipe or radiator to the left-
hand telegraph instrument, bind one bare end of this wire firmly to a clean
part of the pipe and bring the other end toward the instrument. Before
attaching this other end to the free binding post of the left-hand
instrument, be sure to open the switch beside the telegraph key by
pushing it to your right. Close the switch on the other instrument. Now
attach the free ground wire to the free binding post of your telegraph
instrument, and press the key. Does the other instrument click? If not,
disconnect the ground wire and examine all connections. Also press the
sounder of each instrument down and see if it springs back readily. It may
be that some screw is too tight, or too loose, or that a spring has come off;

tinker awhile and see if you cannot make the instrument work. If you are
unable to do so, ask for help.

Fig. 142. Telegraphing across the room.

Figure 141 is a diagram of all the connections.

When you want to telegraph, open the switch of the instrument you want
to send from and close the switch of the instrument which is to receive the
message.

Holding the key down a little while, then letting it up, makes a "dash,"
while letting it spring up instantly, makes a "dot."

Practice making dots and dashes. Telegraph the word "cat," using the
alphabet shown on the next page. Telegraph your own name; your
address.

Here is the Morse telegraph code in dots and dashes:

Letters

Numerals

By using the Morse code, telegraph and cable messages are sent all over the
world in a few seconds. The ability to send messages in this way arose from
the simple discovery that when an electric current passes around a piece of
iron, it turns the iron into a magnet.

How a telephone works. A telephone is much like a delicate and complicated
telegraph in which the vibrations started by your voice press the "key," and in
which the sounder can vibrate swiftly in response to the electric currents
passing through the wire. The "key" in the telephone is a thin metal disk that
vibrates easily, back of the rubber mouthpiece. Each time an air vibration from
your voice presses against it, it increases the current flowing in the circuit. And
each time the current in the circuit is increased, the disk in the receiver is
pulled down, just as the sounder of a telegraph is pulled down. So every
vibration of the disk back of the mouthpiece causes a vibration of the disk in
the receiver of the other telephone; this makes the air over it vibrate just as
your voice made the mouthpiece vibrate, and you get the same sound.

To make a difference between slight vibrations and larger ones in telephones,
there are some carbon granules between the mouthpiece disk and a disk behind
it; and there are various other complications, such as the bell-ringing apparatus

and the connections in the central office. But the principle of the telephone is
almost exactly the same as the principle of the telegraph. Both depend entirely
on the fact that an electric current passing around a piece of iron magnetizes
the iron.

Experiment 78. By means of your battery, make an electric bell ring.
Examine the bell and trace the current through it. Notice how the current
passes around two iron bars and magnetizes them, as it did in the
telegraph instrument. Notice that the circuit is completed through a little
metal attachment on the base of the clapper, and that when the clapper is
pulled toward the electromagnet the circuit is broken. The iron bars are
then no longer magnetized. Notice that a spring pulls the clapper back
into place as soon as the iron stops attracting it. This completes the circuit
again and the clapper is pulled down. That breaks the circuit and the
clapper springs back. See how this constant making and breaking of the
circuit causes the bell clapper to fly back and forth.

Fig. 143. The bell is rung by electromagnets.

The electric bell, like the telephone and telegraph, works on the simple
principle that electricity flowing through a wire that is wrapped around and
around a piece of iron will turn that piece of iron into a magnet as long as the
electricity flows.

The electric motor. The motor of a street car is a still more complicated
carrying out of the same principle. In the next experiment you will see the
working of a motor.

Experiment 79. Connect the wires from the laboratory battery to the two
binding posts of the toy motor, and make the motor run. Examine the
motor and see that it is made of several electromagnets which keep
attracting each other around and around.

Motors, and therefore all things that are moved by electricity, including trolley
cars and electric railways, submarines while submerged, electric automobiles,
electric sewing machines, electric vacuum cleaners, and electric player-pianos,
are moved by magnetizing a piece of iron and letting this pull on another piece
of iron. And the iron is magnetized by letting a current of electricity flow
around and around it.

Fig. 144. A toy electric motor that goes.

The making of various kinds of electromagnets and putting currents of
electricity to work is becoming one of the great industries of mankind.
Waterfalls are being hitched up to dynamos everywhere, and the water power
that once turned the mill wheels now turns millions of coils of wire between
the poles of powerful magnets. The current generated in this way is used for all
kinds of work—not only for furnishing light to cities, and cooking meals,

heating homes, and ironing clothes,
but for running powerful motors in
factories, for driving interurban
trains swiftly across the country,
for carrying people back and forth
to work in city street cars, for
lifting great pieces of iron and steel
in the yards where huge
electromagnets are used,—for
countless pieces of work in all parts
of the globe. Yet the use of
electricity is still only in its
beginning. Tremendous amounts of
water power are still running to
waste; there is almost no limit to
the amount of electricity we shall Fig. 145. An electric motor of
be able to generate as we use the commercial size.
world's water power to turn our
dynamos.

