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Title: Conversations on Chemistry, V. 1-2

Author: Mrs. Marcet

Release date: October 13, 2008 [eBook #26908]
Most recently updated: November 11, 2023

Language: English

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CONVERSATIONS
ON

C H E M I S T R Y;
IN WHICH

THE ELEMENTS OF THAT SCIENCE
ARE

FAMILIARLY EXPLAINED
AND

ILLUSTRATED BY EXPERIMENTS.

IN TWO VOLUMES.

The Fifth Edition, revised, corrected, and considerably enlarged.

LONDON:
PRINTED FOR LONGMAN, HURST, REES, ORME, AND BROWN,
PATERNOSTER-ROW.

1817.

Printed by A. Strahan,
Printers-Street, London.

ADVERTISEMENT.

The Author, in this fifth edition, has endeavoured to give an account of the
principal discoveries which have been made within the last four years in
Chemical Science, and of the various important applications, such as the
gas-lights, and the miner’s-lamp, to which they have given rise. But in
regard to doctrines or principles, the work has undergone no material
alteration.
London, July, 1817.

PREFACE.

In venturing to offer to the public, and more particularly to the female sex,
an Introduction to Chemistry, the author, herself a woman, conceives that
some explanation may be required; and she feels it the more necessary to
apologise for the present undertaking, as her knowledge of the subject is but
recent, and as she can have no real claims to the title of chemist.

On attending for the first time experimental lectures, the author found it
almost impossible to derive any clear or satisfactory information from the
rapid demonstrations which are usually, and perhaps necessarily, crowded
into popular courses of this kind. But frequent opportunities having
afterwards occurred of conversing with a friend on the subject of chemistry,
and of repeating a variety of experiments, she became better acquainted
with the principles of that science, and began to feel highly interested in its
pursuit. It was then that she perceived, in attending the excellent lectures
delivered at the Royal Institution, by the present Professor of Chemistry, the
great advantage which her previous knowledge of the subject, slight as it
was, gave her over others who had not enjoyed the same means of private
instruction. Every fact or experiment attracted her attention, and served to
explain some theory to which she was not a total stranger; and she had the
gratification to find that the numerous and elegant illustrations, for which
that school is so much distinguished, seldom failed to produce on her mind
the effect for which they were intended.
Hence it was natural to infer, that familiar conversation was, in studies of
this kind, a most useful auxiliary source of information; and more
especially to the female sex, whose education is seldom calculated to
prepare their minds for abstract ideas, or scientific language.
As, however, there are but few women who have access to this mode of
instruction; and as the author was not acquainted with any book that could
prove a substitute for it, she thought that it might be useful for beginners, as
well as satisfactory to herself, to trace the steps by which she had acquired
her little stock of chemical knowledge, and to record, in the form of
dialogue, those ideas which she had first derived from conversation.
But to do this with sufficient method, and to fix upon a mode of
arrangement, was an object of some difficulty. After much hesitation, and a
degree of embarrassment, which, probably, the most competent chemical
writers have often felt in common with the most superficial, a mode of
division was adopted, which, though the most natural, does not always
admit of being strictly pursued—it is that of treating first of the simplest
bodies, and then gradually rising to the most intricate compounds.

It is not the author’s intention to enter into a minute vindication of this plan.
But whatever may be its advantages or inconveniences, the method adopted
in this work is such, that a young pupil, who should occasionally recur to it,
with a view to procure information on particular subjects, might often find
it obscure or unintelligible; for its various parts are so connected with each
other as to form an uninterrupted chain of facts and reasonings, which will
appear sufficiently clear and consistent to those only who may have
patience to go through the whole work, or have previously devoted some
attention to the subject.
It will, no doubt, be observed, that in the course of these Conversations,
remarks are often introduced, which appear much too acute for the young
pupils, by whom they are supposed to be made. Of this fault the author is
fully aware. But, in order to avoid it, it would have been necessary either to
omit a variety of useful illustrations, or to submit to such minute
explanations and frequent repetitions, as would have rendered the work
tedious, and therefore less suited to its intended purpose.
In writing these pages, the author was more than once checked in her
progress by the apprehension that such an attempt might be considered by
some, either as unsuited to the ordinary pursuits of her sex, or ill-justified
by her own recent and imperfect knowledge of the subject. But, on the one
hand, she felt encouraged by the establishment of those public institutions,
open to both sexes, for the dissemination of philosophical knowledge,
which clearly prove that the general opinion no longer excludes women
from an acquaintance with the elements of science; and, on the other, she
flattered herself that whilst the impressions made upon her mind, by the
wonders of Nature, studied in this new point of view, were still fresh and
strong, she might perhaps succeed the better in communicating to others the
sentiments she herself experienced.
The reader will soon perceive, in perusing this work, that he is often
supposed to have previously acquired some slight knowledge of natural
philosophy, a circumstance, indeed, which appears very desirable. The
author’s original intention was to commence this work by a small tract,
explaining, on a plan analogous to this, the most essential rudiments of that
science. This idea she has since abandoned; but the manuscript was ready,

and might, perhaps, have been printed at some future period, had not an
elementary work of a similar description, under the tide of “Scientific
Dialogues,” been pointed out to her, which, on a rapid perusal, she thought
very ingenious, and well calculated to answer its intended object.

CONTENTS
Page numbers have been retained to give an idea of the relative length of
each Conversation.

CONVERSATION I.
ON THE GENERAL PRINCIPLES OF CHEMISTRY. Page 1

CONVERSATION II.
ON LIGHT AND HEAT. 26

CONVERSATION III.
CONTINUATION OF THE SUBJECT. 70

CONVERSATION IV.
ON COMBINED CALORIC, COMPREHENDING SPECIFIC HEAT AND LATENT 122
HEAT.

CONVERSATION V.
ON THE CHEMICAL AGENCIES OF ELECTRICITY. 160

CONVERSATION VI.
ON OXYGEN AND NITROGEN. 181

CONVERSATION VII.
ON HYDROGEN. 214

CONVERSATION VIII.
ON SULPHUR AND PHOSPHORUS. 256

CONVERSATION IX.
ON CARBON. 282

CONVERSATION X.
ON METALS. 314

CONVERSATION XIII.
ON THE ATTRACTION OF COMPOSITION. 1

CONVERSATION XIV.
ON ALKALIES. 19

CONVERSATION XV.
ON EARTHS. 44

CONVERSATION XVI.
ON ACIDS. 69

CONVERSATION XVII.
OF THE SULPHURIC AND PHOSPHORIC ACIDS: OR, THE COMBINATIONS OF 80
OXYGEN WITH SULPHUR AND WITH PHOSPHORUS; AND OF THE
SULPHATS AND PHOSPHATS.

CONVERSATION XVIII.
OF THE NITRIC AND CARBONIC ACIDS: OR THE COMBINATION OF OXYGEN 100
WITH NITROGEN AND WITH CARBON; AND OF THE NITRATS AND
CARBONATS.

CONVERSATION XIX.
ON THE BORACIC, FLUORIC, MURIATIC, AND OXYGENATED MURIATIC 131
ACIDS; AND ON MURIATS.

CONVERSATION XX.
ON THE NATURE AND COMPOSITION OF VEGETABLES. 162

CONVERSATION XXI.
ON THE DECOMPOSITION OF VEGETABLES. 202

CONVERSATION XXII.
HISTORY OF VEGETATION. 243

CONVERSATION XXIII.
ON THE COMPOSITION OF ANIMALS. 276

CONVERSATION XXIV.
ON THE ANIMAL ECONOMY. 297

CONVERSATION XXV.
ON ANIMALISATION, NUTRITION, AND RESPIRATION. 314

CONVERSATION XXVI.
ON ANIMAL HEAT; AND OF VARIOUS ANIMAL PRODUCTS. 336

ERRATA.
Vol. I. page 56. last line but one, for “caloric,” read “calorific.”
179. Note, for “Plate XII.” r. “Plate XIII.”

CONVERSATIONS
ON

C H E M I S T R Y;
IN WHICH

THE ELEMENTS OF THAT SCIENCE
ARE

FAMILIARLY EXPLAINED
AND

ILLUSTRATED BY EXPERIMENTS.

IN TWO VOLUMES.

The Fifth Edition, revised, corrected, and considerably enlarged.

VOL. I.
ON SIMPLE BODIES.

LONDON:
PRINTED FOR LONGMAN, HURST, REES, ORME, AND BROWN,
PATERNOSTER-ROW.

1817.

CONTENTS
OF

THE FIRST VOLUME.

ON SIMPLE BODIES.

CONVERSATION I.
ON THE GENERAL PRINCIPLES OF CHEMISTRY. Page 1
Connexion between Chemistry and Natural Philosophy.—Improved State of modern
Chemistry.—Its use in the Arts.—The general Objects of Chemistry.—Definition of
Elementary Bodies.—Definition of Decomposition.—Integrant and Constituent
Particles.—Distinction between Simple and Compound Bodies.—Classification of
Simple Bodies.—Of Chemical Affinity, or Attraction of Composition.—Examples of
Composition and Decomposition.

CONVERSATION II.
ON LIGHT AND HEAT. 26
Light and Heat capable of being separated.—Dr. Herschel’s Experiments.—
Phosphorescence.—Of Caloric.—Its two Modifications.—Free Caloric.—Of the three
different States of Bodies, solid, fluid, and aeriform.—Dilatation of solid Bodies.—
Pyrometer.—Dilatation of Fluids.—Thermometer.—Dilatation of Elastic Fluids.—Air
Thermometer.—Equal Diffusion of Caloric.—Cold a Negative Quality.—Professor
Prevost’s Theory of the Radiation of Heat.—Professor Pictet’s Experiments on the
Reflexion of Heat.—Mr. Leslie’s Experiments on the Radiation of Heat.

CONVERSATION III.
CONTINUATION OF THE SUBJECT. 70
Of the different Power of Bodies to conduct Heat.—Attempt to account for this Power.—
Count Rumford’s Theory of the non-conducting Power of Fluids.—Phenomena of
Boiling.—Of Solution in general.—Solvent Power of Water.—Difference between
Solution and Mixture.—Solvent Power of Caloric.—Of Clouds, Rain, Dr. Wells’ theory
of Dew, Evaporation, &c.—Influence of Atmospherical Pressure on Evaporation.—
Ignition.

CONVERSATION IV.
ON COMBINED CALORIC, COMPREHENDING SPECIFIC HEAT AND LATENT 122
HEAT.
Of Specific Heat.—Of the different Capacities of Bodies for Heat.—Specific Heat not
perceptible by the Senses.—How to be ascertained.—Of Latent Heat.—Distinction
between Latent and Specific Heat.—Phenomena attending the Melting of Ice and the
Formation of Vapour.—Phenomena attending the Formation of Ice, and the
Condensation of Elastic Fluids.—Instances of Condensation, and consequent
Disengagement of Heat, produced by Mixtures, by the Slaking of Lime.—General
Remarks on Latent Heat.—Explanation of the Phenomena of Ether boiling, and Water
freezing, at the same Temperature.—Of the Production of Cold by Evaporation.—
Calorimeter.—Meteorological Remarks.

CONVERSATION V.
ON THE CHEMICAL AGENCIES OF ELECTRICITY. 160

Of Positive and Negative Electricity.—Galvani’s Discoveries.—Voltaic Battery.—
Electrical Machine.—Theory of Voltaic Excitement.

CONVERSATION VI.
ON OXYGEN AND NITROGEN. 181
The Atmosphere composed of Oxygen and Nitrogen in the State of Gas.—Definition of
Gas.—Distinction between Gas and Vapour.—Oxygen essential to Combustion and
Respiration.—Decomposition of the Atmosphere by Combustion.—Nitrogen Gas
obtained by this Process.—Of Oxygenation in general.—Of the Oxydation of Metals.—
Oxygen Gas obtained from Oxyd of Manganese.—Description of a Water-Bath for
collecting and preserving Gases.—Combustion of Iron Wire in Oxygen Gas.—Fixed
and volatile Products of Combustion.—Patent Lamps.—Decomposition of the
Atmosphere by Respiration.—Recomposition of the Atmosphere.

CONVERSATION VII.
ON HYDROGEN. 214
Of Hydrogen.—Of the Formation of Water by the Combustion of Hydrogen.—Of the
Decomposition of Water. —Detonation of Hydrogen Gas.—Description of Lavoisier’s
Apparatus for the formation of Water.—Hydrogen Gas essential to the Production of
Flame.—Musical Tones produced by the Combustion of Hydrogen Gas within a Glass
Tube.—Combustion of Candles explained.—Gas lights.—Detonation of Hydrogen Gas
in Soap Bubbles.—Air Balloons.—Meteorological Phenomena ascribed to Hydrogen
Gas.—Miner’s Lamp.

The final two pages of the Table of Contents for Volume I were missing
from the available text; everything after “Decomposition of Water” was
supplied from earlier and later editions, compared against the body text.
The section marked “Diamond” (Conv. IX) was called “Diamond is
Carbon(e) in a state of perfect purity” in the 4th edn., “Diamond” alone in
later editions.

CONVERSATION VIII.
ON SULPHUR AND PHOSPHORUS. 256
Natural History of Sulphur.—Sublimation.—Alembic.—Combustion of Sulphur in
Atmospheric Air.—Of Acidification in general.—Nomenclature of the Acids.—
Combustion of Sulphur in Oxygen Gas.—Sulphuric Acid.—Sulphurous Acid.—
Decomposition of Sulphur.—Sulphurated Hydrogen Gas.—Harrogate, or Hydro-
sulphurated Waters.—Phosphorus.—History of its Discovery.—Its Combustion in
Oxygen Gas.—Phosphoric Acid.—Phosphorus Acid.—Eudiometer.—Combination of
Phosphorus with Sulphur.—Phosphorated Hydrogen Gas.—Nomenclature of Binary
Compounds.—Phosphoret of Lime burning under Water.

CONVERSATION IX.

ON CARBON. 282
Method of obtaining pure Charcoal.—Method of making common Charcoal.—Pure
Carbon not to be obtained by Art.—Diamond.—Properties of Carbon.—Combustion of
Carbon.—Production of Carbonic Acid Gas.—Carbon susceptible of only one Degree
of Acidification.—Gaseous Oxyd of Carbon.—Of Seltzer Water and other Mineral
Waters.—Effervescence.—Decomposition of Water by Carbon.—Of Fixed and
Essential Oils.—Of the Combustion of Lamps and Candles.—Vegetable Acids.—Of the
Power of Carbon to revive Metals.

CONVERSATION X.
ON METALS. 314
Natural History of Metals.—Of Roasting, Smelting, &c.—Oxydation of metals by the
Atmosphere.—Change of Colours produced by different degrees of Oxydation.—
Combustion of Metals.—Perfect Metals burnt by Electricity only.—Some Metals
revived by Carbon and other Combustibles.—Perfect Metals revived by Heat alone.—
Of the Oxydation of certain Metals by the Decomposition of Water. Power of Acids to
promote this Effect.—Oxydation of Metals by Acids.—Metallic Neutral Salts.—
Previous oxydation of the Metal requisite.—Crystallisation.—Solution distinguished
from Dissolution.—Five metals susceptible of acidification.—Meteoric Stones.—
Alloys, Soldering, Plating, &c.—Of Arsenic, and of the caustic Effects of Oxygen.—Of
Verdigris, Sympathetic Ink, &c.—Of the new Metals discovered by Sir H. Davy.

CONVERSATIONS

ON

C H E M I S T R Y.

CONVERSATION I.
ON THE GENERAL PRINCIPLES OF CHEMISTRY.

MRS. B.

AS you have now acquired some elementary notions of Natural
Philosophy, I am going to propose to you another branch of science, to
which I am particularly anxious that you should devote a share of your
attention. This is Chemistry, which is so closely connected with Natural
Philosophy, that the study of the one must be incomplete without some
knowledge of the other; for, it is obvious that we can derive but a very
imperfect idea of bodies from the study of the general laws by which they
are governed, if we remain totally ignorant of their intimate nature.
CAROLINE.

To confess the truth, Mrs. B., I am not disposed to form a very favourable
idea of chemistry, nor do I expect to derive much entertainment from it.
I prefer the sciences which exhibit nature on a grand scale, to those that are
confined to the minutiæ of petty details. Can the studies which we have
lately pursued, the general properties of matter, or the revolutions of the
heavenly bodies, be compared to the mixing up of a few insignificant
drugs? I grant, however, there may be entertaining experiments in
chemistry, and should not dislike to try some of them: the distilling, for
instance, of lavender, or rose water . . . . . .
MRS. B.

I rather imagine, my dear Caroline, that your want of taste for chemistry
proceeds from the very limited idea you entertain of its object. You confine
the chemist’s laboratory to the narrow precincts of the apothecary’s and
perfumer’s shops, whilst it is subservient to an immense variety of other

useful purposes. Besides, my dear, chemistry is by no means confined to
works of art. Nature also has her laboratory, which is the universe, and there
she is incessantly employed in chemical operations. You are surprised,
Caroline, but I assure you that the most wonderful and the most interesting
phenomena of nature are almost all of them produced by chemical powers.
What Bergman, in the introduction to his history of chemistry, has said of
this science, will give you a more just and enlarged idea of it. The
knowledge of nature may be divided, he observes, into three periods. The
first was that in which the attention of men was occupied in learning the
external forms and characters of objects, and this is called Natural History.
In the second, they considered the effects of bodies acting on each other by
their mechanical power, as their weight and motion, and this constitutes the
science of Natural Philosophy. The third period is that in which the
properties and mutual action of the elementary parts of bodies was
investigated. This last is the science of Chemistry, and I have no doubt you
will soon agree with me in thinking it the most interesting.
You may easily conceive, therefore, that without entering into the minute
details of practical chemistry, a woman may obtain such a knowledge of the
science as will not only throw an interest on the common occurrences of
life, but will enlarge the sphere of her ideas, and render the contemplation
of nature a source of delightful instruction.
CAROLINE.

If this is the case, I have certainly been much mistaken in the notion I had
formed of chemistry. I own that I thought it was chiefly confined to the
knowledge and preparation of medicines.
MRS. B.

That is only a branch of chemistry which is called Pharmacy; and, though
the study of it is certainly of great importance to the world at large, it
belongs exclusively to professional men, and is therefore the last that I
should advise you to pursue.
EMILY.

But, did not the chemists formerly employ themselves in search of the
philosopher’s stone, or the secret of making gold?

MRS. B.

These were a particular set of misguided philosophers, who dignified
themselves with the name of Alchemists, to distinguish their pursuits from
those of the common chemists, whose studies were confined to the
knowledge of medicines.
But, since that period, chemistry has undergone so complete a revolution,
that, from an obscure and mysterious art, it is now become a regular and
beautiful science, to which art is entirely subservient. It is true, however,
that we are indebted to the alchemists for many very useful discoveries,
which sprung from their fruitless attempts to make gold, and which,
undoubtedly, have proved of infinitely greater advantage to mankind than
all their chimerical pursuits.
The modern chemists, instead of directing their ambition to the vain attempt
of producing any of the original substances in nature, rather aim at
analysing and imitating her operations, and have sometimes succeeded in
forming combinations, or effecting decompositions, no instances of which
occur in the chemistry of Nature. They have little reason to regret their
inability to make gold, whilst, by their innumerable inventions and
discoveries, they have so greatly stimulated industry and facilitated labour,
as prodigiously to increase the luxuries as well as the necessaries of life.
EMILY.

But, I do not understand by what means chemistry can facilitate labour; is
not that rather the province of the mechanic?
MRS. B.

There are many ways by which labour may be rendered more easy,
independently of mechanics; but even the machine, the most wonderful in
its effects, the Steam-engine, cannot be understood without the assistance of
chemistry. In agriculture, a chemical knowledge of the nature of soils, and
of vegetation, is highly useful; and, in those arts which relate to the
comforts and conveniences of life, it would be endless to enumerate the
advantages which result from the study of this science.
CAROLINE.

But, pray, tell us more precisely in what manner the discoveries of chemists
have proved so beneficial to society?
MRS. B.

That would be an injudicious anticipation; for you would not comprehend
the nature of such discoveries and useful applications, as well as you will
do hereafter. Without a due regard to method, we cannot expect to make any
progress in chemistry. I wish to direct your observations chiefly to the
chemical operations of Nature; but those of Art are certainly of too high
importance to pass unnoticed. We shall therefore allow them also some
share of our attention.
EMILY.

Well, then, let us now set to work regularly. I am very anxious to begin.
MRS. B.

The object of chemistry is to obtain a knowledge of the intimate nature of
bodies, and of their mutual action on each other. You find therefore,
Caroline, that this is no narrow or confined science, which comprehends
every thing material within our sphere.
CAROLINE.

On the contrary, it must be inexhaustible; and I am a loss to conceive how
any proficiency can be made in a science whose objects are so numerous.
MRS. B.

If every individual substance were formed of different materials, the study
of chemistry would, indeed, be endless; but you must observe that the
various bodies in nature are composed of certain elementary principles,
which are not very numerous.
CAROLINE.

Yes; I know that all bodies are composed of fire, air, earth, and water;
I learnt that many years ago.
MRS. B.

But you must now endeavour to forget it. I have already informed you what
a great change chemistry has undergone since it has become a regular
science. Within these thirty years especially, it has experienced an entire
revolution, and it is now proved, that neither fire, air, earth, nor water, can
be called elementary bodies. For an elementary body is one that has never
been decomposed, that is to say, separated into other substances; and fire,
air, earth, and water, are all of them susceptible of decomposition.
EMILY.

I thought that decomposing a body was dividing it into its minutest parts.
And if so, I do not understand why an elementary substance is not capable
of being decomposed, as well as any other.
MRS. B.

You have misconceived the idea of decomposition; it is very different from
mere division. The latter simply reduces a body into parts, but the former
separates it into the various ingredients, or materials, of which it is
composed. If we were to take a loaf of bread, and separate the several
ingredients of which it is made, the flour, the yeast, the salt, and the water, it
would be very different from cutting or crumbling the loaf into pieces.
EMILY.

I understand you now very well. To decompose a body is to separate from
each other the various elementary substances of which it consists.
CAROLINE.

But flour, water, and other materials of bread, according to our definition,
are not elementary substances?
MRS. B.

No, my dear; I mentioned bread rather as a familiar comparison, to illustrate
the idea, than as an example.
The elementary substances of which a body is composed are called the
constituent parts of that body; in decomposing it, therefore, we separate its
constituent parts. If, on the contrary, we divide a body by chopping it to
pieces, or even by grinding or pounding it to the finest powder, each of

these small particles will still consist of a portion of the several constituent
parts of the whole body: these are called the integrant parts; do you
understand the difference?
EMILY.

Yes, I think, perfectly. We decompose a body into its constituent parts; and
divide it into its integrant parts.
MRS. B.

Exactly so. If therefore a body consists of only one kind of substance,
though it may be divided into its integrant parts, it is not possible to
decompose it. Such bodies are therefore called simple or elementary, as
they are the elements of which all other bodies are composed. Compound
bodies are such as consist of more than one of these elementary principles.
CAROLINE.

But do not fire, air, earth, and water, consist, each of them, but of one kind
of substance?
MRS. B.

No, my dear; they are every one of them susceptible of being separated into
various simple bodies. Instead of four, chemists now reckon upwards of
forty elementary substances. The existence of most of these is established
by the clearest experiments; but, in regard to a few of them, particularly the
most subtle agents of nature, heat, light, and electricity, there is yet much
uncertainty, and I can only give you the opinion which seems most probably
deduced from the latest discoveries. After I have given you a list of the
elementary bodies, classed according to their properties, we shall proceed to
examine each of them separately, and then consider them in their
combinations with each other.
Excepting the more general agents of nature, heat, light, and electricity, it
would seem that the simple form of bodies is that of a metal.
CAROLINE.

You astonish me! I thought the metals were only one class of minerals, and
that there were besides, earths, stones, rocks, acids, alkalies, vapours, fluids,

and the whole of the animal and vegetable kingdoms.
MRS. B.

You have made a tolerably good enumeration, though I fear not arranged in
the most scientific order. All these bodies, however, it is now strongly
believed, may be ultimately resolved into metallic substances. Your surprise
at this circumstance is not singular, as the decomposition of some of them,
which has been but lately accomplished, has excited the wonder of the
whole philosophical world.
But to return to the list of simple bodies—these being usually found in
combination with oxygen, I shall class them according to their properties
when so combined. This will, I think, facilitate their future investigation.
EMILY.

Pray what is oxygen?
MRS. B.

A simple body; at least one that is supposed to be so, as it has never been
decomposed. It is always found united with the negative electricity. It will
be one of the first of the elementary bodies whose properties I shall explain
to you, and, as you will soon perceive, it is one of the most important in
nature; but it would be irrelevant to enter upon this subject at present. We
must now confine our attention to the enumeration and classification of the
simple bodies in general. They may be arranged as follows:
CLASS I.
Comprehending the imponderable agents, viz.
heat or caloric,
light,
electricity.

CLASS II.
Comprehending agents capable of uniting with inflammable bodies, and in
most instances of effecting their combustion.
oxygen,
chlorine,

iodine. *

CLASS III.
Comprehending bodies capable of uniting with oxygen, and, forming with it
various compounds. This class may be divided as follows:
DIVISION 1.

hydrogen, forming water.
DIVISION 2.

Bodies forming acids.
nitrogen, forming nitric acid.
sulphur, forming sulphuric acid.
phosphorus, forming phosphoric acid.
carbon, forming carbonic acid.
boracium, forming boracic acid.
fluorium, forming fluoric acid.
muriatium, forming muriatic acid.

DIVISION 3.

Metallic bodies forming alkalies.
potassium, forming potash.
sodium, forming soda.
ammonium, forming ammonia.

DIVISION 4.

Metallic bodies forming earths.
calcium, or metal forming lime.
magnium, forming magnesia.
barium, forming barytes.
strontium, forming strontites.
silicium, forming silex.
alumium, forming alumine.
yttrium, forming yttria.
glucium, forming glucina.
zirconium, forming zirconi. *

DIVISION 5.

Metals, either naturally metallic, or yielding their oxygen to carbon or to
heat alone.
Subdivision 1.
Malleable Metals.
gold,
platina,
palladium,
silver *
mercury †
tin,
copper,
iron,
lead,
nickel,
zinc.

Subdiv. 2.
Brittle Metals.
arsenic,
bismuth,
antimony,
manganese,
tellurium,
cobalt,
tungsten,
molybdenum,
titanium,
chrome,
uranium,
columbium or tantalium,
iridium,
osmium,
rhodium. *

CAROLINE.

Oh, what a formidable list! You will have much to do to explain it, Mrs. B.;
for I assure you it is perfectly unintelligible to me, and I think rather
perplexes than assists me.
MRS. B.

Do not let that alarm you, my dear; I hope that hereafter this classification
will appear quite clear, and, so far from perplexing you, will assist you in
arranging your ideas. It would be in vain to attempt forming a division that
would appear perfectly clear to a beginner: for you may easily conceive that
a chemical division being necessarily founded on properties with which you
are almost wholly unacquainted, it is impossible that you should at once be
able to understand its meaning or appreciate its utility.
But, before we proceed further, it will be necessary to give you some idea
of chemical attraction, a power on which the whole science depends.
Chemical Attraction, or the Attraction of Composition, consists in the
peculiar tendency which bodies of a different nature have to unite with each
other. It is by this force that all the compositions, and decompositions, are
effected.
EMILY.

What is the difference between chemical attraction, and the attraction of
cohesion, or of aggregation, which you often mentioned to us, in former
conversations?
MRS. B.

The attraction of cohesion exists only between particles of the same nature,
whether simple or compound; thus it unites the particles of a piece of metal
which is a simple substance, and likewise the particles of a loaf of bread
which is a compound. The attraction of composition, on the contrary, unites
and maintains, in a state of combination, particles of a dissimilar nature; it
is this power that forms each of the compound particles of which bread
consists; and it is by the attraction of cohesion that all these particles are
connected into a single mass.
EMILY.

The attraction of cohesion, then, is the power which unites the integrant
particles of a body: the attraction of composition that which combines the
constituent particles. Is it not so?
MRS. B.

Precisely: and observe that the attraction of cohesion unites particles of a
similar nature, without changing their original properties; the result of such
an union, therefore, is a body of the same kind as the particles of which it is
formed; whilst the attraction of composition, by combining particles of a
dissimilar nature, produces compound bodies, quite different from any of
their constituents. If, for instance, I pour on the piece of copper, contained
in this glass, some of this liquid (which is called nitric acid), for which it
has a strong attraction, every particle of the copper will combine with a
particle of acid, and together they will form a new body, totally different
from either the copper or the acid.
Do you observe the internal commotion that already begins to take place? It
is produced by the combination of these two substances; and yet the acid
has in this case to overcome not only the resistance which the strong
cohesion of the particles of copper opposes to their combination with it, but
also to overcome the weight of the copper, which makes it sink to the
bottom of the glass, and prevents the acid from having such free access to it
as it would if the metal were suspended in the liquid.
EMILY.

The acid seems, however, to overcome both these obstacles without
difficulty, and appears to be very rapidly dissolving the copper.
MRS. B.

By this means it reduces the copper into more minute parts than could
possibly be done by any mechanical power. But as the acid can act only on
the surface of the metal, it will be some time before the union of these two
bodies will be completed.
You may, however, already see how totally different this compound is from
either of its ingredients. It is neither colourless, like the acid, nor hard,
heavy, and yellow like the copper. If you tasted it, you would no longer

perceive the sourness of the acid. It has at present the appearance of a blue
liquid; but when the union is completed, and the water with which the acid
is diluted is evaporated, the compound will assume the form of regular
crystals, of a fine blue colour, and perfectly transparent *. Of these I can
shew you a specimen, as I have prepared some for that purpose.
CAROLINE.

How very beautiful they are, in colour, form, and transparency!
EMILY.

Nothing can be more striking than this example of chemical attraction.
MRS. B.

The term attraction has been lately introduced into chemistry as a substitute
for the word affinity, to which some chemists have objected, because it
originated in the vague notion that chemical combinations depended upon a
certain resemblance, or relationship, between particles that are disposed to
unite; and this idea is not only imperfect, but erroneous, as it is generally
particles of the most dissimilar nature, that have the greatest tendency to
combine.
CAROLINE.

Besides, there seems to be no advantage in using a variety of terms to
express the same meaning; on the contrary it creates confusion; and as we
are well acquainted with the term Attraction in natural philosophy, we had
better adopt it in chemistry likewise.
MRS. B.

If you have a clear idea of the meaning, I shall leave you at liberty to
express it in the terms you prefer. For myself, I confess that I think the word
Attraction best suited to the general law that unites the integrant particles of
bodies; and Affinity better adapted to that which combines the constituent
particles, as it may convey an idea of the preference which some bodies
have for others, which the term attraction of composition does not so well
express.
EMILY.

So I think; for though that preference may not result from any relationship,
or similitude, between the particles (as you say was once supposed), yet, as
it really exists, it ought to be expressed.
MRS. B.

Well, let it be agreed that you may use the terms affinity, chemical
attraction and attraction of composition, indifferently, provided you
recollect that they have all the same meaning.
EMILY.

I do not conceive how bodies can be decomposed by chemical attraction.
That this power should be the means of composing them, is very obvious;
but that it should, at the same time, produce exactly the contrary effect,
appears to me very singular.
MRS. B.

To decompose a body is, you know, to separate its constituent parts, which,
as we have just observed, cannot be done by mechanical means.
EMILY.

No: because mechanical means separate only the integrant particles; they
act merely against the attraction of cohesion, and only divide a compound
into smaller parts.
MRS. B.

The decomposition of a body is performed by chemical powers. If you
present to a body composed of two principles, a third, which has a greater
affinity for one of them than the two first have for each other, it will be
decomposed, that is, its two principles will be separated by means of the
third body. Let us call two ingredients, of which the body is composed,
A and B. If we present to it another ingredient C, which has a greater
affinity for B than that which unites A and B, it necessarily follows that B
will quit A to combine with C. The new ingredient, therefore, has effected a
decomposition of the original body A B; A has been left alone, and a new
compound, B C, has been formed.
EMILY.

We might, I think, use the comparison of two friends, who were very happy
in each other’s society, till a third disunited them by the preference which
one of them gave to the new-comer.
MRS. B.

Very well. I shall now show you how this takes place in chemistry.
Let us suppose that we wish to decompose the compound we have just
formed by the combination of the two ingredients, copper and nitric acid;
we may do this by presenting to it a piece of iron, for which the acid has a
stronger attraction than for copper; the acid will, consequently, quit the
copper to combine with the iron, and the copper will be what the chemists
call precipitated, that is to say, it will be thrown down in its separate state,
and reappear in its simple form.
In order to produce this effect, I shall dip the blade of this knife into the
fluid, and, when I take it out, you will observe, that, instead of being wetted
with a bluish liquid, like that contained in the glass, it will be covered with
a thin coat of copper.
CAROLINE.

So it is really! but then is it not the copper, instead of the acid, that has
combined with the iron blade?
MRS. B.

No; you are deceived by appearances: it is the acid which combines with
the iron, and, in so doing, deposits or precipitates the copper on the surface
of the blade.
EMILY.

But, cannot three or more substances combine together, without any of them
being precipitated?
MRS. B.

That is sometimes the case; but, in general, the stronger affinity destroys the
weaker; and it seldom happens that the attraction of several substances for
each other is so equally balanced as to produce such complicated
compounds.

CAROLINE.

But, pray, Mrs. B., what is the cause of the chemical attraction of bodies for
each other? It appears to me more extraordinary or unnatural, if I may use
the expression, than the attraction of cohesion, which unites particles of a
similar nature.
MRS. B.

Chemical attraction may, like that of cohesion or gravitation, be one of the
powers inherent in matter which, in our present state of knowledge, admits
of no other satisfactory explanation than an immediate reference to a divine
cause. Sir H. Davy, however, whose important discoveries have opened
such improved views in chemistry, has suggested an hypothesis which may
throw great light upon that science. He supposes that there are two kinds of
electricity, with one or other of which all bodies are united. These we
distinguish by the names of positive and negative electricity; those bodies
are disposed to combine, which possess opposite electricities, as they are
brought together by the attraction which these electricities have for each
other. But, whether this hypothesis be altogether founded on truth or not, it
is impossible to question the great influence of electricity in chemical
combinations.
EMILY.

So, that we must suppose that the two electricities always attract each other,
and thus compel the bodies in which they exist to combine?
CAROLINE.

And may not this be also the cause of the attraction of cohesion?
MRS. B.

No, for in particles of the same nature the same electricities must prevail,
and it is only the different or opposite electric fluids that attract each other.
CAROLINE.

These electricities seem to me to be a kind of chemical spirit, which
animates the particles of bodies, and draws them together.
EMILY.

If it is known, then, with which of the electricities bodies are united, it can
be inferred which will, and which will not, combine together?
MRS. B.

Certainly.—I should not omit to mention, that some doubts have been
entertained whether electricity be really a material agent, or whether it
might not be a power inherent in bodies, similar to, or, perhaps identical
with, attraction.
EMILY.

But what then would be the electric spark which is visible, and must
therefore be really material?
MRS. B.

What we call the electric spark, may, Sir H. Davy says, be merely the heat
and light, or fire produced by the chemical combinations with which these
phenomena are always connected. We will not, however, enter more fully
on this important subject at present, but reserve the principal facts which
relate to it to a future conversation.
Before we part, however, I must recommend you to fix in your memory the
names of the simple bodies, against our next interview.

* It has been questioned by some eminent chemists, whether these
two last agents should not be classed among the inflammable
bodies, as they are capable of combining with oxygen, as well as
with inflammable bodies. But they seem to be more distinctly
characterised by their property of supporting combustion than by
any other quality.
* Of all these earths, three or four only have as yet been distinctly
decomposed.
* These first four metals have commonly been distinguished by
the appellation of perfect or noble metals, on account of their
possessing the characteristic properties of ductility, malleability,
inalterability, and great specific gravity, in an eminent degree.

† Mercury, in its liquid state, cannot, of course, be called a
malleable metal. But when frozen, it possesses a considerable
degree of malleability.
* These last four or five metallic bodies are placed under this
class for the sake of arrangement, though some of their properties
have not been yet fully investigated.
* These crystals are more easily obtained from a mixture of
sulphuric with a little nitric acid.

CONVERSATION II.
ON LIGHT AND HEAT OR CALORIC.

CAROLINE.

We have learned by heart the names of all the simple bodies which you
have enumerated, and we are now ready to enter on the examination of each
of them successively. You will begin, I suppose, with light?
MRS. B.

Respecting the nature of light we have little more than conjectures. It is
considered by most philosophers as a real substance, immediately
emanating from the sun, and from all luminous bodies, from which it is
projected in right lines with prodigious velocity. Light, however, being
imponderable, it cannot be confined and examined by itself; and therefore it
is to the effects it produces on other bodies, rather than to its immediate
nature, that we must direct our attention.
The connection between light and heat is very obvious; indeed, it is such,
that it is extremely difficult to examine the one independently of the other.
EMILY.

But, is it possible to separate light from heat; I thought they were only
different degrees of the same thing, fire?
MRS. B.

I told you that fire was not now considered as a simple element. Whether
light and heat be altogether different agents, or not, I cannot pretend to
decide; but, in many cases, light may be separated from heat. The first
discovery of this was made by a celebrated Swedish chemist, Scheele.
Another very striking illustration of the separation of heat and light was
long after pointed out by Dr. Herschell. This philosopher discovered that
these two agents were emitted in the rays of the sun, and that heat was less
refrangible than light; for, in separating the different coloured rays of light
by a prism (as we did some time ago), he found that the greatest heat was
beyond the spectrum, at a little distance from the red rays, which, you may
recollect, are the least refrangible.
EMILY.

I should like to try that experiment.
MRS. B.

It is by no means an easy one: the heat of a ray of light, refracted by a
prism, is so small, that it requires a very delicate thermometer to distinguish
the difference of the degree of heat within and without the spectrum. For in
this experiment the heat is not totally separated from the light, each
coloured ray retaining a certain portion of it, though the greatest part is not
sufficiently refracted to fall within the spectrum.
EMILY.

I suppose, then, that those coloured rays which are the least refrangible,
retain the greatest quantity of heat?
MRS. B.