Application 62. Explain how pressing a telegraph key can make another
instrument click hundreds of miles away, and how you can hear over the
telephone. Is it vibrations of sound or of electricity that go through the
telephone wire, or does your voice travel over it, or does the wire itself
vibrate? Explain how electricity can make a car go.

Inference Exercise

Explain the following:

371. When a fuse blows out, you can get no light.

372. If you lay your ear on a desk, you hear the sounds in the room
clearly.

373. If you touch a live wire with wet hands, you get a much worse shock
than if you touch it with dry hands.

374. A park music stand is backed by a sounding board.

375. The clapper of an electric bell is pulled against the bell when you
push the button.

376. A hot iron tire put on a wagon wheel fits very tightly when it cools.

377. Candy will cool more rapidly in a tin plate than in a china plate.

378. When a trolley wire breaks and falls to the ground it melts and burns
at the point at which it touches the ground.

379. By allowing the electricity from the trolley wire to flow down
through an underground coil of wire, a motorman can open a switch in the
track.

380. The bare ends of the two wires leading to your electric lamp should
never be allowed to touch each other.

CHAPTER NINE
MINGLING OF MOLECULES

Section 41. Solutions and emulsions.

How does soap make your hands clean?

Why will gasoline take a grease spot out of your clothes?

If we were to go back to our convenient imaginary switchboard to turn off
another law, we should find near the heat switches, and not far from the
chemistry ones, a switch labeled Solution. Suppose we turned it off:

The fishes in the sea are among the first creatures to be surprised by our action.
For instantly all the salt in the ocean drops to the bottom like so much sand,
and most salt-water fishes soon perish in the fresh water.

If some one is about to drink a cup of tea and has sweetened it just to his taste,
you can imagine his amazement when, bringing it to his lips, he finds himself
drinking tasteless, white, milky water. Down in the bottom of the cup is a
sediment of sugar, like so much fine gravel, with a brownish dust of tea
covering it.

To see whether or not the trouble is with the sugar itself, he may take some
sugar out of the bowl and taste it,—it is just like white sand. Wondering what
has happened, and whether he or the sugar is at fault, he reaches for the
vinegar cruet. The vinegar is no longer clear, but is a colorless liquid with tiny
specks of brown floating about in it. Tasting it, he thinks it must be dusty
water. Salt, pepper, mustard, onions, or anything he eats, is absolutely tasteless,
although some of the things smell as strong as ever.

To tell the truth, I doubt if the man has a chance to do all of this experimenting.
For the salt in his blood turns to solid hard grains, and the dissolved food in the
blood turns to dustlike particles. His blood flows through him, a muddy stream
of sterile water. The cells of his body get no food, and even before they miss

the food, most of the cells shrivel to drops of muddy water. The whole man
collapses.

Plants are as badly off. The life-giving sap turns to water with specks of the
one-time nourishment floating uselessly through it. Most plant cells, like the
cells in the man, turn to water, with fibers and dust flecks making it cloudy.
Within a few seconds there is not a living thing left in the world, and the
saltless waves dash up on a barren shore.

Probably we had better let the Solution switch alone, after all. Instead, here
are a couple of experiments that will help to make clear what happens when
anything dissolves to make a solution.

Experiment 80. Fill a test tube one fourth full of cold water. Slowly stir
in salt until no more will dissolve. Add half a teaspoonful more of salt
than will dissolve. Dry the outside of the test tube and heat the salty water
over the Bunsen burner. Will hot water dissolve things more readily or
less readily than cold? Why do you wash dishes in hot water?

Experiment 81. Fill a test tube
one fourth full of any kind of oil,
and one fourth full of water.
Hold your thumb over the top of
the test tube and shake it hard for
a minute or two. Now look at it.
Pour it out, and shake some
prepared cleanser into the test
tube, adding a little more water.
Shake the test tube thoroughly
and rinse. Put it away clean.

When you shake the oil with the
water, the oil breaks up into tiny
droplets. These droplets are so
small that they reflect the light
that strikes them and so look
Fig. 146. Will heating the water
white, or pale yellow. This milky
make more salt dissolve?
mixture is called an emulsion.
Milk is an emulsion; there are

tiny droplets of butter fat and other substances scattered all through the
milk. The butter fat is not dissolved in the rest of the milk, and the oil is
not dissolved in the water. But the droplets may be so small that an
emulsion acts almost exactly like a solution.

But when you shake or stir salt or sugar in water, the particles divide up
into smaller and smaller pieces, until probably each piece is just a single
molecule of the salt or sugar. And these molecules get into the spaces
between the water molecules and bounce around among them. They
therefore act like the water and let the light through. This is a solution.
The salt or sugar is dissolved in the water. Any liquid mixture which
remains clear is a solution, no matter what the color. Most red ink, most
blueing, clear coffee, tea, and ocean water are solutions. If a liquid is
clear, no matter what the color, you can be sure that whatever things may
be in it are dissolved.

Fig. 147. Will the volume be doubled when the alcohol and water are
poured together?