They do so.
EMILY.

Though I no longer doubt that light and heat can be separated, Dr.
Herschell’s experiment does not appear to me to afford sufficient proof that

they are essentially different; for light, which you call a simple body, may
likewise be divided into the various coloured rays.
MRS. B.

No doubt there must be some difference in the various coloured rays. Even
their chemical powers are different. The blue rays, for instance, have the
greatest effect in separating oxygen from bodies, as was found by Scheele;
and there exist also, as Dr. Wollaston has shown, rays more refrangible than
the blue, which produce the same chemical effect, and, what is very
remarkable, are invisible.
EMILY.

Do you think it possible that heat may be merely a modification of light?
MRS. B.

That is a supposition which, in the present state of natural philosophy, can
neither be positively affirmed nor denied. Let us, therefore, instead of
discussing theoretical points, be contented with examining what is known
respecting the chemical effects of light.
Light is capable of entering into a kind of transitory union with certain
substances, and this is what has been called phosphorescence. Bodies that
are possessed of this property, after being exposed to the sun’s rays, appear
luminous in the dark. The shells of fish, the bones of land animals, marble,
limestone, and a variety of combinations of earths, are more or less
powerfully phosphorescent.
CAROLINE.

I remember being much surprised last summer with the phosphorescent
appearance of some pieces of rotten wood, which had just been dug out of
the ground; they shone so bright that I at first supposed them to be glow-
worms.
EMILY.

And is not the light of a glow-worm of a phosphorescent nature?
MRS. B.

It is a very remarkable instance of phosphorescence in living animals; this
property, however, is not exclusively possessed by the glow-worm. The
insect called the lanthorn-fly, which is peculiar to warm climates, emits
light as it flies, producing in the dark a remarkably sparkling appearance.
But it is more common to see animal matter in a dead state possessed of a
phosphorescent quality; sea fish is often eminently so.
EMILY.

I have heard that the sea has sometimes had the appearance of being
illuminated, and that the light is supposed to proceed from the spawn of
fishes floating on its surface.
MRS. B.

This light is probably owing to that or some other animal matter. Sea water
has been observed to become luminous from the substance of a fresh
herring having been immersed in it; and certain insects, of the Medusa kind,
are known to produce similar effects.
But the strongest phosphorescence is produced by chemical compositions
prepared for the purpose, the most common of which consists of oyster
shells and sulphur, and is known by the name of Canton’s Phosphorus.
EMILY.

I am rather surprised, Mrs. B., that you should have said so much of the
light emitted by phosphorescent bodies without taking any notice of that
which is produced by burning bodies.
MRS. B.

The light emitted by the latter is so intimately connected with the chemical
history of combustion, that I must defer all explanation of it till we come to
the examination of that process, which is one of the most interesting in
chemical science.
Light is an agent capable of producing various chemical changes. It is
essential to the welfare both of the animal and vegetable kingdoms; for men
and plants grow pale and sickly if deprived of its salutary influence. It is
likewise remarkable for its property of destroying colour, which renders it
of great consequence in the process of bleaching.

EMILY.

Is it not singular that light, which in studying optics we were taught to
consider as the source and origin of colours, should have also the power of
destroying them?
CAROLINE.

It is a fact, however, that we every day experience; you know how it fades
the colours of linens and silks.
EMILY.

Certainly. And I recollect that endive is made to grow white instead of
green, by being covered up so as to exclude the light. But by what means
does light produce these effects?
MRS. B.

This I cannot attempt to explain to you until you have obtained a further
knowledge of chemistry. As the chemical properties of light can be
accounted for only in their reference to compound bodies, it would be
useless to detain you any longer on this subject; we may therefore pass on
to the examination of heat, or caloric, with which we are somewhat better
acquainted.
Heat and Light may be always distinguished by the different sensations
they produce, Light affects the sense of sight; Caloric that of feeling; the
one produces Vision, the other the sensation of Heat.
Caloric is found to exist in a variety of forms or modifications, and I think it
will be best to consider it under the two following heads, viz.

1. free or radiant caloric.
2. combined caloric.

The first, free or radiant caloric, is also called heat of temperature; it
comprehends all heat which is perceptible to the senses, and affects the
thermometer.
EMILY.

You mean such as the heat of the sun, of fire, of candles, of stoves; in short,
of every thing that burns?
MRS. B.

And likewise of things that do not burn, as, for instance, the warmth of the
body; in a word, all heat that is sensible, whatever may be its degree, or the
source from which it is derived.
CAROLINE.

What then are the other modifications of caloric? It must be a strange kind
of heat that cannot be perceived by our senses.
MRS. B.

None of the modifications of caloric should properly be called heat; for
heat, strictly speaking, is the sensation produced by caloric, on animated
bodies; this word, therefore, in the accurate language of science, should be
confined to express the sensation. But custom has adapted it likewise to
inanimate matter, and we say the heat of an oven, the heat of the sun,
without any reference to the sensation which they are capable of exciting.
It was in order to avoid the confusion which arose from thus confounding
the cause and effect, that modern chemists adopted the new word caloric, to
denote the principle which produces heat; yet they do not always, in
compliance with their own language, limit the word heat to the expression
of the sensation, since they still frequently employ it in reference to the
other modifications of caloric which are quite independent of sensation.
CAROLINE.

But you have not yet explained to us what these other modifications of
caloric are.
MRS. B.

Because you are not acquainted with the properties of free caloric, and you
know that we have agreed to proceed with regularity.
One of the most remarkable properties of free caloric is its power of
dilating bodies. This fluid is so extremely subtle, that it enters and pervades
all bodies whatever, forces itself between their particles, and not only

separates them, but frequently drives them asunder to a considerable
distance from each other. It is thus that caloric dilates or expands a body so
as to make it occupy a greater space than it did before.
EMILY.

The effect it has on bodies, therefore, is directly contrary to that of the
attraction of cohesion; the one draws the particles together, the other drives
them asunder.
MRS. B.

Precisely. There is a continual struggle between the attraction of
aggregation, and the expansive power of caloric; and from the action of
these two opposite forces, result all the various forms of matter, or degrees
of consistence, from the solid, to the liquid and aëriform state. And
accordingly we find that most bodies are capable of passing from one of
these forms to the other, merely in consequence of their receiving different
quantities of caloric.
CAROLINE.

That is very curious; but I think I understand the reason of it. If a great
quantity of caloric is added to a solid body, it introduces itself between the
particles in such a manner as to overcome, in a considerable degree, the
attraction of cohesion; and the body, from a solid, is then converted into a
fluid.
MRS. B.

This is the case whenever a body is fused or melted; but if you add caloric
to a liquid, can you tell me what is the consequence?
CAROLINE.

The caloric forces itself in greater abundance between the particles of the
fluid, and drives them to such a distance from each other, that their
attraction of aggregation is wholly destroyed: the liquid is then transformed
into vapour.
MRS. B.

Very well; and this is precisely the case with boiling water, when it is
converted into steam or vapour, and with all bodies that assume an aëriform
state.
EMILY.

I do not well understand the word aëriform?
MRS. B.

Any elastic fluid whatever, whether it be merely vapour or permanent air, is
called aëriform.
But each of these various states, solid, liquid, and aëriform, admit of many
different degrees of density, or consistence, still arising (chiefly at least)
from the different quantities of caloric the bodies contain. Solids are of
various degrees of density, from that of gold, to that of a thin jelly. Liquids,
from the consistence of melted glue, or melted metals, to that of ether,
which is the lightest of all liquids. The different elastic fluids (with which
you are not yet acquainted) are susceptible of no less variety in their
degrees of density.
EMILY.

But does not every individual body also admit of different degrees of
consistence, without changing its state?
MRS. B.

Undoubtedly; and this I can immediately show you by a very simple
experiment. This piece of iron now exactly fits the frame, or ring, made to
receive it; but if heated red hot, it will no longer do so, for its dimensions
will be so much increased by the caloric that has penetrated into it, that it
will be much too large for the frame.
The iron is now red hot; by applying it to the frame, we shall see how much
it is dilated.
EMILY.

Considerably so indeed! I knew that heat had this effect on bodies, but I did
not imagine that it could be made so conspicuous.

MRS. B.

By means of this instrument (called a Pyrometer) we may estimate, in the
most exact manner, the various dilatations of any solid body by heat. The
body we are now going to submit to trial is this small iron bar; I fix it to this
apparatus, (Plate I. Fig. 1.) and then heat it by lighting the three lamps
beneath it: when the bar expands, it increases in length as well as thickness;
and, as one end communicates with this wheel-work, whilst the other end is
fixed and immoveable, no sooner does it begin to dilate than it presses
against the wheel-work, and sets in motion the index, which points out the
degrees of dilatation on the dial-plate.
Vol. I. p. 38. Plate I.

Fig. 1 A.A Bar of Metal. 1.2.3 Lamps burning. B.B Wheel work. C Index.
Fig. 2 A.A Glass tubes with bulbs. B.B Glasses of water in which they are immersed.
Larger view

EMILY.

This is, indeed, a very curious instrument; but I do not understand the use of
the wheels: would it not be more simple, and answer the purpose equally
well, if the bar, in dilating, pressed against the index, and put it in motion
without the intervention of the wheels?
MRS. B.

The use of the wheels is merely to multiply the motion, and therefore render
the effect of the caloric more obvious; for if the index moved no more than
the bar increased in length, its motion would scarcely be perceptible; but by
means of the wheels it moves in a much greater proportion, which therefore
renders the variations far more conspicuous.
By submitting different bodies to the test of the pyrometer, it is found that
they are far from dilating in the same proportion. Different metals expand in
different degrees, and other kinds of solid bodies vary still more in this
respect. But this different susceptibility of dilatation is still more
remarkable in fluids than in solid bodies, as I shall show you. I have here
two glass tubes, terminated at one end by large bulbs. We shall fill the
bulbs, the one with spirit of wine, the other with water. I have coloured both
liquids, in order that the effect may be more conspicuous. The spirit of
wine, you see, dilates by the warmth of my hand as I hold the bulb.
EMILY.

It certainly does, for I see it is rising into the tube. But water, it seems, is
not so easily affected by heat; for scarcely any change is produced on it by
the warmth of the hand.
MRS. B.

True; we shall now plunge the bulbs into hot water, (Plate I. Fig. 2.) and
you will see both liquids rise in the tubes; but the spirit of wine will ascend
highest.
CAROLINE.

How rapidly it expands! Now it has nearly reached the top of the tube,
though the water has hardly begun to rise.
EMILY.

The water now begins to dilate. Are not these glass tubes, with liquids rising
within them, very like thermometers?
MRS. B.

A thermometer is constructed exactly on the same principle, and these tubes
require only a scale to answer the purpose of thermometers: but they would

be rather awkward in their dimensions. The tubes and bulbs of
thermometers, though of various sizes, are in general much smaller than
these; the tube too is hermetically closed, and the air excluded from it. The
fluid most generally used in thermometers is mercury, commonly called
quicksilver, the dilatations and contractions of which correspond more
exactly to the additions, and subtractions, of caloric, than those of any other
fluid.
CAROLINE.

Yet I have often seen coloured spirit of wine used in thermometers.
MRS. B.

The expansions and contractions of that liquid are not quite so uniform as
those of mercury; but in cases in which it is not requisite to ascertain the
temperature with great precision, spirit of wine will answer the purpose
equally well, and indeed in some respects better, as the expansion of the
latter is greater, and therefore more conspicuous. This fluid is used likewise
in situations and experiments in which mercury would be frozen; for
mercury becomes a solid body, like a piece of lead or any other metal, at a
certain degree of cold: but no degree of cold has ever been known to freeze
spirit of wine.
A thermometer, therefore, consists of a tube with a bulb, such as you see
here, containing a fluid whose degrees of dilatation and contraction are
indicated by a scale to which the tube is fixed. The degree which indicates
the boiling point, simply means that, when the fluid is sufficiently dilated to
rise to this point, the heat is such that water exposed to the same
temperature will boil. When, on the other hand, the fluid is so much
condensed as to sink to the freezing point, we know that water will freeze at
that temperature. The extreme points of the scales are not the same in all
thermometers, nor are the degrees always divided in the same manner. In
different countries philosophers have chosen to adopt different scales and
divisions. The two thermometers most used are those of Fahrenheit, and of
Reaumur; the first is generally preferred by the English, the latter by the
French.
EMILY.

The variety of scale must be very inconvenient, and I should think liable to
occasion confusion, when French and English experiments are compared.
MRS. B.

The inconvenience is but very trifling, because the different gradations of
the scales do not affect the principle upon which thermometers are
constructed. When we know, for instance, that Fahrenheit’s scale is divided
into 212 degrees, in which 32° corresponds with the freezing point, and
212° with the point of boiling water: and that Reaumur’s is divided only
into 80 degrees, in which 0° denotes the freezing point, and 80° that of
boiling water, it is easy to compare the two scales together, and reduce the
one into the other. But, for greater convenience, thermometers are
sometimes constructed with both these scales, one on either side of the tube;
so that the correspondence of the different degrees of the two scales is thus
instantly seen. Here is one of these scales, (Plate II. Fig. 1.) by which you
can at once perceive that each degree of Reaumur’s corresponds to 2¼ of
Fahrenheit’s division. But I believe the French have, of late, given the
preference to what they call the centigrade scale, in which the space
between the freezing and the boiling point is divided into 100 degrees.
Vol. I. p. 42. Plate II.

Larger view

CAROLINE.

That seems to me the most reasonable division, and I cannot guess why the
freezing point is called 32°, or what advantage is derived from it.
MRS. B.

There really is no advantage in it; and it originated in a mistaken opinion of
the instrument-maker, Fahrenheit, who first constructed these
thermometers. He mixed snow and salt together, and produced by that
means a degree of cold which he concluded was the greatest possible, and

therefore made his scale begin from that point. Between that and boiling
water he made 212 degrees, and the freezing point was found to be at 32°.
EMILY.

Are spirit of wine, and mercury, the only liquids used in the construction of
thermometers?
MRS. B.

I believe they are the only liquids now in use, though some others, such as
linseed oil, would make tolerable thermometers: but for experiments in
which a very quick and delicate test of the changes of temperature is
required, air is the fluid sometimes employed. The bulb of air thermometers
is filled with common air only, and its expansion and contraction are
indicated by a small drop of any coloured liquor, which is suspended within
the tube, and moves up and down, according as the air within the bulb and
tube expands or contracts. But in general, air thermometers, however
sensible to changes of temperature, are by no means accurate in their
indications.
I can, however, show you an air thermometer of a very peculiar
construction, which is remarkably well adapted for some chemical
experiments, as it is equally delicate and accurate in its indications.
CAROLINE.

It looks like a double thermometer reversed, the tube being bent, and having
a large bulb at each of its extremities. (Plate II. Fig. 2.)
EMILY.

Why do you call it an air thermometer; the tube contains a coloured liquid?
MRS. B.

But observe that the bulbs are filled with air, the liquid being confined to a
portion of the tube, and answering only the purpose of showing, by its
motion in the tube, the comparative dilatation or contraction of the air
within the bulbs, which afford an indication of their relative temperature.
Thus if you heat the bulb A, by the warmth of your hand, the fluid will rise

towards the bulb B, and the contrary will happen if you reverse the
experiment.
But if, on the contrary, both tubes are of the same temperature, as is the case
now, the coloured liquid, suffering an equal pressure on each side, no
change of level takes place.
CAROLINE.

This instrument appears, indeed, uncommonly delicate. The fluid is set in
motion by the mere approach of my hand.
MRS. B.

You must observe, however, that this thermometer cannot indicate the
temperature of any particular body, or of the medium in which it is
immersed; it serves only to point out the difference of temperature between
the two bulbs, when placed under different circumstances. For this reason it
has been called differential thermometer. You will see by-and-bye to what
particular purposes this instrument applies.
EMILY.

But do common thermometers indicate the exact quantity of caloric
contained either in the atmosphere, or in any body with which they are in
contact?
MRS. B.

No: first, because there are other modifications of caloric which do not
affect the thermometer; and, secondly, because the temperature of a body, as
indicated by the thermometer, is only relative. When, for instance, the
thermometer remains stationary at the freezing point, we know that the
atmosphere (or medium in which it is placed, whatever it may be) is as cold
as freezing water; and when it stands at the boiling point, we know that this
medium is as hot as boiling water; but we do not know the positive quantity
of heat contained either in freezing or boiling water, any more than we
know the real extremes of heat and cold; and consequently we cannot
determine that of the body in which the thermometer is placed.
CAROLINE.

I do not quite understand this explanation.
MRS. B.

Let us compare a thermometer to a well, in which the water rises to
different heights, according as it is more or less supplied by the spring
which feeds it: if the depth of the well is unfathomable, it must be
impossible to know the absolute quantity of water it contains; yet we can
with the greatest accuracy measure the number of feet the water has risen or
fallen in the well at any time, and consequently know the precise quantity
of its increase or diminution, without having the least knowledge of the
whole quantity of water it contains.
CAROLINE.

Now I comprehend it very well; nothing appears to me to explain a thing so
clearly as a comparison.
EMILY.

But will thermometers bear any degree of heat?
MRS. B.

No; for if the temperature were much above the highest degree marked on
the scale of the thermometer, the mercury would burst the tube in an
attempt to ascend. And at any rate, no thermometer can be applied to
temperatures higher than the boiling point of the liquid used in its
construction, for the steam, on the liquid beginning to boil, would burst the
tube. In furnaces, or whenever any very high temperature is to be measured,
a pyrometer, invented by Wedgwood, is used for that purpose. It is made of
a certain composition of baked clay, which has the peculiar property of
contracting by heat, so that the degree of contraction of this substance
indicates the temperature to which it has been exposed.
EMILY.

But is it possible for a body to contract by heat? I thought that heat dilated
all bodies whatever.
MRS. B.

This is not an exception to the rule. You must recollect that the bulk of the
clay is not compared, whilst hot, with that which it has when cold; but it is
from the change which the clay has undergone by having been heated that
the indications of this instrument are derived. This change consists in a
beginning fusion which tends to unite the particles of clay more closely,
thus rendering it less pervious or spongy.
Clay is to be considered as a spongy body, having many interstices or pores,
from its having contained water when soft. These interstices are by heat
lessened, and would by extreme heat be entirely obliterated.
CAROLINE.

And how do you ascertain the degrees of contraction of Wedgwood’s
pyrometer?
MRS. B.

The dimensions of a piece of clay are measured by a scale graduated on the
side of a tapered groove, formed in a brass ruler; the more the clay is
contracted by the heat, the further it will descend into the narrow part of the
tube.
Before we quit the subject of expansion, I must observe to you that, as
liquids expand more readily than solids, so elastic fluids, whether air or
vapour, are the most expansible of all bodies.
It may appear extraordinary that all elastic fluids whatever, undergo the
same degree of expansion from equal augmentations of temperature.
EMILY.

I suppose, then, that all elastic fluids are of the same density?
MRS. B.

Very far from it; they vary in density, more than either liquids or solids. The
uniformity of their expansibility, which at first may appear singular, is,
however, readily accounted for. For if the different susceptibilities of
expansion of bodies arise from their various degrees of attraction of
cohesion, no such difference can be expected in elastic fluids, since in these
the attraction of cohesion does not exist, their particles being on the

contrary possessed of an elastic or repulsive power; they will therefore all
be equally expanded by equal degrees of caloric.
EMILY.