Experiment 82. Pour alcohol into a test tube (square-bottomed test tubes
are best for this experiment), standing the tube up beside a ruler. When
the alcohol is just 1 inch high in the tube, stop pouring. Put exactly the
same amount of water in another test tube of the same size. When you
pour them together, how many inches high do you think the mixture will
be? Pour the water into the alcohol, shake the mixture a little, and
measure to see how high it comes in the test tube. Did you notice the
warmth when you shook the tube?

If you use denatured alcohol, you are likely to have an emulsion as a
result of the mixing. The alcohol part of the denatured alcohol dissolves
in the water well enough, but the denaturing substance in the alcohol will
not dissolve in water; so it forms tiny droplets that make the mixture of
alcohol and water cloudy.

The purpose of this experiment is to show that the molecules of water get into
the spaces between the molecules of alcohol. It is as if you were to add a pail
of pebbles to a pail of apples. The pebbles would fill in between the apples,
and the mixture would not nearly fill two pails.

The most important difference between a solution and an emulsion is that the
particles in an emulsion are very much larger than those in a solution; but for
practical purposes that often does not make much difference. You dissolve a
grease spot from your clothes with gasoline; you make an emulsion when you
take it off with soap and water; but by either method you remove the spot. You
dissolve part of the coffee or tea in boiling water; you make an emulsion with
cocoa; but in both cases the flavor is distributed through the liquid. Milk is an
emulsion, vinegar is a solution; but in both, the particles are so thoroughly
mixed with the water that the flavor is the same throughout. Therefore in
working out inferences that are explained in terms of solutions and emulsions,
it is not especially important for you to decide whether you have a solution or
an emulsion if you know that it is one or the other.

How precious stones are formed. Colored glass is made by dissolving
coloring matter in the glass while it is molten. Rubies, sapphires, emeralds,
topazes, and amethysts were colored in the same way, but by nature. When the
part of the earth where they are found was hot enough to melt stone, the liquid
ruby or sapphire or emerald, or whatever the stone was to be, happened to be
near some coloring matter that dissolved in it and gave it color. Several of

these stones are made of exactly the same kind of material, but different kinds
of coloring matter dissolved in them when they were melted.

Many articles are much used chiefly because they are good emulsifiers or good
solvents (dissolve things well). Soap is a first-rate emulsifier; water is the best
solvent in the world; but it will not dissolve oil and gummy things sufficiently
to be of use when we want them dissolved. Turpentine, alcohol, and gasoline
find one of their chief uses as solvents for gums and oils. Almost all cleaning is
simply a process of dissolving or emulsifying the dirt you want to get rid of,
and washing it away with the liquid. Do not forget that heat helps to dissolve
most things.

Application 63. Explain why clothes are washed in hot suds; why sugar
disappears in hot coffee or tea; why it does not disappear as quickly in
cold lemonade; why you cannot see through milk as you can through
water.

Inference Exercise

Explain the following:

381. A kind of lamp bracket is made with a rubber cup. When you press
this cup against the wall or against a piece of furniture and exhaust the air
from the cup, the cup sticks fast to the wall and supports the lamp bracket.

382. You can take a vaseline stain out with kerosene.

383. If the two poles of an electric battery are connected with a copper
wire, the battery soon becomes discharged.

384. Electric bells have iron bars wound around and around with
insulated copper wire.

385. Piano keys may be cleaned with alcohol.

386. Linemen working with live wires wear heavy rubber gloves.

387. Crayon will not write on the smooth, glazed parts of a blackboard.

388. Varnish and shellac may be thinned with alcohol.

389. Filtering will take mud out of water, but it will not remove salt.

390. Explain why only one wire is needed to telegraph between two
stations.

Section 42. Crystals.

How is rock candy made?

Why is there sugar around the mouth of a syrup jug?

How are jewels formed in the earth?

You can learn how crystals are formed—and many gems and rock candy and
the sugar on a syrup jug are all crystals—by making some. Try this
experiment:

Experiment 83. Fill a test tube one fourth full of powdered alum; cover
the alum with boiling water; hold the tube over a flame so that the
mixture will boil gently; and slowly add boiling-hot water until all of the
alum is dissolved. Do not add any more water than you have to, and keep
stirring the alum with a glass rod while you are adding the water. Pour
half of the solution into another test tube for the next experiment. Hang a
string in the first test tube so that it touches the bottom of the tube. Set it
aside to cool, uncovered. The next day examine the string and the bottom
of the tube.

Experiment 84. While the solution of alum in the second test tube
(Experiment 83) is still hot, hold the tube in a pan of cold water and shake
or stir it until it cools. When white specks appear in the clear solution,
pour off as much of the clear part of the liquid as you can; then pour a
little of the rest on a glass slide, and examine the specks under a
microscope.

Fig. 148. Alum crystals.

In both of the above experiments, the hot water was able to dissolve more of
the alum than the cold water could possibly hold. So when the water cooled it
could no longer hold the alum in solution. Therefore part of the alum turned to
solid particles.

When the string was in the cooling liquid, it attracted the particles of alum as
they crystallized out of the solution. The force of adhesion drew the near-by
molecules to the string, then these drew the next, and these drew more, and so
on until the crystals were formed. But when you kept stirring the liquid while
it cooled, the crystals never had time to grow large before they were jostled
around to some other part of the liquid or were broken by your stirring rod.
Therefore they were small instead of large. Stirring or shaking a solution will
always make crystals form more quickly, but it will also make them smaller.

How rock candy is made. Rock candy is made by hanging a string in a strong
sugar solution or syrup and letting the water evaporate slowly until there is not
enough water to hold all the sugar in solution. Then the sugar crystals gather
slowly around the string, forming the large, clear pieces of rock candy. The
sugar around the mouth of a syrup jug is formed in the same way.

You always get crystallization when you make a liquid too cool to hold the
solid thing in solution, or when you evaporate so much of the liquid that there

is not enough left to keep the solid thing dissolved.

When you make fudge, the sugar forms small crystals as the liquid cools.
When a boat has been on the ocean, salt crystals form on the sails when the
spray that has wet them evaporates.

But crystals may form also in the air. There is always some moisture in the air,
and when it becomes very cold, some of this moisture forms crystals of ice. If
they form up in the clouds, they fall as snow. If they form around blades of
grass or on the sidewalk, as the alum crystals formed on the string, we have
frost.

Still another place that crystals occur is in the earth. When the rocks in the
earth were hot enough to be melted and then began to cool, certain substances
in the rocks crystallized. Some of these crystals that are especially hard and
clear constitute precious and semi-precious stones.

Application 64. Explain why you beat fudge as it cools; why the paper
around butter becomes encrusted with salt if it is exposed to the air for
some time.

Inference Exercise

Explain the following:

391. Dynamos have copper brushes to lead the current from the coils of
wire to the line wires.

392. A megaphone makes the voice carry farther than usual.

393. Copper wire is used to conduct electricity, although iron wire costs
much less.

394. A flute gives notes that differ in pitch according to the stops that are
opened.

395. There are usually solid pieces of sugar around the mouth of a syrup
jar.

396. You can beat eggs quickly with a Dover egg beater.

397. When ocean water stands in shallow open tanks for some time, salt
begins to form before the water has all evaporated.

398. In a coffee percolator the boiling water goes up through a tube. As
this water drips back through the ground coffee beans, it becomes brown
and flavored, and the coffee is made.

399. Kerosene will clean off the rim of soap and grease that forms in
bathtubs.

400. Beating cake frosting or candy causes it to sugar.

Section 43. Diffusion.

How does food get into the blood?

Why can you so quickly smell gas that is escaping at the opposite side of
a room?

On our imaginary switchboard the Diffusion switch would not be safe to
tamper with. It would be near the Solution switch, and almost as dangerous.
For if you were to make diffusion cease in the world, the dissolved food and
oxygen in your blood would do no good; it could not get out of the blood
vessels or into the cells of your body. You might breathe all you liked, but
breathing would not help you; the air could not get through the walls of your
lungs into the blood. Plants would begin to wither and droop, although they
would not die quite as quickly as animals and fishes and people. But no sap
could enter their roots and none could pass from cell to cell. The plants would
be as little able to breathe through their leaves as we through our lungs.

If gas escaped in the room where you were, you could not smell it even if you
stayed alive long enough to try; the gas would rise to the top of the room and
stay there. All gases and all liquids would stay as they were, and neither would
ever form mixtures.

It would not make so much difference in the dead parts of the world if
diffusion ceased; the rocks, mountains, earth, and sea would not be changed at
all at first. To be sure, the rivers where they flowed into the oceans would

make big spaces of saltless water; and when water evaporated from the ocean
the vapor would push aside the air and stay in a layer over the ocean, instead
of mixing with the air and rising to great heights. But the real disaster would
be to living things. All of them would be smothered and starved to death as
soon as diffusion ceased.

Here is an experiment that shows how gases diffuse:

Experiment 85. Take two test tubes with mouths of the same size so that
you can fit them snugly against each other when you want to. Fill one to
the brim with water and hold your thumb or a piece of cardboard over its
mouth while you place it upside down in a pan of water. Take the free end
of a rubber tube that is attached to a gas pipe and put it into the test tube a
short distance, so that the gas will go up into the tube, as shown in Figure
149. Now turn on the gas gently. When all the water has been forced out
of the tube and the gas bubbles begin to come up on the outside, turn off
the gas. Put a piece of cardboard, about an inch or so square, over the
mouth of the tube so that no air can get into it, and take the tube out of the
water, keeping the mouth down and covered. Bring the empty test tube,
which of course is full of air, mouth up under the test tube full of gas,
making the mouths of the two tubes meet with the cardboard between
them, as shown in Figure 150. Now have some one pull the cardboard
gently from between the two test tubes, so that the mouths of the tubes
will be pressed against each other and so that practically no gas will
escape. Hold them quietly this way, the tube of gas uppermost, for not
less than one full minute by the clock. A minute and a half is not too
much time. Now have some one light a match for you, or else go to a
lighted Bunsen burner.

Fig. 149. Filling a test tube with gas.