True; as there is no power opposed to the expansive force of caloric in
elastic bodies, its effect must be the same in all of them.
MRS. B.

Let us now proceed to examine the other properties of free caloric.
Free caloric always tends to diffuse itself equally, that is to say, when two
bodies are of different temperatures, the warmer gradually parts with its
heat to the colder, till they are both brought to the same temperature. Thus,
when a thermometer is applied to a hot body, it receives caloric; when to a
cold one, it communicates part of its own caloric, and this communication
continues until the thermometer and the body arrive at the same
temperature.
EMILY.

Cold, then, is nothing but a negative quality, simply implying the absence of
heat.
MRS. B.

Not the total absence, but a diminution of heat; for we know of no body in
which some caloric may not be discovered.
CAROLINE.

But when I lay my hand on this marble table I feel it positively cold, and
cannot conceive that there is any caloric in it.
MRS. B.

The cold you experience consists in the loss of caloric that your hand
sustains in an attempt to bring its temperature to an equilibrium with the
marble. If you lay a piece of ice upon it, you will find that the contrary
effect will take place; the ice will be melted by the heat which it abstracts
from the marble.
CAROLINE.

Is it not in this case the air of the room, which being warmer than the
marble, melts the ice?
MRS. B.

The air certainly acts on the surface which is exposed to it, but the table
melts that part with which it is in contact.
CAROLINE.

But why does caloric tend to an equilibrium? It cannot be on the same
principle as other fluids, since it has no weight?
MRS. B.

Very true, Caroline, that is an excellent objection. You might also, with
some propriety, object to the term equilibrium being applied to a body that
is without weight; but I know of no expression that would explain my
meaning so well. You must consider it, however, in a figurative rather than
a literal sense; its strict meaning is an equal diffusion. We cannot, indeed,
well say by what power it diffuses itself equally, though it is not surprising
that it should go from the parts which have the most to those which have the
least. This subject is best explained by a theory suggested by Professor
Prevost of Geneva, which is now, I believe, generally adopted.
According to this theory, caloric is composed of particles perfectly separate
from each other, every one of which moves with a rapid velocity in a certain
direction. These directions vary as much as imagination can conceive, the
result of which is, that there are rays or lines of these particles moving with
immense velocity in every possible direction. Caloric is thus universally
diffused, so that when any portion of space happens to be in the
neighbourhood of another, which contains more caloric, the colder portion
receives a quantity of calorific rays from the latter, sufficient to restore an
equilibrium of temperature. This radiation does not only take place in free
space, but extends also to bodies of every kind. Thus you may suppose all
bodies whatever constantly radiating caloric: those that are of the same
temperature give out and absorb equal quantities, so that no variation of
temperature is produced in them; but when one body contains more free
caloric than another, the exchange is always in favour of the colder body,

until an equilibrium is effected; this you found to be the case when the
marble table cooled your hand, and again when it melted the ice.
CAROLINE.

This reciprocal radiation surprises me extremely; I thought, from what you
first said, that the hotter bodies alone emitted rays of caloric which were
absorbed by the colder; for it seems unnatural that a hot body should
receive any caloric from a cold one, even though it should return a greater
quantity.
MRS. B.

It may at first appear so, but it is no more extraordinary than that a candle
should send forth rays of light to the sun, which, you know, must
necessarily happen.
CAROLINE.

Well, Mrs. B—, I believe that I must give up the point. But I wish I could
see these rays of caloric; I should then have greater faith in them.
MRS. B.

Will you give no credit to any sense but that of sight? You may feel the rays
of caloric which you receive from any body of a temperature higher than
your own; the loss of the caloric you part with in return, it is true, is not
perceptible; for as you gain more than you lose, instead of suffering a
diminution, you are really making an acquisition of caloric. It is, therefore,
only when you are parting with it to a body of a lower temperature, that you
are sensible of the sensation of cold, because you then sustain an absolute
loss of caloric.
EMILY.

And in this case we cannot be sensible of the small quantity of heat we
receive in exchange from the colder body, because it serves only to
diminish the loss.
MRS. B.

Very well, indeed, Emily. Professor Pictet, of Geneva, has made some very
interesting experiments, which prove not only that caloric radiates from all

bodies whatever, but that these rays may be reflected, according to the laws
of optics, in the same manner as light. I shall repeat these experiments
before you, having procured mirrors fit for the purpose; and it will afford us
an opportunity of using the differential thermometer, which is particularly
well adapted for these experiments.—I place an iron bullet, (Plate III.
Fig. 1.) about two inches in diameter, and heated to a degree not sufficient
to render it luminous, in the focus of this large metallic concave mirror. The
rays of heat which fall on this mirror are reflected, agreeably to the property
of concave mirrors, in a parallel direction, so as to fall on a similar mirror,
which, you see, is placed opposite to the first, at the distance of about ten
feet; thence the rays converge to the focus of the second mirror, in which I
place one of the bulbs of this thermometer. Now, observe in what manner it
is affected by the caloric which is reflected on it from the heated bullet.—
The air is dilated in the bulb which we placed in the focus of the mirror, and
the liquor rises considerably in the opposite leg.
Vol. I. p. 54 Plate III.

A.A. & B.B Concave mirrors fixed on stands. C Heated Bullet placed in the focus of the
mirror A. D Thermometer, with its bulb placed in the focus of the mirror B.
1.2.3.4 Rays of Caloric radiating from the bullet & falling on the mirror A. 5.6.7.8 The
same rays reflected from the mirror A to the mirror B. 9.10.11.12 The same rays
reflected by the mirror B to the Thermometer.
Larger view

EMILY.

But would not the same effect take place, if the rays of caloric from the
heated bullet fell directly on the thermometer, without the assistance of the
mirrors?
MRS. B.

The effect would in that case be so trifling, at the distance at which the
bullet and the thermometer are from each other, that it would be almost
imperceptible. The mirrors, you know, greatly increase the effect, by
collecting a large quantity of rays into a focus; place your hand in the focus
of the mirror, and you will find it much hotter there than when you remove
it nearer to the bullet.
EMILY.

That is very true; it appears extremely singular to feel the heat diminish in
approaching the body from which it proceeds.
CAROLINE.

And the mirror which produces so much heat, by converging the rays, is
itself quite cold.
MRS. B.

The same number of rays that are dispersed over the surface of the mirror
are collected by it into the focus; but, if you consider how large a surface
the mirror presents to the rays, and, consequently, how much they are
diffused in comparison to what they are at the focus, which is little more
than a point, I think you can no longer wonder that the focus should be so
much hotter than the mirror.
The principal use of the mirrors in this experiment is, to prove that the
calorific emanation is reflected in the same manner as light.
CAROLINE.

And the result, I think, is very conclusive.
MRS. B.

The experiment may be repeated with a wax taper instead of the bullet, with
a view of separating the light from the caloric. For this purpose a

transparent plate of glass must be interposed between the mirrors; for light,
you know, passes with great facility through glass, whilst the transmission
of caloric is almost wholly impeded by it. We shall find, however, in this
experiment, that some few of the calorific rays pass through the glass
together with the light, as the thermometer rises a little; but, as soon as the
glass is removed, and a free passage left to the caloric, it will rise
considerably higher.
EMILY.

This experiment, as well as that of Dr. Herschell’s, proves that light and
heat may be separated; for in the latter experiment the separation was not
perfect, any more than in that of Mr. Pictet.
CAROLINE.

I should like to repeat this experiment, with the difference of substituting a
cold body instead of the hot one, to see whether cold would not be reflected
as well as heat.
MRS. B.

That experiment was proposed to Mr. Pictet by an incredulous philosopher
like yourself, and he immediately tried it by substituting a piece of ice in the
place of the heated bullet.
CAROLINE.

Well, Mrs. B., and what was the result?
MRS. B.

That we shall see; I have procured some ice for the purpose.
EMILY.

The thermometer falls considerably!
CAROLINE.

And does not that prove that cold is not merely a negative quality, implying
simply an inferior degree of heat? The cold must be positive, since it is
capable of reflection.
MRS. B.

So it at first appeared to Mr. Pictet; but upon a little consideration he found
that it afforded only an additional proof of the reflection of heat: this I shall
endeavour to explain to you.
According to Mr. Prevost’s theory, we suppose that all bodies whatever
radiate caloric; the thermometer used in these experiments therefore emits
calorific rays in the same manner as any other substance. When its
temperature is in equilibrium with that of the surrounding bodies, it receives
as much caloric as it parts with, and no change of temperature is produced.
But when we introduce a body of a lower temperature, such as a piece of
ice, which parts with less caloric than it receives, the consequence is, that
its temperature is raised, whilst that of the surrounding bodies is
proportionally lowered.
EMILY.

If, for instance, I was to bring a large piece of ice into this room, the ice
would in time be melted, by absorbing caloric from the general radiation
which is going on throughout the room; and as it would contribute very
little caloric in return for what is absorbed, the room would necessarily be
cooled by it.
MRS. B.

Just so; and as in consequence of the mirrors, a more considerable exchange
of rays takes place between the ice and the thermometer, than between these
and any of the surrounding bodies, the temperature of the thermometer must
be more lowered than that of any other adjacent object.
CAROLINE.

I confess I do not perfectly understand your explanation.
MRS. B.

This experiment is exactly similar to that made with the heated bullet: for, if
we consider the thermometer as the hot body (which it certainly is in
comparison to the ice), you may then easily understand that it is by the loss
of the calorific rays which the thermometer sends to the ice, and not by any
cold rays received from it, that the fall of the mercury is occasioned: for the

ice, far from emitting rays of cold, sends forth rays of caloric, which
diminish the loss sustained by the thermometer.
Let us say, for instance, that the radiation of the thermometer towards the
ice is equal to 20, and that of the ice towards the thermometer to 10: the
exchange in favour of the ice is as 20 is to 10, or the thermometer
absolutely loses 10, whilst the ice gains 10.
CAROLINE.

But if the ice actually sends rays of caloric to the thermometer, must not the
latter fall still lower when the ice is removed?
MRS. B.

No; for the space that the ice occupied, admits rays from all the surrounding
bodies to pass through it; and those being of the same temperature as the
thermometer, will not affect it, because as much heat now returns to the
thermometer as radiates from it.
CAROLINE.

I must confess that you have explained this in so satisfactory a manner, that
I cannot help being convinced now that cold has no real claim to the rank of
a positive being.
MRS. B.

Before I conclude the subject of radiation I must observe to you that
different bodies, (or rather surfaces,) possess the power of radiating caloric
in very different degrees.
Some very curious experiments have been made by Mr. Leslie on this
subject, and it was for this purpose that he invented the differential
thermometer; with its assistance he ascertained that black surfaces radiate
most, glass next, and polished surfaces the least of all.
EMILY.

Supposing these surfaces, of course, to be all of the same temperature.
MRS. B.

Undoubtedly. I will now show you the very simple and ingenious apparatus,
by means of which he made these experiments. This cubical tin vessel or
canister, has each of its sides externally covered with different materials;
the one is simply blackened; the next is covered with white paper; the third
with a pane of glass, and in the fourth the polished tin surface remains
uncovered. We shall fill this vessel with hot water, so that there can be no
doubt but that all its sides will be of the same temperature. Now let us place
it in the focus of one of the mirrors, making each of its sides front it in
succession. We shall begin with the black surface.
CAROLINE.

It makes the thermometer which is in the focus of the other mirror rise
considerably. Let us turn the paper surface towards the mirror. The
thermometer falls a little, therefore of course this side cannot emit or radiate
so much caloric as the blackened side.
EMILY.

This is very surprising; for the sides are exactly of the same size, and must
be of the same temperature. But let us try the glass surface.
MRS. B.

The thermometer continues falling, and with the plain surface it falls still
lower; these two surfaces therefore radiate less and less.
CAROLINE.

I think I have found out the reason of this.
MRS. B.

I should be very happy to hear it, for it has not yet (to my knowledge) been
accounted for.
CAROLINE.

The water within the vessel gradually cools, and the thermometer in
consequence gradually falls.
MRS. B.

It is true that the water cools, but certainly in much less proportion than the
thermometer descends, as you will perceive if you now change the tin
surface for the black one.
CAROLINE.

I was mistaken certainly, for the thermometer rises again now that the black
surface fronts the mirror.
MRS. B.

And yet the water in the vessel is still cooling, Caroline.
EMILY.

I am surprised that the tin surface should radiate the least caloric, for a
metallic vessel filled with hot water, a silver teapot, for instance, feels much
hotter to the hand than one of black earthen ware.
MRS. B.

That is owing to the different power which various bodies possess for
conducting caloric, a property which we shall presently examine. Thus,
although a metallic vessel feels warmer to the hand, a vessel of this kind is
known to preserve the heat of the liquid within, better than one of any other
materials; it is for this reason that silver teapots make better tea than those
of earthen ware.
EMILY.

According to these experiments, light-coloured dresses, in cold weather,
should keep us warmer than black clothes, since the latter radiate so much
more than the former.
MRS. B.

And that is actually the case.
EMILY.

This property, of different surfaces to radiate in different degrees, appears to
me to be at variance with the equilibrium of caloric; since it would imply
that those bodies which radiate most, must ultimately become coldest.

Suppose that we were to vary this experiment, by using two metallic vessels
full of boiling water, the one blackened, the other not; would not the black
one cool the first?
CAROLINE.

True; but when they were both brought down to the temperature of the
room, the interchange of caloric between the canisters and the other bodies
of the room being then equal, their temperatures would remain the same.
EMILY.

I do not see why that should be the case; for if different surfaces of the same
temperature radiate in different degrees when heated, why should they not
continue to do so when cooled down to the temperature of the room?
MRS. B.

You have started a difficulty, Emily, which certainly requires explanation. It
is found by experiment that the power of absorption corresponds with and is
proportional to that of radiation; so that under equal temperatures, bodies
compensate for the greater loss they sustain in consequence of their greater
radiation by their greater absorption; so that if you were to make your
experiment in an atmosphere heated like the canisters, to the temperature of
boiling water, though it is true that the canisters would radiate in different
degrees, no change of temperature would be produced in them, because
they would each absorb caloric in proportion to their respective radiation.
EMILY.

But would not the canisters of boiling water also absorb caloric in different
degrees in a room of the common temperature?
MRS. B.

Undoubtedly they would. But the various bodies in the room would not, at a
lower temperature, furnish either of the canisters with a sufficiency of
caloric to compensate for the loss they undergo; for, suppose the black
canister to absorb 400 rays of caloric, whilst the metallic one absorbed only
200; yet if the former radiate 800, whilst the latter radiates only 400, the
black canister will be the first cooled down to the temperature of the room.
But from the moment the equilibrium of temperature has taken place, the

black canister, both receiving and giving out 400 rays, and the metallic one
200, no change of temperature will take place.
EMILY.

I now understand it extremely well. But what becomes of the surplus of
calorific rays, which good radiators emit and bad radiators refuse to receive;
they must wander about in search of a resting-place?
MRS. B.

They really do so; for they are rejected and sent back, or, in other words,
reflected by the bodies which are bad radiators of caloric; and they are thus
transmitted to other bodies which happen to lie in their way, by which they
are either absorbed or again reflected, according as the property of
reflection, or that of absorption, predominates in these bodies.
CAROLINE.

I do not well understand the difference between radiating and reflecting
caloric, for the caloric that is reflected from a body proceeds from it in
straight lines, and may surely be said to radiate from it?
MRS. B.

It is true that there at first appears to be a great analogy between radiation
and reflection, as they equally convey the idea of the transmission of
caloric.
But if you consider a little, you will perceive that when a body radiates
caloric, the heat which it emits not only proceeds from, but has its origin in
the body itself. Whilst when a body reflects caloric, it parts with none of its
own caloric, but only reflects that which it receives from other bodies.
EMILY.

Of this difference we have very striking examples before us, in the tin
vessel of water, and the concave mirrors; the first radiates its own heat, the
latter reflect the heat which they receive from other bodies.
CAROLINE.

Now, that I understand the difference, it no longer surprises me that bodies
which radiate, or part with their own caloric freely, should not have the
power of transmitting with equal facility that which they receive from other
bodies.
EMILY.

Yet no body can be said to possess caloric of its own, if all caloric is
originally derived from the sun.
MRS. B.

When I speak of a body radiating its own caloric, I mean that which it has
absorbed and incorporated either immediately from the sun’s rays, or
through the medium of any other substance.
CAROLINE.

It seems natural enough that the power of absorption should be in
opposition to that of reflection, for the more caloric a body receives, the less
it will reject.
EMILY.

And equally so that the power of radiation should correspond with that of
absorption. It is, in fact, cause and effect; for a body cannot radiate heat
without having previously absorbed it; just as a spring that is well fed flows
abundantly.
MRS. B.

Fluids are in general very bad radiators of caloric; and air neither radiates
nor absorbs caloric in any sensible degree.
We have not yet concluded our observations on free caloric. But I shall
defer, till our next meeting, what I have further to say on this subject.
I believe it will afford us ample conversation for another interview.

CONVERSATION III.
CONTINUATION OF THE SUBJECT.

MRS. B.

In our last conversation, we began to examine the tendency of caloric to
restore an equilibrium of temperature. This property, when once well
understood, affords the explanation of a great variety of facts which
appeared formerly unaccountable. You must observe, in the first place, that
the effect of this tendency is gradually to bring all bodies that are in contact
to the same temperature. Thus, the fire which burns in the grate,
communicates its heat from one object to another, till every part of the room
has an equal proportion of it.
EMILY.

And yet this book is not so cold as the table on which it lies, though both
are at an equal distance from the fire, and actually in contact with each
other, so that, according to your theory, they should be exactly of the same
temperature.
CAROLINE.

And the hearth, which is much nearer the fire than the carpet, is certainly
the colder of the two.
MRS. B.

If you ascertain the temperature of these several bodies by a thermometer
(which is a much more accurate test than your feeling), you will find that it
is exactly the same.
CAROLINE.

But if they are of the same temperature, why should the one feel colder than
the other?
MRS. B.

The hearth and the table feel colder than the carpet or the book, because the
latter are not such good conductors of heat as the former. Caloric finds a
more easy passage through marble and wood, than through leather and
worsted; the two former will therefore absorb heat more rapidly from your
hand, and consequently give it a stronger sensation of cold than the two
latter, although they are all of them really of the same temperature.
CAROLINE.

So, then, the sensation I feel on touching a cold body, is in proportion to the
rapidity with which my hand yields its heat to that body?
MRS. B.

Precisely; and, if you lay your hand successively on every object in the
room, you will discover which are good, and which are bad conductors of
heat, by the different degrees of cold you feel. But, in order to ascertain this
point, it is necessary that the several substances should be of the same
temperature, which will not be the case with those that are very near the
fire, or those that are exposed to a current of cold air from a window or
door.
EMILY.

But what is the reason that some bodies are better conductors of heat than
others?
MRS. B.

This is a point not well ascertained. It has been conjectured that a certain
union or adherence takes place between the caloric and the particles of the
body through which it passes. If this adherence be strong, the body detains
the heat, and parts with it slowly and reluctantly; if slight, it propagates it
freely and rapidly. The conducting power of a body is therefore, inversely,
as its tendency to unite with caloric.
EMILY.