Fig. 150. The lower test tube is full of air; the upper, of gas. What will
happen when the cardboard is withdrawn?

Take the test tubes apart gently and hold the lower one, which was full of
air, with its mouth to the flame. What has the gas in the upper tube done?
Now hold the flame to the upper test tube, which was full of gas. What
happens? Has all the gas gone out of it?

As you well know, gas is much lighter than air; you can make a balloon rise by
filling it with gas. Yet part of the gas went down into the lower tube. The
explanation is that the molecules of gas and those of air were flying around at
such a rate that many of the gas molecules went shooting down among the air
molecules, and many of the molecules of air went shooting up among those of
gas, so that the gas and the air became mixed.

Diffusion in liquids. Diffusion takes place in liquids, as you know. For when
you put sugar in coffee or tea and do not stir it, although the upper part of the
tea or coffee is not sweetened, the part nearer the sugar is very sweet. If you
should let the coffee or tea, with the sugar in the bottom, stand for a few
months, it would get sweet all through. Diffusion is slower in liquids than in
gases, because the molecules are so very much closer together.

Osmosis. One of the most striking and important facts about diffusion is that it
can take place right through a membrane. Try this experiment:

Experiment 86. With a rubber band fasten a piece of parchment paper,
made into a little bag, to the end of a piece of glass tubing about 10 inches
long. Or make a small hole in one end of a raw egg and empty the shell;
then, to get the hard part off the shell, soak it overnight in strong vinegar
or hydrochloric acid diluted about 1 to 4. This will leave a membranous
bag that can be used in place of the parchment bag. Fill a tumbler half full
of water colored with red ink, and add enough cornstarch to make the
water milky. Pour into the tube enough of a strong sugar solution to fill
the membranous bag at its base and to rise half an inch in the tube. Put the
membranous bag down into the pink, milky water, supporting the tube by
passing it through a square cardboard and clamping it with a spring
clothespin as shown in Figure 151. Every few minutes look to see what is
happening. Does any of the red ink pass through the membrane? Does
any of the cornstarch pass through?

This is an example of diffusion through a membrane. The process is called
osmosis, and the pressure that forces the liquid up the tube is called osmotic
pressure. It is by this sort of diffusion that chicks which are being incubated
get air, and that growing plants get food. It is in this way that the cells of our
body secure food and oxygen and get rid of their wastes. There are no little
holes in our blood vessels to let the air get into them from our lungs. The air
simply diffuses through the thin walls of the blood vessels. There are no holes

from the intestinal tract into the blood vessels. Yet the dissolved food diffuses
right through the intestinal wall and through the walls of the blood vessels.
And later on, when it reaches the body cells that need nourishment, the
dissolved food diffuses out through the walls of the blood vessels again and
through the cell walls into the cells. Waste is taken out of the cells into the
blood and passes from the blood into the lungs and kidneys by this same
process of diffusion. So you can readily see why everything would die if
diffusion stopped.

Fig. 151. Pouring the syrup into the "osmosis tube."

Application 65. Explain how the roots of a plant can take in water and
food when there are no holes from the outside of the root to the inside;
how bees can smell flowers for a considerable distance.

Inference Exercise

Explain the following:

401. A shell in the bottom of a teakettle gathers most of the scale around it
and so keeps the scale from caking at the bottom of the kettle.

402. There is oxygen dissolved in water. When the water comes in contact
with the fine blood vessels in a fish's gills, some of this oxygen passes
through the walls of the blood vessels into the blood. Explain how it does
so.

403. Asphalt becomes soft in summer.

404. When the trolley comes off the wire the car soon stops.

405. You cannot see stars in the daytime on earth, yet you could see them
in the daytime on the airless moon.

406. Although the carbon dioxid you breathe out is heavier than the rest
of the air, part of it goes up and mixes with the air above.

407. On a cold day wood does not feel as cold as iron.

408. To make mayonnaise dressing, the oil, egg, and vinegar are
thoroughly beaten together.

409. A solution of iodin becomes stronger if it is allowed to stand open to
the air.

410. A drop of milk in a glass of water clouds all the water slightly.

Section 44. Clouds, rain, and dew: Humidity.

Why is it that you can see your breath on a cold day?

Where do rain and snow come from?

What makes the clouds?

There is water vapor in the air all around us—invisible water vapor, its
molecules mingling with those of the air—water that has evaporated from the
oceans and lakes and all wet places.

This water vapor changes into droplets of water when it gets cool enough. And
those droplets of water make up our clouds and fogs; they join together to form
our rain and snow high in the air, or gather as dew or frost on the grass at
night.

If the water vapor should suddenly lose its power of changing into droplets of
water when it cooled,—well, let us pretend it has lost this power but that any
amount of water can evaporate, and see what happens:

What fine weather it is! There is not a cloud in the sky. As evening closes in,
the stars come out with intense brightness. The whole sky is gleaming with
stars—more than we have ever seen at night before.

The next morning we find no dew or frost on the grass. All the green things
look dry. As the day goes on, they begin to wilt and wither. We all wish the day
were not quite so fine—a little rain would help things wonderfully. Not a cloud
appears, however, and we water as much of our gardens as we can. They drink
the water greedily, and that night, again no dew or fog, and not the faintest
cloud or mist to dim the stars. And the new day once more brings the blazing
sun further to parch the land and plants. Day after day and night after night the
drought gets worse. The rivers sink low; brooks run dry; the edges of the lakes
become marshes. The marshes dry out to hardened mud. The dry leaves of the
trees rustle and crumble. All the animals and wood creatures gather around the
muddy pools that once were lakes or rivers. People begin saving water and
buying it and selling it as the most precious of articles.

As the months go by, winter freezes the few pools that remain. No snow falls.
Living creatures die by the tens of thousands. But the winter is less cold than
usual, because there is now so much water vapor in the air that it acts like a
great blanket holding in the earth's heat.

With spring no showers come. The dead trees send forth no buds. No birds
herald the coming of warm weather. The continents of the world have become
vast, uninhabitable deserts. People have all moved to the shores of the ocean,
where their chemists are extracting salt from the water in order to give them
something to drink. By using this saltless water they can irrigate the land near
the oceans and grow some food to live on. Each continent is encircled by a
strip of irrigated land and densely populated cities close to the water's edge.

It is many years before the oceans disappear. But in time they too are
transformed into water vapor, and no more life as we know it is possible in the
world. The earth has become a great rocky and sandy ball, whirling through
space, lifeless and utterly dry.

That which prevents this from really happening is very simple: In the world as
it is, water vapor condenses and changes to drops of water whenever it gets
cool enough.

How water vapor gets into the air. The water vapor gets into the air by
evaporation. When we say that water evaporates, we mean that it changes into
water vapor. As you already know, it is heat that makes water evaporate; that is
why you hang wet clothes in the sun or by the fire to dry: you want to change
the water in them to water vapor. The sun does not suck up the water from the
ocean, as some people say; but it warms the water and turns part of it to vapor.

What happens down among the molecules when water evaporates is this: The
heat makes the molecules dance around faster and faster; then the ones with
the swiftest motion near the top shoot off into the air. The molecules that have
shot off into the air make up the water vapor.

The water vapor is entirely invisible. No matter how much of it there is, you
cannot see it. The weather is just as clear when there is a great deal of water
vapor in the air as when there is very little, as long as none of the vapor
condenses.

How clouds are formed. But when water vapor condenses, it forms into
extremely small drops of real water. Each of these drops is so small that it is
usually impossible to see one; they are so tiny that you could lay about 3000 of
them side by side in one inch! Yet, small as they are, when there are many of
them they become distinctly visible. We see them floating around us
sometimes and call them fog or mist. And when there are millions of them
floating in the air high above us, we call them a cloud.

The reason clouds form so high in the air is this: When air or any gas expands,
it cools. Do you remember Experiment 31, where you let the gas from a tank
expand into a wet test tube and it became so cold that the water on the test tube
froze? Well, it is much the same way with rising air. When air rises, there is

less air above it to keep it compressed; so it expands and cools. Then the water
vapor in it condenses into droplets of water, and these form a cloud.

Each droplet forms a gathering place for more condensing water vapor, and
therefore grows. When the droplets of water in a cloud are very close together,
some may be jostled against one another by the wind. And when they touch
each other, they stick together, forming a larger drop. When a drop grows large
enough it begins to fall through the cloud, gathering up the small droplets as it
goes. By the time it gets out of the cloud it has grown to a full-sized raindrop,
and falls to earth. The complete story of rain, then, is this:

How rain is caused. The surface of the oceans and lakes is warmed by the
sun. The water evaporates, turning to invisible water vapor. This water vapor
mingles with the air. After a while the air is caught in a rising current and
swept up high, carrying the water vapor with it. As the air rises, there is less air
above it to press down on it; so it expands. When air expands it cools, and the
water vapor which is mingled with it likewise cools. When the water vapor
gets cool enough it condenses, changing to myriads of extremely small drops
of water. These make a cloud.

A wind comes along; that is, the air in which the cloud is floating moves. The
wind carries the cloud along with it. More rising air, full of evaporated water
from the ocean, joins the cloud and cools, and the water forms into more tiny
droplets. The droplets get so close together that they shut out the sun's light
from the earth, and people say that the sky is darkening.

Meanwhile some of the droplets begin to touch each other and to stick
together. Little by little the drops grow bigger by joining together. Pretty soon
they get so big and heavy that they can no longer float high in the air, and they
fall to the ground as rain.

Part of the rain soaks into the ground. Some of it gradually seeps down
through the ground to an underground stream. This has its outlet in a spring or
well, or in an open lake or the ocean. But the rain does not all soak in. After
the storm, some of the water again evaporates from the top of the ground and
mixes with the warm air, and it goes through the same round. Other raindrops
join on the ground to form rivulets that trickle along until they meet and join
other rivulets; and all go on together as a brook. The brook joins others until
the brooks form a river; and the river flows into a lake or into the ocean.