That is to say, that the best conductors are those that have the least affinity
for caloric.
MRS. B.

Yes; but the term affinity is objectionable in this case, because, as that word
is used to express a chemical attraction (which can be destroyed only by
decomposition), it cannot be applicable to the slight and transient union that
takes place between free caloric and the bodies through which it passes; an
union which is so weak, that it constantly yields to the tendency which
caloric has to an equilibrium. Now you clearly understand, that the passage
of caloric, through bodies that are good conductors, is much more rapid
than through those that are bad conductors, and that the former both give
and receive it more quickly, and therefore, in a given time, more
abundantly, than bad conductors, which makes them feel either hotter or
colder, though they may be, in fact, both of the same temperature.
CAROLINE.

Yes, I understand it now; the table, and the book lying upon it, being really
of the same temperature, would each receive, in the same space of time, the
same quantity of heat from my hand, were their conducting powers equal;
but as the table is the best conductor of the two, it will absorb the heat from
my hand more rapidly, and consequently produce a stronger sensation of
cold than the book.
MRS. B.

Very well, my dear; and observe, likewise, that if you were to heat the table
and the book an equal number of degrees above the temperature of your
body, the table, which before felt the colder, would now feel the hotter of
the two; for, as in the first case it took the heat most rapidly from your hand,
so it will now impart heat most rapidly to it. Thus the marble table, which
seems to us colder than the mahogany one, will prove the hotter of the two
to the ice; for, if it takes heat more rapidly from our hands, which are
warmer, it will give out heat more rapidly to the ice, which is colder. Do
you understand the reason of these apparently opposite effects?
EMILY.

Perfectly. A body which is a good conductor of caloric, affords it a free
passage; so that it penetrates through that body more rapidly than through
one which is a bad conductor; and consequently, if it is colder than your

hand, you lose more caloric, and if it is hotter, you gain more than with a
bad conductor of the same temperature.
MRS. B.

But you must observe that this is the case only when the conductors are
either hotter or colder than your hand; for, if you heat different conductors
to the temperature of your body, they will all feel equally warm, since the
exchange of caloric between bodies of the same temperature is equal. Now,
can you tell me why flannel clothing, which is a very bad conductor of heat,
prevents our feeling cold?
CAROLINE.

It prevents the cold from penetrating . . . . . . . .
MRS. B.

But you forget that cold is only a negative quality.
CAROLINE.

True; it only prevents the heat of our bodies from escaping so rapidly as it
would otherwise do.
MRS. B.

Now you have explained it right; the flannel rather keeps in the heat, than
keeps out the cold. Were the atmosphere of a higher temperature than our
bodies, it would be equally efficacious in keeping their temperature at the
same degree, as it would prevent the free access of the external heat, by the
difficulty with which it conducts it.
EMILY.

This, I think, is very clear. Heat, whether external or internal, cannot easily
penetrate flannel; therefore in cold weather it keeps us warm; and if the
weather was hotter than our bodies, it would keep us cool.
MRS. B.

The most dense bodies are, generally speaking, the best conductors of heat;
probably because the denser the body the greater are the number of points
or particles that come in contact with caloric. At the common temperature

of the atmosphere a piece of metal will feel much colder than a piece of
wood, and the latter than a piece of woollen cloth; this again will feel colder
than flannel; and down, which is one of the lightest, is at the same time one
of the warmest bodies.
CAROLINE.

This is, I suppose, the reason that the plumage of birds preserves them so
effectually from the influence of cold in winter?
MRS. B.

Yes; but though feathers in general are an excellent preservative against
cold, down is a kind of plumage peculiar to aquatic birds, and covers their
chest, which is the part most exposed to the water; for though the surface of
the water is not of a lower temperature than the atmosphere, yet, as it is a
better conductor of heat, it feels much colder, consequently the chest of the
bird requires a warmer covering than any other part of its body. Besides, the
breasts of aquatic birds are exposed to cold not only from the temperature
of the water, but also from the velocity with which the breast of the bird
strikes against it; and likewise from the rapid evaporation occasioned in that
part by the air against which it strikes, after it has been moistened by
dipping from time to time into the water.
If you hold a finger of one hand motionless in a glass of water, and at the
same time move a finger of the other hand swiftly through water of the
same temperature, a different sensation will be soon perceived in the
different fingers.
Most animal substances, especially those which Providence has assigned as
a covering for animals, such as fur, wool, hair, skin, &c. are bad conductors
of heat, and are, on that account, such excellent preservatives against the
inclemency of winter, that our warmest apparel is made of these materials.
EMILY.

Wood is, I dare say, not so good a conductor as metal, and it is for that
reason, no doubt, that silver teapots have always wooden handles.
MRS. B.

Yes; and it is the facility with which metals conduct caloric that made you
suppose that a silver pot radiated more caloric than an earthen one. The
silver pot is in fact hotter to the hand when in contact with it; but it is
because its conducting power more than counterbalances its deficiency in
regard to radiation.
We have observed that the most dense bodies are in general the best
conductors; and metals, you know, are of that class. Porous bodies, such as
the earths and wood, are worse conductors, chiefly, I believe, on account of
their pores being filled with air; for air is a remarkably bad conductor.
CAROLINE.

It is a very fortunate circumstance that air should be a bad conductor, as it
tends to preserve the heat of the body when exposed to cold weather.
MRS. B.

It is one of the many benevolent dispensations of Providence, in order to
soften the inclemency of the seasons, and to render almost all climates
habitable to man.
In fluids of different densities, the power of conducting heat varies no less
remarkably; if you dip your hand into this vessel full of mercury, you will
scarcely conceive that its temperature is not lower than that of the
atmosphere.
CAROLINE.

Indeed I know not how to believe it, it feels so extremely cold.—But we
may easily ascertain its true temperature by the thermometer.—It is really
not colder than the air;—the apparent difference then is produced merely by
the difference of the conducting power in mercury and in air.
MRS. B.

Yes; hence you may judge how little the sense of feeling is to be relied on
as a test of the temperature of bodies, and how necessary a thermometer is
for that purpose.
It has indeed been doubted whether fluids have the power of conducting
caloric in the same manner as solid bodies. Count Rumford, a very few

years since, attempted to prove, by a variety of experiments, that fluids,
when at rest, were not at all endowed with this property.
CAROLINE.

How is that possible, since they are capable of imparting cold or heat to us;
for if they did not conduct heat, they would neither take it from, nor give it
to us?
MRS. B.

Count Rumford did not mean to say that fluids would not communicate
their heat to solid bodies; but only that heat does not pervade fluids, that is
to say, is not transmitted from one particle of a fluid to another, in the same
manner as in solid bodies.
EMILY.

But when you heat a vessel of water over the fire, if the particles of water
do not communicate heat to each other, how does the water become hot
throughout?
MRS. B.

By constant agitation. Water, as you have seen, expands by heat in the same
manner as solid bodies; the heated particles of water, therefore, at the
bottom of the vessel, become specifically lighter than the rest of the liquid,
and consequently ascend to the surface, where, parting with some of their
heat to the colder atmosphere, they are condensed, and give way to a fresh
succession of heated particles ascending from the bottom, which having
thrown off their heat at the surface, are in their turn displaced. Thus every
particle is successively heated at the bottom, and cooled at the surface of
the liquid; but as the fire communicates heat more rapidly than the
atmosphere cools the succession of surfaces, the whole of the liquid in time
becomes heated.
CAROLINE.

This accounts most ingeniously for the propagation of heat upwards. But
suppose you were to heat the upper surface of a liquid, the particles being
specifically lighter than those below, could not descend: how therefore
would the heat be communicated downwards?

MRS. B.

If there were no agitation to force the heated surface downwards, Count
Rumford assures us that the heat would not descend. In proof of this he
succeeded in making the upper surface of a vessel of water boil and
evaporate, while a cake of ice remained frozen at the bottom.
CAROLINE.

That is very extraordinary indeed!
MRS. B.

It appears so, because we are not accustomed to heat liquids by their upper
surface; but you will understand this theory better if I show you the internal
motion that takes place in liquids when they experience a change of
temperature. The motion of the liquid itself is indeed invisible from the
extreme minuteness of its particles; but if you mix with it any coloured
dust, or powder, of nearly the same specific gravity as the liquid, you may
judge of the internal motion of the latter by that of the coloured dust it
contains.—Do you see the small pieces of amber moving about in the liquid
contained in this phial?
CAROLINE.

Yes, perfectly.
MRS. B.

We shall now immerse the phial in a glass of hot water, and the motion of
the liquid will be shown, by that which it communicates to the amber.
EMILY.

I see two currents, the one rising along the sides of the phial, the other
descending in the centre: but I do not understand the reason of this.
MRS. B.

The hot water communicates its caloric, through the medium of the phial, to
the particles of the fluid nearest to the glass; these dilate and ascend
laterally to the surface, where, in parting with their heat, they are
condensed, and in descending, form the central current.

CAROLINE.

This is indeed a very clear and satisfactory experiment; but how much
slower the currents now move than they did at first?
MRS. B.

It is because the circulation of particles has nearly produced an equilibrium
of temperature between the liquid in the glass and that in the phial.
CAROLINE.

But these communicate laterally, and I thought that heat in liquids could be
propagated only upwards.
MRS. B.

You do not take notice that the heat is imparted from one liquid to the other,
through the medium of the phial itself, the external surface of which
receives the heat from the water in the glass, whilst its internal surface
transmits it to the liquid it contains. Now take the phial out of the hot water,
and observe the effect of its cooling.
EMILY.

The currents are reversed; the external current now descends, and the
internal one rises.—I guess the reason of this change:—the phial being in
contact with cold air instead of hot water, the external particles are cooled
instead of being heated; they therefore descend and force up the central
particles, which, being warmer, are consequently lighter.
MRS. B.

It is just so. Count Rumford hence infers that no alteration of temperature
can take place in a fluid, without an internal motion of its particles, and as
this motion is produced only by the comparative levity of the heated
particles, heat cannot be propagated downwards.
But though I believe that Count Rumford’s theory as to heat being
incapable of pervading fluids is not strictly correct, yet there is, no doubt,
much truth in his observation, that the communication is materially
promoted by a motion of the parts; and this accounts for the cold that is
found to prevail at the bottom of the lakes in Switzerland, which are fed by

rivers issuing from the snowy Alps. The water of these rivers being colder,
and therefore more dense than that of the lakes, subsides to the bottom,
where it cannot be affected by the warmer temperature of the surface; the
motion of the waves may communicate this temperature to some little
depth, but it can descend no further than the agitation extends.
EMILY.

But when the atmosphere is colder than the lake, the colder surface of the
water will descend, for the very reason that the warmer will not.
MRS. B.

Certainly: and it is on this account that neither a lake, nor any body of water
whatever, can be frozen until every particle of the water has risen to the
surface to give off its caloric to the colder atmosphere; therefore the deeper
a body of water is, the longer will be the time it requires to be frozen.
EMILY.

But if the temperature of the whole body of water be brought down to the
freezing point, why is only the surface frozen?
MRS. B.

The temperature of the whole body is lowered, but not to the freezing point.
The diminution of heat, as you know, produces a contraction in the bulk of
fluids, as well as of solids. This effect, however, does not take place in
water below the temperature of 40 degrees, which is 8 degrees above the
freezing point. At that temperature, therefore, the internal motion,
occasioned by the increased specific gravity of the condensed particles,
ceases; for when the water at the surface no longer condenses, it will no
longer descend, and leave a fresh surface exposed to the atmosphere: this
surface alone, therefore, will be further exposed to its severity, and will
soon be brought down to the freezing point, when it becomes ice, which
being a bad conductor of heat, preserves the water beneath a long time from
being affected by the external cold.
CAROLINE.

And the sea does not freeze, I suppose, because its depth is so great, that a
frost never lasts long enough to bring down the temperature of such a great

body of water to 40 degrees?
MRS. B.

That is one reason why the sea, as a large mass of water, does not freeze.
But, independently of this, salt water does not freeze till it is cooled much
below 32 degrees, and with respect to the law of condensation, salt water is
an exception, as it condenses even many degrees below the freezing point.
When the caloric of fresh water, therefore, is imprisoned by the ice on its
surface, the ocean still continues throwing off heat into the atmosphere,
which is a most signal dispensation of Providence to moderate the intensity
of the cold in winter.
CAROLINE.

This theory of the non-conducting power of liquids, does not, I suppose,
hold good with respect to air, otherwise the atmosphere would not be heated
by the rays of the sun passing through it?
MRS. B.

Nor is it heated in that way. The pure atmosphere is a perfectly transparent
medium, which neither radiates, absorbs, nor conducts caloric, but transmits
the rays of the sun to us without in any way diminishing their intensity. The
air is therefore not more heated, by the sun’s rays passing through it, than
diamond, glass, water, or any other transparent medium.
CAROLINE.

That is very extraordinary! Are glass windows not heated then by the sun
shining on them?
MRS. B.

No; not if the glass be perfectly transparent. A most convincing proof that
glass transmits the rays of the sun without being heated by them is afforded
by the burning lens, which by converging the rays to a focus will set
combustible bodies on fire, without its own temperature being raised.
EMILY.

Yet, Mrs. B., if I hold a piece of glass near the fire it is almost immediately
warmed by it; the glass therefore must retain some of the caloric radiated by

the fire? Is it that the solar rays alone pass freely through glass without
paying tribute? It seems unaccountable that the radiation of a common fire
should have power to do what the sun’s rays cannot accomplish.
MRS. B.

It is not because the rays from the fire have more power, but rather because
they have less, that they heat glass and other transparent bodies. It is true,
however, that as you approach the source of heat the rays being nearer each
other, the heat is more condensed, and can produce effects of which the
solar rays, from the great distance of their source, are incapable. Thus we
should find it impossible to roast a joint of meat by the sun’s rays, though it
is so easily done by culinary heat. Yet caloric emanated from burning
bodies, which is commonly called culinary heat, has neither the intensity
nor the velocity of solar rays. All caloric, we have said, is supposed to
proceed originally from the sun; but after having been incorporated with
terrestrial bodies, and again given out by them, though its nature is not
essentially altered, it retains neither the intensity nor the velocity with
which it first emanated from that luminary; it has therefore not the power of
passing through transparent mediums, such as glass and water, without
being partially retained by those bodies.
EMILY.

I recollect that in the experiment on the reflection of heat, the glass skreen
which you interposed between the burning taper and the mirror, arrested the
rays of caloric, and suffered only those of light to pass through it.
CAROLINE.

Glass windows, then, though they cannot be heated by the sun shining on
them, may be heated internally by a fire in the room? But, Mrs. B., since the
atmosphere is not warmed by the solar rays passing through it, how does it
obtain heat; for all the fires that are burning on the surface of the earth
would contribute very little towards warming it?
EMILY.

The radiation of heat is not confined to burning bodies: for all bodies, you
know, have that property; therefore, not only every thing upon the surface

of the earth, but the earth itself, must radiate heat; and this terrestrial
caloric, not having, I suppose, sufficient power to traverse the atmosphere,
communicates heat to it.
MRS. B.

Your inference is extremely well drawn, Emily; but the foundation on
which it rests is not sound; for the fact is, that terrestrial or culinary heat,
though it cannot pass through the denser transparent mediums, such as glass
or water, without loss, traverses the atmosphere completely: so that all the
heat which the earth radiates, unless it meet with clouds or any foreign body
to intercept its passage, passes into the distant regions of the universe.
CAROLINE.

What a pity that so much heat should be wasted!
MRS. B.

Before you are tempted to object to any law of nature, reflect whether it
may not prove to be one of the numberless dispensations of Providence for
our good. If all the heat which the earth has received from the sun, since the
creation had been accumulated in it, its temperature by this time would, no
doubt, have been more elevated than any human being could have borne.
CAROLINE.

I spoke indeed very inconsiderately. But, Mrs. B., though the earth, at such
a high temperature, might have scorched our feet, we should always have
had a cool refreshing air to breathe, since the radiation of the earth does not
heat the atmosphere.
EMILY.

The cool air would have afforded but very insufficient refreshment, whilst
our bodies were exposed to the burning radiation of the earth.
MRS. B.

Nor should we have breathed a cool air; for though it is true that heat is not
communicated to the atmosphere by radiation, yet the air is warmed by
contact with heated bodies, in the same manner as solids or liquids. The
stratum of air which is immediately in contact with the earth is heated by it;

it becomes specifically lighter and rises, making way for another stratum of
air which is in its turn heated and carried upwards; and thus each successive
stratum of air is warmed by coming in contact with the earth. You may
perceive this effect in a sultry day, if you attentively observe the strata of air
near the surface of the earth; they appear in constant agitation, for though it
is true the air is itself invisible, yet the sun shining on the vapours floating
in it, render them visible, like the amber dust in the water. The temperature
of the surface of the earth is therefore the source from whence the
atmosphere derives its heat, though it is communicated neither by radiation,
nor transmitted from one particle of it to another by the conducting power;
but every particle of air must come in contact with the earth in order to
receive heat from it.
EMILY.

Wind then by agitating the air should contribute to cool the earth and warm
the atmosphere, by bringing a more rapid succession of fresh strata of air in
contact with the earth, and yet in general wind feels cooler than still air?
MRS. B.

Because the agitation of the air carries off heat from the surface of our
bodies more rapidly than still air, by occasioning a greater number of points
of contact in a given time.
EMILY.

Since it is from the earth and not the sun that the atmosphere receives its
heat, I no longer wonder that elevated regions should be colder than plains
and valleys; it was always a subject of astonishment to me, that in
ascending a mountain and approaching the sun, the air became colder
instead of being more heated.
MRS. B.

At the distance of about a hundred million of miles, which we are from the
sun, the approach of a few thousand feet makes no sensible difference,
whilst it produces a very considerable effect with regard to the warming the
atmosphere at the surface of the earth.
CAROLINE.

Yet as the warm air rises from the earth and the cold air descends to it,
I should have supposed that heat would have accumulated in the upper
regions of the atmosphere, and that we should have felt the air warmer as
we ascended?
MRS. B.

The atmosphere, you know, diminishes in density, and consequently in
weight, as it is more distant from the earth; the warm air, therefore, rises
only till it meets with a stratum of air of its own density; and it will not
ascend into the upper regions of the atmosphere until all the parts beneath
have been previously heated. The length of summer even in warm climates
does not heat the air sufficiently to melt the snow which has accumulated
during the winter on very high mountains, although they are almost
constantly exposed to the heat of the sun’s rays, being too much elevated to
be often enveloped in clouds.
EMILY.

These explanations are very satisfactory; but allow me to ask you one more
question respecting the increased levity of heated liquids. You said that
when water was heated over the fire, the particles at the bottom of the
vessel ascended as soon as heated, in consequence of their specific levity:
why does not the same effect continue when the water boils, and is
converted into steam? and why does the steam rise from the surface, instead
of the bottom of the liquid?
MRS. B.

The steam or vapour does ascend from the bottom, though it seems to arise
from the surface of the liquid. We shall boil some water in this Florence
flask, (Plate IV. Fig. 1.) in order that you may be well acquainted with the
process of ebullition;—you will then see, through the glass, that the vapour
rises in bubbles from the bottom. We shall make it boil by means of a lamp,
which is more convenient for this purpose than the chimney fire.
Vol. I. p. 84. Plate IV.