Then again the sun warms the surface of the ocean or lake; the water
evaporates and mixes with the air, which rises, expands, and cools; the droplets
form and make clouds; the droplets join, forming big drops, and they fall once
more as rain. The rain soaks into the ground or runs off in rivulets, and sooner
or later it is once more evaporated. And so the cycle is repeated again and
again.

And all this is accounted for by the simple fact that when water evaporates its
vapor mingles with the air; and when this vapor is sufficiently cooled it
condenses and forms droplets of water.

The barometer. In predicting the weather a great deal of use is made of an
instrument called the barometer. The barometer shows how hard the air around
it is pressing. If the air is pressing hard, the mercury in the barometer rises. If
the air is not pressing hard the mercury sinks. Just before a storm, the air
usually does not press so hard on things as at other times; so usually, just
before a storm, the mercury in the barometer is lower than in clear weather.
You will understand the barometer better after you make one. Here are the
directions for making a barometer:

Experiment 87. To be done by the class with the aid of the teacher. Use a
piece of glass tubing not less than 32 inches long, sealed at one end. Fill
this tube to the brim with mercury (quicksilver), by pouring the mercury
into it through a paper funnel. Have the sealed end of the tube in a cup, to
catch any mercury that spills.7 When the tube is full, pour mercury into
the cup until there is at least half an inch of it at the bottom. Now put your
forefinger very tightly over the open end of the tube, take hold of the
sealed end with your other hand, and turn the tube over. Lower the open
end, with your finger over it, into the cup. When the mercury in the cup
completely covers your finger and the end of the tube, remove your finger
carefully so that no air can get up into the tube of mercury. Let the open
end of the tube rest gently on the bottom of the cup, and hold the tube
upright with your hand or by clamping it to a ring stand. Hold a yardstick
or meter stick beside the tube, remembering to keep the tube straight up
and down. Measure accurately the height of the mercury column from the
surface of the mercury in the cup. Then go to the regular barometer
hanging on the wall, and read it.

Fig. 152. Filling the barometer tube with mercury.

The reason your barometer may not read exactly the same as the
expensive laboratory instrument is that a little air and water vapor stick to
the inside of the tube and rise into the "vacuum" above the mercury; also,
the tube may not be quite straight up and down. Otherwise the readings
would be the same.
Footnote 7: If mercury spills on the floor or table during this experiment, gather it
all into a piece of paper by brushing even the tiny droplets together with a soft
brush; squeeze it through a towel into a cup to clean it. It is expensive; so try not to
lose any of it.

Of course you understand what holds the mercury up in the tube. If you could
put the cup of mercury into a vacuum, the mercury in the tube would sink
down into the cup. But the pressure of the air on the surface of the mercury in
the cup keeps the mercury from flowing out of the tube and so leaving a
vacuum in there. If the air pushes down hard on the mercury in the cup, the
mercury will stand high in the tube. This is called high pressure. If the air does
not press hard on the mercury in the cup, the mercury stands low in the tube.
This is called low pressure.

How weather is forecast. Weather
forecasters make a great deal of use of the
barometer, for storms are usually
accompanied by low pressure, and clear
weather nearly always goes with high
pressure.

The reason storms are usually accompanied
by low pressure is this: A storm is almost
always due to the rising of air, for the rising
air expands and cools, and if there is much
water vapor in it, this condenses when it
cools and forms clouds and rain. Now air
rises only when there is comparatively little
pressure from above. Therefore, before and
during a storm there is not so much
pressure on the mercury of the barometer
and the barometer is low.
Fig. 153. Inverting the filled Clear weather, on the other hand, is often
tube in the cup of mercury. the result of air being compressed, for
compressing air warms it. When air is
being warmed, the water vapor in it will
not condense; so the air remains clear. But when the air is being compressed, it
presses hard on the mercury of the barometer; the pressure is high, and the
mercury in the barometer rises high. Therefore when the mercury in the
barometer is rising, the weather is usually clear.

These two statements are true only in a very general way, however. If weather
forecasters had only their own barometers to go by, they would not be of much
value; for one thing, they could not tell us that a storm was coming much
before it reached us. But there are weather stations all over the civilized world,
and they keep in touch with each other by telegraph. It is known that storms
travel from west to east in our part of the world. If one weather man reports a
storm at his station, and tells how his barometer stands, the weather men to the
east of him know that the storm is coming their way. From several such reports
the weather men to the east can tell how fast the storm is traveling and exactly
which way it is going. Then they can tell when it will reach their station and
can make the correct prediction.

Weather men do not have to wait
for an actual storm to be reported.
If the reports from the west show
that the air is rising as it swirls
along—that is, if the barometer
readings in the west are low—they
know that this low-pressure air is
approaching them. And they know
that low pressure usually means air
that is rising and cooling and
therefore likely to drop its
moisture. In the same way, if the
barometers to the west show high
pressure, the eastern weather men
know that the air that is blowing
toward them is being compressed
and warmed, and is therefore not at Fig. 154. Finding the pressure of the
all likely to drop its moisture; so air by measuring the height of the
they predict fair weather. mercury in the tube.

The weather man is not ever certain
of his forecasts, however.
Sometimes the air will begin to rise
just before it gets to him. Then
there may be a shower of rain when
he has predicted fair weather. Or
sometimes the air that has been
rising to the west, and which has
made him predict bad weather, may
stop rising; the storm may be over
before it reaches his station. Then
his prediction of bad weather is
wrong. Or sometimes the storm
unexpectedly changes its path.
There are many ways in which a
weather prophecy may go wrong;
and then we blame the weather
man. We are likely to remember the

Fig. 155. The kind of mercury times that his prophecy is mistaken
barometer that you buy. and to forget the many, many times
when it is right.

Fig. 156. An aneroid barometer is more convenient than one made with
mercury. The walls are forced in or spring back out according to the
pressure of the air. This movement of the walls forces the hand around.

How snow is formed. The difference between the ways in which snow and
rain are formed is very slight. In both cases water evaporates and its vapor
mingles with the warm air. The warm air rises and expands. It cools as it
expands, and when it gets cool enough the water vapor begins to condense. But
if the air as it expands becomes very cold, so cold that the droplets of water
freeze as they form and gather together to make delicate crystals of ice, snow
is formed. The ice crystals found in snow are always six-sided or six-pointed,
because, probably, the water or ice molecules pull from six directions and
therefore gather each other together along the six lines of this pull. At any rate,
the tiny crystals of frozen water are formed and come floating down to the
ground; and we call them snowflakes. After the snow melts it goes through the
same cycle as the rain, most of it finally getting back to the ocean through
rivers, and there, in time, being evaporated once more.

Fig. 157. Different forms of snowflakes. Each snowflake is a collection of
small ice crystals.

Hail is rain that happens to be caught in a powerful current of rising air as it
forms, and is carried up so high that it freezes in the cold, expanding air into
little balls of ice, or hail stones, which fall to the ground before they have time
to melt.

Why one side of a mountain range usually has rainfall. When air that is
moving along reaches a mountain range, it either would have to stop, or rise
and go over the mountain. The pressure of the air behind it, moving in the
same direction, keeps it from stopping, and so it has to go up the slopes and
over the range. But as it goes up, there is less air above it to push down on it;
so it expands. This makes it cool, and the water vapor in it begins to condense
and form snow or rain. Therefore the side of mountain ranges against which
the wind usually blows, almost always has plenty of rainfall.

It is different on the farther side of the mountain range. For here the air is
sinking. As it sinks it is being compressed. And as it is compressed it is heated.
If you hold your finger over the mouth of a bicycle pump and compress the air
in the pump by pushing down on the handle, you will find that the pump is
decidedly warmed. When the air, sinking down on the farther side of the
mountain range, is heated, the water vapor in it is not at all likely to condense.
Therefore rain seldom falls on the side of the mountains which is turned away
from the prevailing winds.

How dew and frost are formed. The heat of the earth radiates out into the air
and on out into space. At night, when the earth loses its heat this way and does
not receive heat from the sun, it becomes cooler. When the air, carrying its
water vapor, touches the cool leaves and flowers, the water vapor is condensed
by the coolness and forms drops of dew upon them. Or, if the night is colder,

the droplets freeze as they form, and in the morning we see the grass and
shrubs all covered with frost.

The cause of fogs. When warm air is cooled while it is down around us, the
water vapor in it condenses into myriads of droplets that float in the air and
make it foggy. The air may be cooled by blowing in from the warm lake or
ocean in the early morning, for at night the land cools more rapidly than the
water does. This accounts for the early morning fogs in many cities that are on
the coasts.

Likewise when the wind has been blowing over a warm ocean current, the
surface of the warm water evaporates and fills the air with water vapor. Then
when this air passes over a cold current, the cold current cools the air so much
that the moisture in it condenses and forms fog. That is why there are fog
banks, dangerous to navigation, in parts of the ocean, particularly off Labrador.

Why you can see your breath on cold days. You really make a little fog
when you breathe on a cold morning. The air in your lungs is warm. The
moisture in the lungs evaporates into this warm air, and you breathe it out. If
the outside air is cold, your breath is cooled; so some of the water vapor in it
condenses into very small droplets, and you see your breath.

Here are two experiments in condensing water vapor by cooling the air with
which it is mixed. Both work best if the weather is warm or the air damp.

Experiment 88. Put the bell jar on the plate of the air pump and begin to
pump the air out of it. Watch the air in the jar. If the day is warm or damp,
a slight mist will form.

As part of the air is pumped out, the rest expands and cools, as warm air does
when it rises and is no longer pressed on so hard by the air above it. And as in
the case of the rising warm air, the water vapor condenses when it cools, and
forms the mist that you see. This mist, like all clouds and fog, consists of
thousands of extremely small droplets.

Experiment 89. Hold a saucer of ice just below your mouth. Open your
mouth wide and breathe gently over the ice. Can you see your breath?

Now put the ice into half a glass of water and cover the glass. Be sure the
outside of the glass is thoroughly dry. Set it aside and look at it again in a