Fig. 2. Boiling water in a flask over a Patent lamp.
Larger view (complete Plate)

EMILY.

I see some small bubbles ascend, and a great many appear all over the
inside of the flask; does the water begin to boil already?
MRS. B.

No; what you now see are bubbles of air, which were either dissolved in the
water, or attached to the inner surface of the flask, and which, being rarefied
by the heat, ascend in the water.
EMILY.

But the heat which rarefies the air inclosed in the water must rarefy the
water at the same time; therefore, if it could remain stationary in the water
when both were cold, I do not understand why it should not when both are
equally heated?
MRS. B.

Air being much less dense than water, is more easily rarefied; the former,
therefore, expands to a great extent, whilst the latter continues to occupy
nearly the same space; for water dilates comparatively but very little

without changing its state and becoming vapour. Now that the water in the
flask begins to boil, observe what large bubbles rise from the bottom of it.
EMILY.

I see them perfectly; but I wonder that they have sufficient power to force
themselves through the water.
CAROLINE.

They must rise, you know, from their specific levity.
MRS. B.

You are right, Caroline; but vapour has not in all liquids (when brought to
the degree of vaporization) the power of overcoming the pressure of the less
heated surface. Metals, for instance, mercury excepted, evaporate only from
the surface; therefore no vapour will ascend from them till the degree of
heat which is necessary to form it has reached the surface; that is to say, till
the whole of the liquid is brought to a state of ebullition.
EMILY.

I have observed that steam, immediately issuing from the spout of a
teakettle, is less visible than at a further distance from it; yet it must be
more dense when it first evaporates, than when it begins to diffuse itself in
the air.
MRS. B.

When the steam is first formed, it is so perfectly dissolved by caloric, as to
be invisible. In order however to understand this, it will be necessary for me
to enter into some explanation respecting the nature of solution. Solution
takes place whenever a body is melted in a fluid. In this operation the body
is reduced to such a minute state of division by the fluid, as to become
invisible in it, and to partake of its fluidity; but in common solutions this
happens without any decomposition, the body being only divided into its
integrant particles by the fluid in which it is melted.
CAROLINE.

It is then a mode of destroying the attraction of aggregation.

MRS. B.

Undoubtedly.—The two principal solvent fluids are water, and caloric. You
may have observed that if you melt salt in water, it totally disappears, and
the water remains clear, and transparent as before; yet though the union of
these two bodies appears so perfect, it is not produced by any chemical
combination; both the salt and the water remain unchanged; and if you were
to separate them by evaporating the latter, you would find the salt in the
same state as before.
EMILY.

I suppose that water is a solvent for solid bodies, and caloric for liquids?
MRS. B.

Liquids of course can only be converted into vapour by caloric. But the
solvent power of this agent is not at all confined to that class of bodies;
a great variety of solid substances are dissolved by heat: thus metals, which
are insoluble in water, can be dissolved by intense heat, being first fused or
converted into a liquid, and then rarefied into an invisible vapour. Many
other bodies, such as salt, gums, &c. yield to either of these solvents.
CAROLINE.

And that, no doubt, is the reason why hot water will melt them so much
better than cold water?
MRS. B.

It is so. Caloric may, indeed, be considered as having, in every instance,
some share in the solution of a body by water, since water, however low its
temperature may be, always contains more or less caloric.
EMILY.

Then, perhaps, water owes its solvent power merely to the caloric contained
in it?
MRS. B.

That, probably, would be carrying the speculation too far; I should rather
think that water and caloric unite their efforts to dissolve a body, and that
the difficulty or facility of effecting this, depend both on the degree of

attraction of aggregation to be overcome, and on the arrangement of the
particles which are more or less disposed to be divided and penetrated by
the solvent.
EMILY.

But have not all liquids the same solvent power as water?
MRS. B.

The solvent power of other liquids varies according to their nature, and that
of the substances submitted to their action. Most of these solvents, indeed,
differ essentially from water, as they do not merely separate the integrant
particles of the bodies which they dissolve, but attack their constituent
principles by the power of chemical attraction, thus producing a true
decomposition. These more complicated operations we must consider in
another place, and confine our attention at present to the solutions by water
and caloric.
CAROLINE.

But there are a variety of substances which, when dissolved in water, make
it thick and muddy, and destroy its transparency.
MRS. B.

In this case it is not a solution, but simply a mixture. I shall show you the
difference between a solution and a mixture, by putting some common salt
into one glass of water, and some powder of chalk into another; both these
substances are white, but their effect on the water will be very different.
CAROLINE.

Very different indeed! The salt entirely disappears and leaves the water
transparent, whilst the chalk changes it into an opaque liquid like milk.
EMILY.

And would lumps of chalk and salt produce similar effects on water?
MRS. B.

Yes, but not so rapidly; salt is, indeed, soon melted though in a lump; but
chalk, which does not mix so readily with water, would require a much

greater length of time; I therefore preferred showing you the experiment
with both substances reduced to powder, which does not in any respect alter
their nature, but facilitates the operation merely by presenting a greater
quantity of surface to the water.
I must not forget to mention a very curious circumstance respecting
solutions, which is, that a fluid is not nearly so much increased in bulk by
holding a body in solution, as it would by mere mixture with the body.
CAROLINE.

That seems impossible; for two bodies cannot exist together in the same
space.
MRS. B.

Two bodies may, by condensation, occupy less space when in union than
when separate, and this I can show you by an easy experiment.
This phial, which contains some salt, I shall fill with water, pouring it in
quickly, so as not to dissolve much of the salt; and when it is quite full I
cork it.—If I now shake the phial till the salt is dissolved, you will observe
that it is no longer full.
CAROLINE.

I shall try to add a little more salt.—But now, you see, Mrs. B., the water
runs over.
MRS. B.

Yes; but observe that the last quantity of salt you put in remains solid at the
bottom, and displaces the water; for it has already melted all the salt it is
capable of holding in solution. This is called the point of saturation; and the
water in this case is said to be saturated with salt.
EMILY.

I think I now understand the solution of a solid body by water perfectly: but
I have not so clear an idea of the solution of a liquid by caloric.
MRS. B.

It is probably of a similar nature; but as caloric is an invisible fluid, its
action as a solvent is not so obvious as that of water. Caloric, we may
conceive, dissolves water, and converts it into vapour by the same process
as water dissolves salt; that is to say, the particles of water are so minutely
divided by the caloric as to become invisible. Thus, you are now enabled to
understand why the vapour of boiling water, when it first issues from the
spout of a kettle, is invisible; it is so, because it is then completely dissolved
by caloric. But the air with which it comes in contact, being much colder
than the vapour, the latter yields to it a quantity of its caloric. The particles
of vapour being thus in a great measure deprived of their solvent, gradually
collect, and become visible in the form of steam, which is water in a state of
imperfect solution; and if you were further to deprive it of its caloric, it
would return to its original liquid state.
CAROLINE.

That I understand very well. If you hold a cold plate over a tea-urn, the
steam issuing from it will be immediately converted into drops of water by
parting with its caloric to the plate; but in what state is the steam, when it
becomes invisible by being diffused in the air?
MRS. B.

It is not merely diffused, but is again dissolved by the air.
EMILY.

The air, then, has a solvent power, like water and caloric?
MRS. B.

This was formerly believed to be the case. But it appears from more recent
enquiries that the solvent power of the atmosphere depends solely upon the
caloric contained in it. Sometimes the watery vapour diffused in the
atmosphere is but imperfectly dissolved, as is the case in the formation of
clouds and fogs; but if it gets into a region sufficiently warm, it becomes
perfectly invisible.
EMILY.

Can any water dissolve in the atmosphere without its being previously
converted into vapour by boiling?

MRS. B.

Unquestionably; and this constitutes the difference between vaporization
and evaporation. Water, when heated to the boiling point, can no longer
exist in the form of water, and must necessarily be converted into vapour or
steam, whatever may be the state and temperature of the surrounding
medium; this is called vaporization. But the atmosphere, by means of the
caloric it contains, can take up a certain portion of water at any temperature,
and hold it in a state of solution. This is simply evaporation. Thus the
atmosphere is continually carrying off moisture from the surface of the
earth, until it is saturated with it.
CAROLINE.

That is the case, no doubt, when we feel the atmosphere damp.
MRS. B.

On the contrary, when the moisture is well dissolved it occasions no
humidity: it is only when in a state of imperfect solution and floating in the
atmosphere, in the form of watery vapour, that it produces dampness. This
happens more frequently in winter than in summer; for the lower the
temperature of the atmosphere, the less water it can dissolve; and in reality
it never contains so much moisture as in a dry hot summer’s day.
CAROLINE.

You astonish me! But why, then, is the air so dry in frosty weather, when its
temperature is at the lowest?
EMILY.

This, I conjecture, proceeds not so much from the moisture being dissolved,
as from its being frozen; is not that the case?
MRS. B.

It is; and the freezing of the watery vapour which the atmospheric heat
could not dissolve, produces what is called a hoar frost; for the particles
descend in freezing, and attach themselves to whatever they meet with on
the surface of the earth.

The tendency of free caloric to an equilibrium, together with its solvent
power, are likewise connected with the phenomena of rain, of dew, &c.
When moist air of a certain temperature happens to pass through a colder
region of the atmosphere, it parts with a portion of its heat to the
surrounding air; the quantity of caloric, therefore, which served to keep the
water in a state of vapour, being diminished, the watery particles approach
each other, and form themselves into drops of water, which being heavier
than the atmosphere, descend to the earth. There are also other
circumstances, and particularly the variation in the weight of the
atmosphere, which may contribute to the formation of rain. This, however,
is an intricate subject, into which we cannot more fully enter at present.
EMILY.

In what manner do you account for the formation of dew?
MRS. B.

Dew is a deposition of watery particles or minute drops from the
atmosphere, precipitated by the coolness of the evening.
CAROLINE.

This precipitation is owing, I suppose, to the cooling of the atmosphere,
which prevents its retaining so great a quantity of watery vapour in solution
as during the heat of the day.
MRS. B.

Such was, from time immemorial, the generally received opinion respecting
the cause of dew; but it has been very recently proved by a course of
ingenious experiments of Dr. Wells, that the deposition of dew is produced
by the cooling of the surface of the earth, which he has shown to take place
previously to the cooling of the atmosphere; for on examining the
temperature of a plot of grass just before the dew-fall, he found that it was
considerably colder than the air a few feet above it, from which the dew
was shortly after precipitated.
EMILY.

But why should the earth cool in the evening sooner than the atmosphere?

MRS. B.

Because it parts with its heat more readily than the air; the earth is an
excellent radiator of caloric, whilst the atmosphere does not possess that
property, at least in any sensible degree. Towards evening, therefore, when
the solar heat declines, and when after sunset it entirely ceases, the earth
rapidly cools by radiating heat towards the skies; whilst the air has no
means of parting with its heat but by coming into contact with the cooled
surface of the earth, to which it communicates its caloric. Its solvent power
being thus reduced, it is unable to retain so large a portion of watery vapour,
and deposits those pearly drops which we call dew.
EMILY.

If this be the cause of dew, we need not be apprehensive of receiving any
injury from it; for it can be deposited only on surfaces that are colder than
the atmosphere, which is never the case with our bodies.
MRS. B.

Very true; yet I would not advise you for this reason to be too confident of
escaping all the ill effects which may arise from exposure to the dew; for it
may be deposited on your clothes, and chill you afterwards by its
evaporation from them. Besides, whenever the dew is copious, there is a
chill in the atmosphere which it is not always safe to encounter.
CAROLINE.

Wind, then, must promote the deposition of dew, by bringing a more rapid
succession of particles of air in contact with the earth, just as it promotes
the cooling of the earth and warming of the atmosphere during the heat of
the day?
MRS. B.

Yes; provided the wind be unattended with clouds, for these accumulations
of moisture not only prevent the free radiation of the earth towards the
upper regions, but themselves radiate towards the earth; under these
circumstances much less dew is formed than on fine clear nights, when the
radiation of the earth passes without obstacle through the atmosphere to the
distant regions of space, whence it receives no caloric in exchange. The dew

continues to be deposited during the night, and is generally most abundant
towards morning, when the contrast between the temperature of the earth
and that of the air is greatest. After sunrise the equilibrium of temperature
between these two bodies is gradually restored by the solar rays passing
freely through the atmosphere to the earth; and later in the morning the
temperature of the earth gains the ascendency, and gives out caloric to the
air by contact, in the same manner as it receives it from the air during the
night.—Can you tell me, now, why a bottle of wine taken fresh from the
cellar (in summer particularly), will soon be covered with dew; and even
the glasses into which the wine is poured will be moistened with a similar
vapour?
EMILY.

The bottle being colder than the surrounding air, must absorb caloric from
it; the moisture therefore which that air contained becomes visible, and
forms the dew which is deposited on the bottle.
MRS. B.

Very well, Emily. Now, Caroline, can you inform me why, in a warm room,
or close carriage, the contrary effect takes place; that is to say, that the
inside of the windows is covered with vapour?
CAROLINE.

I have heard that it proceeds from the breath of those within the room or the
carriage; and I suppose it is occasioned by the windows which, being colder
than the breath, deprive it of part of its caloric, and by this means convert it
into watery vapour.
MRS. B.

You have both explained it extremely well. Bodies attract dew in proportion
as they are good radiators of caloric, as it is this quality which reduces their
temperature below that of the atmosphere; hence we find that little or no
dew is deposited on rocks, sand, water; while grass and living vegetables, to
which it is so highly beneficial, attract it in abundance—another remarkable
instance of the wise and bountiful dispensations of Providence.
EMILY.

And we may again observe it in the abundance of dew in summer, and in
hot climates, when its cooling effects are so much required; but I do not
understand what natural cause increases the dew in hot weather?
MRS. B.

The more caloric the earth receives during the day, the more it will radiate
afterwards, and consequently the more rapidly its temperature will be
reduced in the evening, in comparison to that of the atmosphere. In the
West-Indies especially, where the intense heat of the day is strongly
contrasted with the coolness of the evening, the dew is prodigiously
abundant. During a drought, the dew is less plentiful, as the earth is not
sufficiently supplied with moisture to be able to saturate the atmosphere.
CAROLINE.

I have often observed, Mrs. B., that when I walk out in frosty weather, with
a veil over my face, my breath freezes upon it. Pray what is the reason of
that?
MRS. B.

It is because the cold air immediately seizes on the caloric of your breath,
and, by robbing it of its solvent, reduces it to a denser fluid, which is the
watery vapour that settles on your veil, and there it continues parting with
its caloric till it is brought down to the temperature of the atmosphere, and
assumes the form of ice.
You may, perhaps, have observed that the breath of animals, or rather the
moisture contained in it, is visible in damp weather, or during a frost. In the
former case, the atmosphere being over-saturated with moisture, can
dissolve no more. In the latter, the cold condenses it into visible vapour; and
for the same reason, the steam arising from water that is warmer than the
atmosphere, becomes visible. Have you never taken notice of the vapour
rising from your hands after having dipped them into warm water?
CAROLINE.

Frequently, especially in frosty weather.
MRS. B.

We have already observed that pressure is an obstacle to evaporation: there
are liquids that contain so great a quantity of caloric, and whose particles
consequently adhere so slightly together, that they may be rapidly converted
into vapour without any elevation of temperature, merely by taking off the
weight of the atmosphere. In such liquids, you perceive, it is the pressure of
the atmosphere alone that connects their particles, and keeps them in a
liquid state.
CAROLINE.

I do not well understand why the particles of such fluids should be disunited
and converted into vapour, without any elevation of temperature, in spite of
the attraction of cohesion.
MRS. B.

It is because the degree of heat at which we usually observe these fluids is
sufficient to overcome their attraction of cohesion. Ether is of this
description; it will boil and be converted into vapour, at the common
temperature of the air, if the pressure of the atmosphere be taken off.
EMILY.

I thought that ether would evaporate without either the pressure of the
atmosphere being taken away, or heat applied; and that it was for that
reason so necessary to keep it carefully corked up?
MRS. B.

It is true it will evaporate, but without ebullition; what I am now speaking
of is the vaporization of ether, or its conversion into vapour by boiling. I am
going to show you how suddenly the ether in this phial will be converted
into vapour, by means of the air-pump.—Observe with what rapidity the
bubbles ascend, as I take off the pressure of the atmosphere.
CAROLINE.

It positively boils: how singular to see a liquid boil without heat!
MRS. B.

Now I shall place the phial of ether in this glass, which it nearly fits, so as
to leave only a small space, which I fill with water; and in this state I put it

again under the receiver. (Plate IV. Fig. 1.) * You will observe, as I exhaust
the air from it, that whilst the ether boils, the water freezes.
Vol. I. p. 84. Plate IV.

Fig. 1. Ether evaporated & water frozen in the air pump. A Phial of Ether. B Glass
vessel containing water. C.C Thermometers, one in the Ether, the other in the water.
Larger view (complete Plate)

CAROLINE.

It is indeed wonderful to see water freeze in contact with a boiling fluid!
EMILY.

I am at a loss to conceive how the ether can pass to the state of vapour
without an addition of caloric. Does it not contain more caloric in a state of
vapour, than in a state of liquidity?
MRS. B.

It certainly does; for though it is the pressure of the atmosphere which
condenses it into a liquid, it is by forcing out the caloric that belongs to it
when in an aëriform state.
EMILY.

You have, therefore, two difficulties to explain, Mrs. B.—First, from
whence the ether obtains the caloric necessary to convert it into vapour

when it is relieved from the pressure of the atmosphere; and, secondly, what
is the reason that the water, in which the bottle of ether stands, is frozen?
CAROLINE.

Now, I think, I can answer both these questions. The ether obtains the
addition of caloric required, from the water in the glass; and the loss of
caloric, which the latter sustains, is the occasion of its freezing.
MRS. B.

You are perfectly right; and if you look at the thermometer which I have
placed in the water, whilst I am working the pump, you will see that every
time bubbles of vapour are produced, the mercury descends; which proves
that the heat of the water diminishes in proportion as the ether boils.
EMILY.

This I understand now very well; but if the water freezes in consequence of
yielding its caloric to the ether, the equilibrium of heat must, in this case, be
totally destroyed. Yet you have told us, that the exchange of caloric between
two bodies of equal temperature, was always equal; how, then, is it that the
water, which was originally of the same temperature as the ether, gives out
caloric to it, till the water is frozen, and the ether made to boil?
MRS. B.

I suspected that you would make these objections; and, in order to remove
them, I enclosed two thermometers in the air-pump; one which stands in the
glass of water, the other in the phial of ether; and you may see that the
equilibrium of temperature is not destroyed; for as the thermometer
descends in the water, that in the ether sinks in the same manner; so that
both thermometers indicate the same temperature, though one of them is in
a boiling, the other in a freezing liquid.
EMILY.

The ether, then, becomes colder as it boils? This is so contrary to common
experience, that I confess it astonishes me exceedingly.
CAROLINE.

It is, indeed, a most extraordinary circumstance. But pray, how do you
account for it?
MRS. B.

I cannot satisfy your curiosity at present; for before we can attempt to
explain this apparent paradox, it is necessary to become acquainted with the
subject of latent heat: and that, I think, we must defer till our next
interview.
CAROLINE.

I believe, Mrs. B., that you are glad to put off the explanation; for it must be
a very difficult point to account for.
MRS. B.

I hope, however, that I shall do it to your complete satisfaction.
EMILY.

But before we part, give me leave to ask you one question. Would not
water, as well as ether, boil with less heat, if deprived of the pressure of the
atmosphere?
MRS. B.

Undoubtedly. You must always recollect that there are two forces to
overcome, in order to make a liquid boil or evaporate; the attraction of
aggregation, and the weight of the atmosphere. On the summit of a high
mountain (as Mr. De Saussure ascertained on Mount Blanc) much less heat
is required to make water boil, than in the plain, where the weight of the
atmosphere is greater. * Indeed if the weight of the atmosphere be entirely
removed by means of a good air-pump, and if water be placed in the
exhausted receiver, it will evaporate so fast, however cold it maybe, as to
give it the appearance of boiling from the surface. But without the
assistance of the air-pump, I can show you a very pretty experiment, which
proves the effect of the pressure of the atmosphere in this respect.
Observe, that this Florence flask is about half full of water, and the upper
half of invisible vapour, the water being in the act of boiling.—I take it

from the lamp, and cork it carefully—the water, you see, immediately
ceases boiling.—I shall now dip the flask into a bason of cold water. †
CAROLINE.

But look, Mrs. B., the hot water begins to boil again, although the cold
water must rob it more and more of its caloric! What can be the reason of
that?
MRS. B.

Let us examine its temperature. You see the thermometer immersed in it
remains stationary at 180 degrees, which is about 30 degrees below the
boiling point. When I took the flask from the lamp, I observed to you that
the upper part of it was filled with vapour; this being compelled to yield its
caloric to the cold water, was again condensed into water—What, then,
filled the upper part of the flask?
EMILY.

Nothing; for it was too well corked for the air to gain admittance, and
therefore the upper part of the flask must be a vacuum.
MRS. B.

The water below, therefore, no longer sustains the pressure of the
atmosphere, and will consequently boil at a much lower temperature. Thus,
you see, though it had lost many degrees of heat, it began boiling again the
instant the vacuum was formed above it. The boiling has now ceased, the
temperature of the water being still farther reduced; if it had been ether,
instead of water, it would have continued boiling much longer, for ether
boils, under the usual atmospheric pressure, at a temperature as low as 100
degrees; and in a vacuum it boils at almost any temperature; but water being
a more dense fluid, requires a more considerable quantity of caloric to make
it evaporate quickly, even when the pressure of the atmosphere is removed.
EMILY.

What proportion of vapour can the atmosphere contain in a state of
solution?
MRS. B.

I do not know whether it has been exactly ascertained by experiment; but at
any rate this proportion must vary, both according to the temperature and
the weight of the atmosphere; for the lower the temperature, and the greater
the pressure, the smaller must be the proportion of vapour that the
atmosphere can contain.
To conclude the subject of free caloric, I should mention Ignition, by which
is meant that emission of light which is produced in bodies at a very high
temperature, and which is the effect of accumulated caloric.
EMILY.

You mean, I suppose, that light which is produced by a burning body?
MRS. B.

No: ignition is quite independent of combustion. Clay, chalk, and indeed all
incombustible substances, may be made red hot. When a body burns, the
light emitted is the effect of a chemical change which takes place, whilst
ignition is the effect of caloric alone, and no other change than that of
temperature is produced in the ignited body.
All solid bodies, and most liquids, are susceptible of ignition, or, in other
words, of being heated so as to become luminous; and it is remarkable that
this takes place pretty nearly at the same temperature in all bodies, that is, at
about 800 degrees of Fahrenheit’s scale.
EMILY.

But how can liquids attain so high a temperature, without being converted
into vapour?
MRS. B.

By means of confinement and pressure. Water confined in a strong iron
vessel (called Papin’s digester) can have its temperature raised to upwards
of 400 degrees. Sir James Hall has made some very curious experiments on
the effects of heat assisted by pressure; by means of strong gun-barrels, he
succeeded in melting a variety of substances which were considered as
infusible: and it is not unlikely that, by similar methods, water itself might
be heated to redness.

EMILY.

I am surprised at that: for I thought that the force of steam was such as to
destroy almost all mechanical resistance.
MRS. B.

The expansive force of steam is prodigious; but in order to subject water to
such high temperatures, it is prevented by confinement from being
converted into steam, and the expansion of heated water is comparatively
trifling.—But we have dwelt so long on the subject of free caloric, that we
must reserve the other modifications of that agent to our next meeting,
when we shall endeavour to proceed more rapidly.

* Two pieces of thin glass tubes, sealed at one end, might answer
this purpose better. The experiment, however, as here described,
is difficult, and requires a very nice apparatus. But if, instead of
phials or tubes, two watch-glasses be used, water may be frozen
almost instantly in the same manner. The two glasses are placed
over one another, with a few drops of water interposed between
them, and the uppermost glass is filled with ether. After working
the pump for a minute or two, the glasses are found to adhere
strongly together, and a thin layer of ice is seen between them.
* On the top of Mount Blanc, water boiled when heated only to
187 degrees, instead of 212 degrees.
† The same effect may be produced by wrapping a cold wet linen
cloth round the upper part of the flask. In order to show how
much the water cools whilst it is boiling, a thermometer,
graduated on the tube itself, may be introduced into the bottle
through the cork.

CONVERSATION IV.
ON COMBINED CALORIC, COMPREHENDING SPECIFIC AND
LATENT HEAT.

MRS. B.

We are now to examine the other modifications of caloric.
CAROLINE.

I am very curious to know of what nature they can be; for I have no notion
of any kind of heat that is not perceptible to the senses.
MRS. B.

In order to enable you to understand them, it will be necessary to enter into
some previous explanations.
It has been discovered by modern chemists, that bodies of a different nature,
heated to the same temperature, do not contain the same quantity of caloric.
CAROLINE.

How could that be ascertained? Have you not told us that it is impossible to
discover the absolute quantity of caloric which bodies contain?
MRS. B.

True; but at the same time I said that we were enabled to form a judgment
of the proportions which bodies bore to each other in this respect. Thus it is
found that, in order to raise the temperature of different bodies the same
number of degrees, different quantities of caloric are required for each of
them. If, for instance, you place a pound of lead, a pound of chalk, and a
pound of milk, in a hot oven, they will be gradually heated to the
temperature of the oven; but the lead will attain it first, the chalk next, and
the milk last.
CAROLINE.

That is a natural consequence of their different bulks; the lead being the
smallest body, will be heated soonest, and the milk, which is the largest,
will require the longest time.
MRS. B.

That explanation will not do, for if the lead be the least in bulk, it offers also
the least surface to the caloric, the quantity of heat therefore which can
enter into it in the same space of time is proportionally smaller.
EMILY.

Why, then, do not the three bodies attain the temperature of the oven at the
same time?

MRS. B.

It is supposed to be on account of the different capacity of these bodies for
caloric.
CAROLINE.

What do you mean by the capacity of a body for caloric?
MRS. B.

I mean a certain disposition of bodies to require more or less caloric for
raising their temperature to any degree of heat. Perhaps the fact may be thus
explained:
Let us put as many marbles into this glass as it will contain, and pour some
sand over them—observe how the sand penetrates and lodges between
them. We shall now fill another glass with pebbles of various forms—you
see that they arrange themselves in a more compact manner than the
marbles, which, being globular, can touch each other by a single point only.
The pebbles, therefore, will not admit so much sand between them; and
consequently one of these glasses will necessarily contain more sand than
the other, though both of them be equally full.
CAROLINE.

This I understand perfectly. The marbles and the pebbles represent two
bodies of different kinds, and the sand the caloric contained in them; it
appears very plain, from this comparison, that one body may admit of more
caloric between its particles than another.
MRS. B.

You can no longer be surprised, therefore, that bodies of a different capacity
for caloric should require different proportions of that fluid to raise their
temperatures equally.
EMILY.

But I do not conceive why the body that contains the most caloric should
not be of the highest temperature; that is to say, feel hot in proportion to the
quantity of caloric it contains?
MRS. B.

The caloric that is employed in filling the capacity of a body, is not free
caloric; but is imprisoned as it were in the body, and is therefore
imperceptible: for we can feel only the caloric which the body parts with,
and not that which it retains.
CAROLINE.

It appears to me very extraordinary that heat should be confined in a body
in such a manner as to be imperceptible.
MRS. B.

If you lay your hand on a hot body, you feel only the caloric which leaves
it, and enters your hand; for it is impossible that you should be sensible of
that which remains in the body. The thermometer, in the same manner, is
affected only by the free caloric which a body transmits to it, and not at all
by that which it does not part with.
CAROLINE.

I begin to understand it: but I confess that the idea of insensible heat is so
new and strange to me, that it requires some time to render it familiar.
MRS. B.

Call it insensible caloric, and the difficulty will appear much less
formidable. It is indeed a sort of contradiction to call it heat, when it is so
situated as to be incapable of producing that sensation. Yet this modification
of caloric is commonly called specific heat.
CAROLINE.

But it certainly would have been more correct to have called it specific
caloric.
EMILY.

I do not understand how the term specific applies to this modification of
caloric?
MRS. B.

It expresses the relative quantity of caloric which different species of bodies
of the same weight and temperature are capable of containing. This

modification is also frequently called heat of capacity, a term perhaps
preferable, as it explains better its own meaning.
You now understand, I suppose, why the milk and chalk required a longer
portion of time than the lead to raise their temperature to that of the oven?
EMILY.

Yes: the milk and chalk having a greater capacity for caloric than the lead,
a greater proportion of that fluid became insensible in those bodies: and the
more slowly, therefore, their temperature was raised.
CAROLINE.

But might not this difference proceed from the different conducting powers
of heat in these three bodies, since that which is the best conductor must
necessarily attain the temperature of the oven first?
MRS. B.

Very well observed, Caroline. This objection would be insurmountable, if
we could not, by reversing the experiment, prove that the milk, the chalk,
and the lead, actually absorbed different quantities of caloric, and we know
that if the different time they took in heating, proceeded merely from their
different conducting powers, they would each have acquired an equal
quantity of caloric.
CAROLINE.

Certainly. But how can you reverse this experiment?
MRS. B.

It may be done by cooling the several bodies to the same degree in an
apparatus adapted to receive and measure the caloric which they give out.
Thus, if you plunge them into three equal quantities of water, each at the
same temperature, you will be able to judge of the relative quantity of
caloric which the three bodies contained, by that, which, in cooling, they
communicated to their respective portions of water: for the same quantity of
caloric which they each absorbed to raise their temperature, will abandon
them in lowering it; and on examining the three vessels of water, you will
find the one in which you immersed the lead to be the least heated; that

which held the chalk will be the next; and that which contained the milk
will be heated the most of all. The celebrated Lavoisier has invented a
machine to estimate, upon this principle, the specific heat of bodies in a
more perfect manner; but I cannot explain it to you, till you are acquainted
with the next modification of caloric.
EMILY.

The more dense a body is, I suppose, the less is its capacity for caloric?
MRS. B.

This is not always the case with bodies of different nature; iron, for
instance, contains more specific heat than tin, though it is more dense. This
seems to show that specific heat does hot merely depend upon the
interstices between the particles; but, probably, also upon some peculiar
constitution of the bodies which we do not comprehend.
EMILY.

But, Mrs. B., it would appear to me more proper to compare bodies by
measure, rather than by weight, in order to estimate their specific heat.
Why, for instance, should we not compare pints of milk, of chalk, and of
lead, rather than pounds of those substances; for equal weights may be
composed of very different quantities?
MRS. B.

You are mistaken, my dear; equal weight must contain equal quantities of
matter; and when we wish to know what is the relative quantity of caloric,
which substances of various kinds are capable of containing under the same
temperature, we must compare equal weights, and not equal bulks of those
substances. Bodies of the same weight may undoubtedly be of very
different dimensions; but that does not change their real quantity of matter.
A pound of feathers does not contain one atom more than a pound of lead.
CAROLINE.

I have another difficulty to propose. It appears to me, that if the temperature
of the three bodies in the oven did not rise equally, they would never reach
the same degree; the lead would always keep its advantage over the chalk
and milk, and would perhaps be boiling before the others had attained the

temperature of the oven. I think you might as well say that, in the course of
time, you and I should be of the same age?
MRS. B.

Your comparison is not correct, Caroline. As soon as the lead reached the
temperature of the oven, it would remain stationary; for it would then give
out as much heat as it would receive. You should recollect that the exchange
of radiating heat, between two bodies of equal temperature, is equal: it
would be impossible, therefore, for the lead to accumulate heat after having
attained the temperature of the oven; and that of the chalk and milk
therefore would ultimately arrive at the same standard. Now I fear that this
will not hold good with respect to our ages, and that, as long as I live,
I shall never cease to keep my advantage over you.
EMILY.

I think that I have found a comparison for specific heat, which is very
applicable. Suppose that two men of equal weight and bulk, but who
required different quantities of food to satisfy their appetites, sit down to
dinner, both equally hungry; the one would consume a much greater
quantity of provisions than the other, in order to be equally satisfied.
MRS. B.

Yes, that is very fair; for the quantity of food necessary to satisfy their
respective appetites, varies in the same manner as the quantity of caloric
requisite to raise equally the temperature of different bodies.
EMILY.

The thermometer, then, affords no indication of the specific heat of bodies?
MRS. B.

None at all: no more than satiety is a test of the quantity of food eaten. The
thermometer, as I have repeatedly said, can be affected only by free caloric,
which alone raises the temperature of bodies.
But there is another mode of proving the existence of specific heat, which
affords a very satisfactory illustration of that modification. This, however,
I did not enlarge upon before, as I thought it might appear to you rather

complicated.—If you mix two fluids of different temperatures, let us say the
one at 50 degrees, and the other at 100 degrees, of what temperature do you
suppose the mixture will be?
CAROLINE.

It will be no doubt the medium between the two, that is to say, 75 degrees.
MRS. B.

That will be the case if the two bodies happen to have the same capacity for
caloric; but if not, a different result will be obtained. Thus, for instance, if
you mix together a pound of mercury, heated at 50 degrees, and a pound of
water heated at 100 degrees, the temperature of the mixture, instead of
being 75 degrees, will be 80 degrees; so that the water will have lost only
12 degrees, whilst the mercury will have gained 38 degrees; from which
you will conclude that the capacity of mercury for heat is less than that of
water.
CAROLINE.

I wonder that mercury should have so little specific heat. Did we not see it
was a much better conductor of heat than water?
MRS. B.

And it is precisely on that account that its specific heat is less. For since the
conductive power of bodies depends, as we have observed before, on their
readiness to receive heat and part with it, it is natural to expect that those
bodies which are the worst conductors should absorb the most caloric
before they are disposed to part with it to other bodies. But let us now
proceed to latent heat.
CAROLINE.

And pray what kind of heat is that?
MRS. B.

It is another modification of combined caloric, which is so analogous to
specific heat, that most chemists make no distinction between them; but Mr.
Pictet, in his Essay on Fire, has so clearly discriminated them, that I am
induced to adopt his view of the subject. We therefore call latent heat that

portion of insensible caloric which is employed in changing the state of
bodies; that is to say, in converting solids into liquids, or liquids; into
vapour. When a body changes its state from solid to liquid, or from liquid to
vapour, its expansion occasions a sudden and considerable increase of
capacity for heat, in consequence of which it immediately absorbs a
quantity of caloric, which becomes fixed in the body which it has
transformed; and, as it is perfectly concealed from our senses, it has
obtained the name of latent heat.
CAROLINE.

I think it would be much more correct to call this modification latent caloric
instead of latent heat, since it does not excite the sensation of heat.
MRS. B.

This modification of heat was discovered and named by Dr. Black long
before the French chemists introduced the term caloric, and we must not
presume to alter it, as it is still used by much better chemists than ourselves.
And, besides, you are not to suppose that the nature of heat is altered by
being variously modified: for if latent heat and specific heat do not excite
the same sensations as free caloric, it is owing to their being in a state of
confinement, which prevents them from acting upon our organs; and
consequently, as soon as they are extricated from the body in which they are
imprisoned, they return to their state of free caloric.
EMILY.

But I do not yet clearly see in what respect latent heat differs from specific
heat; for they are both of them imprisoned and concealed in bodies.
MRS. B.

Specific heat is that which is employed in filling the capacity of a body for
caloric, in the state in which this body actually exists; while latent heat is
that which is employed only in effecting a change of state, that is, in
converting bodies from a solid to a liquid, or from a liquid to an aëriform
state. But I think that, in a general point of view, both these modifications
might be comprehended under the name of heat of capacity, as in both
cases the caloric is equally engaged in filling the capacities of bodies.

I shall now show you an experiment, which I hope will give you a clear
idea of what is understood by latent heat.
The snow which you see in this phial has been cooled by certain chemical
means (which I cannot well explain to you at present), to 5 or 6 degrees
below the freezing point, as you will find indicated by the thermometer
which is placed in it. We shall expose it to the heat of a lamp, and you will
see the thermometer gradually rise, till it reaches the freezing point——
EMILY.

But there it stops, Mrs. B., and yet the lamp burns just as well as before.
Why is not its heat communicated to the thermometer?
CAROLINE.

And the snow begins to melt, therefore it must be rising above the freezing
point?
MRS. B.

The heat no longer affects the thermometer, because it is wholly employed
in converting the ice into water. As the ice melts, the caloric becomes latent
in the new-formed liquid, and therefore cannot raise its temperature; and the
thermometer will consequently remain stationary, till the whole of the ice
be melted.
CAROLINE.

Now it is all melted, and the thermometer begins to rise again.
MRS. B.

Because the conversion of the ice into water being completed, the caloric no
longer becomes latent; and therefore the heat which the water now receives
raises its temperature, as you find the thermometer indicates.
EMILY.

But I do not think that the thermometer rises so quickly in the water as it
did in the ice, previous to its beginning to melt, though the lamp burns
equally well?
MRS. B.

That is owing to the different specific heat of ice and water. The capacity of
water for caloric being greater than that of ice, more heat is required to raise
its temperature, and therefore the thermometer rises slower in the water
than in the ice.
EMILY.

True; you said that a solid body always increased its capacity for heat by
becoming fluid; and this is an instance of it.
MRS. B.

Yes, and the latent heat is that which is absorbed in consequence of the
greater capacity which the water has for heat, in comparison to ice.
I must now tell you a curious calculation founded on that consideration.
I have before observed to you that though the thermometer shows us the
comparative warmth of bodies, and enables us to determine the same point
at different times and places, it gives us no idea of the absolute quantity of
heat in any body. We cannot tell how low it ought to fall by the privation of
all heat, but an attempt has been made to infer it in the following manner. It
has been found by experiment, that the capacity of water for heat, when
compared with that of ice, is as 10 to 9, so that, at the same temperature, ice
contains one tenth of caloric less than water. By experiment also it is
observed, that in order to melt ice, there must be added to it as much heat,
as would, if it did not melt it, raise its temperature 140 degrees. This
quantity of heat is therefore absorbed when the ice, by being converted into
water, is made to contain one-ninth more caloric than it did before.
Therefore 140 degrees is a ninth part of the heat contained in ice at 30
degrees; and the point of zero, or the absolute privation of heat, must
consequently be 1260 degrees below 32 degrees.
This mode of investigating so curious a question is ingenious, but its
correctness is not yet established by similar calculations for other bodies.
The points of absolute cold, indicated by this method in various bodies, are
very remote from each other; it is however possible, that this may arise
from some imperfection in the experiments.
CAROLINE.

It is indeed very ingenious—but we must now attend to our present
experiment. The water begins to boil, and the thermometer is again
stationary.
MRS. B.

Well, Caroline, it is your turn to explain the phenomenon.
CAROLINE.

It is wonderfully curious! The caloric is now busy in changing the water
into steam, in which it hides itself, and becomes insensible. This is another
example of latent heat, producing a change of form. At first it converted a
solid body into a liquid, and now it turns the liquid into vapour!
MRS. B.

You see, my dear, how easily you have become acquainted with these
modifications of insensible heat, which at first appeared so unintelligible. If,
now, we were to reverse these changes, and condense the vapour into water,
and the water into ice, the latent heat would re-appear entirely, in the form
of free caloric.
EMILY.

Pray do let us see the effect of latent heat returning to its free state.
MRS. B.

For the purpose of showing this, we need simply conduct the vapour
through this tube into this vessel of cold water, where it will part with its
latent heat and return to its liquid form.
EMILY.

How rapidly the steam heats the water!
MRS. B.

That is because it does not merely impart its free caloric to the water, but
likewise its latent heat. This method of heating liquids, has been turned to
advantage, in several economical establishments. The steam-kitchens,
which are getting into such general use, are upon the same principle. The
steam is conveyed through a pipe in a similar manner, into the several

vessels which contain the provisions to be dressed, where it communicates
to them its latent caloric, and returns to the state of water. Count Rumford
makes great use of this principle in many of his fire-places: his grand
maxim is to avoid all unnecessary waste of caloric, for which purpose he
confines the heat in such a manner, that not a particle of it shall
unnecessarily escape; and while he economises the free caloric, he takes
care also to turn the latent heat to advantage. It is thus that he is enabled to
produce a degree of heat superior to that which is obtained in common fire-
places, though he employs less fuel.
EMILY.

When the advantages of such contrivances are so clear and plain, I cannot
understand why they are not universally used.
MRS. B.

A long time is always required before innovations, however useful, can be
reconciled with the prejudices of the vulgar.
EMILY.

What a pity it is that there should be a prejudice against new inventions;
how much more rapidly the world would improve, if such useful
discoveries were immediately and universally adopted!
MRS. B.

I believe, my dear, that there are as many novelties attempted to be
introduced, the adoption of which would be prejudicial to society, as there
are of those which would be beneficial to it. The well-informed, though by
no means exempt from error, have an unquestionable advantage over the
illiterate, in judging what is likely or not to prove serviceable; and therefore
we find the former more ready to adopt such discoveries as promise to be
really advantageous, than the latter, who having no other test of the value of
a novelty but time and experience, at first oppose its introduction. The well-
informed, however, are frequently disappointed in their most sanguine
expectations, and the prejudices of the vulgar, though they often retard the
progress of knowledge, yet sometimes, it must be admitted, prevent the
propagation of error.—But we are deviating from our subject.

We have converted steam into water, and are now to change water into ice,
in order to render the latent heat sensible, as it escapes from the water on its
becoming solid. For this purpose we must produce a degree of cold that will
make water freeze.
CAROLINE.

That must be very difficult to accomplish in this warm room.
MRS. B.

Not so much as you think. There are certain chemical mixtures which
produce a rapid change from the solid to the fluid state, or the reverse, in
the substances combined, in consequence of which change latent heat is
either extricated or absorbed.
EMILY.

I do not quite understand you.
MRS. B.

This snow and salt, which you see me mix together, are melting rapidly;
heat, therefore, must be absorbed by the mixture, and cold produced.
CAROLINE.

It feels even colder than ice, and yet the snow is melted. This is very
extraordinary.
MRS. B.

The cause of the intense cold of the mixture is to be attributed to the change
from a solid to a fluid state. The union of the snow and salt produces a new
arrangement of their particles, in consequence of which they become liquid;
and the quantity of caloric, required to effect this change, is seized upon by
the mixture wherever it can be obtained. This eagerness of the mixture for
caloric, during its liquefaction, is such, that it converts part of its own free
caloric into latent heat, and it is thus that its temperature is lowered.
EMILY.

Whatever you put in this mixture, therefore, would freeze?
MRS. B.

Yes; at least any fluid that is susceptible of freezing at that temperature.
I have prepared this mixture of salt and snow for the purpose of freezing the
water from which you are desirous of seeing the latent heat escape. I have
put a thermometer in the glass of water that is to be frozen, in order that you
may see how it cools.
CAROLINE.

The thermometer descends, but the heat which the water is now losing, is its
free, not its latent heat.
MRS. B.

Certainly; it does not part with its latent heat till it changes its state and is
converted into ice.
EMILY.

But here is a very extraordinary circumstance! The thermometer is fallen
below the freezing point, and yet the water is not frozen.
MRS. B.

That is always the case previous to the freezing of water when it is in a state
of rest. Now it begins to congeal, and you may observe that the
thermometer again rises to the freezing point.
CAROLINE.

It appears to me very strange that the thermometer should rise the very
moment that the water freezes; for it seems to imply that the water was
colder before it froze than when in the act of freezing.
MRS. B.

It is so; and after our long dissertation on this circumstance, I did not think
it would appear so surprising to you. Reflect a little, and I think you will
discover the reason of it.
CAROLINE.

It must be, no doubt, the extrications of latent heat, at the instant the water
freezes, that raises the temperature.
MRS. B.

Certainly; and if you now examine the thermometer, you will find that its
rise was but temporary, and lasted only during the disengagement of the
latent heat—now that all the water is frozen it falls again, and will continue
to fall till the ice and mixture are of an equal temperature.
EMILY.

And can you show us any experiments in which liquids, by being mixed,
become solid, and disengage latent heat?
MRS. B.

I could show you several; but you are not yet sufficiently advanced to
understand them well. I shall, however, try one, which will afford you a
striking instance of the fact. The fluid which you see in this phial consists of
a quantity of a certain salt called muriat of lime, dissolved in water. Now, if
I pour into it a few drops of this other fluid, called sulphuric acid, the
whole, or very nearly the whole, will be instantaneously converted into a
solid mass.
EMILY.

How white it turns! I feel the latent heat escaping, for the bottle is warm,
and the fluid is changed to a solid white substance like chalk!
CAROLINE.

This is, indeed, the most curious experiment we have seen yet. But pray
what is that white vapour that ascends from the mixture?
MRS. B.

You are not yet enough of a chemist to understand that.—But take care,
Caroline, do not approach too near it, for it has a very pungent smell.
I shall show you another instance similar to that of the water, which you
observed to become warmer as it froze. I have in this phial a solution of a
salt called sulphat of soda or Glauber’s salt, made very strong, and corked
up when it was hot, and kept without agitation till it became cold, as you
may feel the phial is. Now when I take out the cork and let the air fall upon
it, (for being closed when boiling, there was a vacuum in the upper part)
observe that the salt will suddenly crystallize. . . .

CAROLINE.

Surprising! how beautifully the needles of salt have shot through the whole
phial!
MRS. B.

Yes, it is very striking—but pray do not forget the object of the experiment.
Feel how warm the phial has become by the conversion of part of the liquid
into a solid.
EMILY.

Quite warm I declare! this is a most curious experiment of the
disengagement of latent heat.
MRS. B.

The slakeing of lime is another remarkable instance of the extrication of
latent heat. Have you never observed how quick-lime smokes when water is
poured upon it, and how much heat it produces?
CAROLINE.

Yes; but I do not understand what change of state takes place in the lime
that occasions its giving out latent heat; for the quick-lime, which is solid, is
(if I recollect right) reduced to powder, by this operation, and is, therefore,
rather expanded than condensed.
MRS. B.

It is from the water, not the lime, that the latent heat is set free. The water
incorporates with, and becomes solid in the lime; in consequence of which,
the heat, which kept it in a liquid state, is disengaged, and escapes in a
sensible form.
CAROLINE.

I always thought that the heat originated in the lime. It seems very strange
that water, and cold water too, should contain so much heat.
EMILY.

After this extrication of caloric, the water must exist in a state of ice in the
lime, since it parts with the heat which kept it liquid.

MRS. B.

It cannot properly be called ice, since ice implies a degree of cold, at least
equal to the freezing point. Yet as water, in combining with lime, gives out
more heat than in freezing, it must be in a state of still greater solidity in the
lime, than it is in the form of ice; and you may have observed that it does
not moisten or liquefy the lime in the smallest degree.
EMILY.

But, Mrs. B., the smoke that rises is white; if it was only pure caloric which
escaped, we might feel, but could not see it.
MRS. B.

This white vapour is formed by some of the particles of lime, in a state of
fine dust, which are carried off by the caloric.
EMILY.

In all changes of state, then, a body either absorbs or disengages latent heat?
MRS. B.

You cannot exactly say absorbs latent heat, as the heat becomes latent only
on being confined in the body; but you may say, generally, that bodies, in
passing from a solid to a liquid form, or from the liquid state to that of
vapour, absorb heat; and that when the reverse takes place, heat is
disengaged. *
EMILY.

We can now, I think, account for the ether boiling, and the water freezing in
vacuo, at the same temperature. †
MRS. B.

Let me hear how you explain it.
EMILY.

The latent heat, which the water gave out in freezing, was immediately
absorbed by the ether, during its conversion into vapour; and therefore,
from a latent state in one liquid, it passed into a latent state in the other.

MRS. B.

But this only partly accounts for the result of the experiment; it remains to
be explained why the temperature of the ether, while in a state of ebullition,
is brought down to the freezing temperature of the water.—It is because the
ether, during its evaporation, reduces its own temperature, in the same
proportion as that of the water, by converting its free caloric into latent heat:
so that, though one liquid boils, and the other freezes, their temperatures
remain in a state of equilibrium.
EMILY.

But why does not water, as well as ether, reduce its own temperature by
evaporating?
MRS. B.

The fact is that it does, though much less rapidly than ether. Thus, for
instance, you may often have observed, in the heat of summer, how much
any particular spot may be cooled by watering, though the water used for
that purpose be as warm as the air itself. Indeed so much cold may be
produced by the mere evaporation of water, that the inhabitants of India, by
availing themselves of the most favourable circumstances for this process
which their warm climate can afford, namely, the cool of the night, and
situations most exposed to the night breeze, succeed in causing water to
freeze, though the temperature of the air be as high as 60 degrees. The water
is put into shallow earthen trays, so as to expose an extensive surface to the
process of evaporation, and in the morning, the water is found covered with
a thin cake of ice, which is collected in sufficient quantity to be used for
purposes of luxury.
CAROLINE.

How delicious it must be to drink liquids so cold in those tropical climates!
But, Mrs. B., could we not try that experiment?
MRS. B.

If we were in the country, I have no doubt but that we should be able to
freeze water, by the same means, and under similar circumstances. But we
can do it immediately, upon a small scale, in this very room, in which the

thermometer stands at 70 degrees. For this purpose we need only place
some water in a little cup under the receiver of the air-pump (Plate V.
fig. 1.), and exhaust the air from it. What will be the consequence,
Caroline?
Vol. I. page 138. Plate V.

Fig. 1. The air-pump & receiver for Mr. Leslie’s experiment. C a saucer with sulphuric
Acid. B a glass or earthen cup containing Water. D a stand for the cup with its legs
made of Glass. A a Thermometer.
Larger view (complete Plate)

CAROLINE.

Of course the water will evaporate more quickly, since there will no longer
be any atmospheric pressure on its surface: but will this be sufficient to
make the water freeze?
MRS. B.

Probably not, because the vapour will not be carried off fast enough; but
this will be accomplished without difficulty if we introduce into the receiver
(fig. 1.), in a saucer, or other large shallow vessel, some strong sulphuric
acid, a substance which has a great attraction for water, whether in the form
of vapour, or in the liquid state. This attraction is such that the acid will
instantly absorb the moisture as it rises from the water, so as to make room
for the formation of fresh vapour; this will of course hasten the process, and
the cold produced from the rapid evaporation of the water, will, in a few

minutes, be sufficient to freeze its surface. * We shall now exhaust the air
from the receiver.
EMILY.

Thousands of small bubbles already rise through the water from the internal
surface of the cup; what is the reason of this?
MRS. B.

These are bubbles of air which were partly attached to the vessel, and partly
diffused in the water itself; and they expand and rise in consequence of the
atmospheric pressure being removed.
CAROLINE.

See, Mrs. B.; the thermometer in the cup is sinking fast; it has already
descended to 40 degrees!
EMILY.

The water seems now and then violently agitated on the surface, as if it was
boiling; and yet the thermometer is descending fast!
MRS. B.

You may call it boiling, if you please, for this appearance is, as well as
boiling, owing to the rapid formation of vapour; but here, as you have just
observed, it takes place from the surface, for it is only when heat is applied
to the bottom of the vessel that the vapour is formed there.—Now crystals
of ice are actually shooting all over the surface of the water.
CAROLINE.

How beautiful it is! The surface is now entirely frozen—but the
thermometer remains at 32 degrees.
MRS. B.

And so it will, conformably with our doctrine of latent heat, until the whole
of the water is frozen; but it will then again begin to descend lower and
lower, in consequence of the evaporation which goes on from the surface of
the ice.
EMILY.

This is a most interesting experiment; but it would be still more striking if
no sulphuric acid were required.
MRS. B.

I will show you a freezing instrument, contrived by Dr. Wollaston, upon the
same principle as Mr. Leslie’s experiment, by which water may be frozen
by its own evaporation alone, without the assistance of sulphuric acid.
This tube, which, as you see (Plate V. fig. 2.), is terminated at each
extremity by a bulb, one of which is half full of water, is internally perfectly
exhausted of air; the consequence of this is, that the water in the bulb is
always much disposed to evaporate. This evaporation, however, does not
proceed sufficiently fast to freeze the water; but if the empty ball be cooled
by some artificial means, so as to condense quickly the vapour which rises
from the water, the process may be thus so much promoted as to cause the
water to freeze in the other ball. Dr. Wollaston has called this instrument
Cryophorus.
Vol. I. page 138. Plate V.

Fig. 2. Dr. Wollaston’s Cryophorus.
Fig. 5. Dr. Marcet’s mode of using the Cryophorus.
Fig. 3. & 4. the different parts of Fig. 5. seen separate.
Larger view (complete Plate)

CAROLINE.

So that cold seems to perform here the same part which the sulphuric acid
acted in Mr. Leslie’s experiment?
MRS. B.

Exactly so; but let us try the experiment.
EMILY.

How will you cool the instrument? You have neither ice nor snow.
MRS. B.

True: but we have other means of effecting this. * You recollect what an
intense cold can be produced by the evaporation of ether in an exhausted
receiver. We shall inclose the bulb in this little bag of fine flannel (fig. 3.),
then soke it in ether, and introduce it into the receiver of the air-pump.
(Fig. 5.) For this purpose we shall find it more convenient to use a
cryophorus of this shape (fig. 4.), as its elongated bulb passes easily through
a brass plate which closes the top of the receiver. If we now exhaust the
receiver quickly, you will see, in less than a minute, the water freeze in the
other bulb, out of the receiver.
EMILY.

The bulb already looks quite dim, and small drops of water are condensing
on its surface.
CAROLINE.

And now crystals of ice shoot all over the water. This is, indeed, a very
curious experiment!
MRS. B.

You will see, some other day, that, by a similar method, even quicksilver
may be frozen.—But we cannot at present indulge in any further digression.
Having advanced so far on the subject of heat, I may now give you an
account of the calorimeter, an instrument invented by Lavoisier, upon the
principles just explained, for the purpose of estimating the specific heat of
bodies. It consists of a vessel, the inner surface of which is lined with ice,

so as to form a sort of hollow globe of ice, in the midst of which the body,
whose specific heat is to be ascertained, is placed. The ice absorbs caloric
from this body, till it has brought it down to the freezing point; this caloric
converts into water a certain portion of the ice which runs out through an
aperture at the bottom of the machine; and the quantity of ice changed to
water is a test of the quantity of caloric which the body has given out in
descending from a certain temperature to the freezing point.
CAROLINE.

In this apparatus, I suppose, the milk, chalk, and lead, would melt different
quantities of ice, in proportion to their different capacities for caloric?
MRS. B.

Certainly: and thence we are able to ascertain, with precision, their
respective capacities for heat. But the calorimeter affords us no more idea
of the absolute quantity of heat contained in a body, than the thermometer;
for though by means of it we extricate both the free and combined caloric,
yet we extricate them only to a certain degree, which is the freezing point;
and we know not how much they contain of either below that point.
EMILY.

According to the theory of latent heat, it appears to me that the weather
should be warm when it freezes, and cold in a thaw: for latent heat is
liberated from every substance that it freezes, and such a large supply of
heat must warm the atmosphere; whilst, during a thaw, that very quantity of
free heat must be taken from the atmosphere, and return to a latent state in
the bodies which it thaws.
MRS. B.

Your observation is very natural; but consider that in a frost the atmosphere
is so much colder than the earth, that all the caloric which it takes from the
freezing bodies is insufficient to raise its temperature above the freezing
point; otherwise the frost must cease. But if the quantity of latent heat
extricated does not destroy the frost, it serves to moderate the suddenness of
the change of temperature of the atmosphere, at the commencement both of
frost, and of a thaw. In the first instance, its extrication diminishes the

severity of the cold; and, in the latter, its absorption moderates the warmth
occasioned by a thaw: it even sometimes produces a discernible chill, at the
breaking up of a frost.
CAROLINE.

But what are the general causes that produce those sudden changes in the
weather, especially from hot to cold, which we often experience?
MRS. B.

This question would lead us into meteorological discussions, to which I am
by no means competent. One circumstance, however, we can easily
understand. When the air has passed over cold countries, it will probably
arrive here at a temperature much below our own, and then it must absorb
heat from every object it meets with, which will produce a general fall of
temperature.
CAROLINE.

But pray, now that we know so much of the effects of heat, will you inform
us whether it is really a distinct body, or, as I have heard, a peculiar kind of
motion produced in bodies?
MRS. B.

As I before told you, there is yet much uncertainty as to the nature of these
subtle agents. But I am inclined to consider heat not as mere motion, but as
a separate substance. Late experiments too appear to make it a compound
body, consisting of the two electricities, and in our next conversation I shall
inform you of the principal facts on which that opinion is founded.

* This rule, if not universal, admits of very few exceptions.
† See page 102.
* This experiment was first devised by Mr. Leslie, and has since
been modified in a variety of forms.
* This mode of making the experiment was proposed, and the
particulars detailed, by Dr. Marcet, in the 34th vol. of Nicholson’s

Journal, page 119.

CONVERSATION V.
ON THE CHEMICAL AGENCIES OF ELECTRICITY.

MRS. B.

Before we proceed further it will be necessary to give you some account of
certain properties of electricity, which have of late years been discovered to
have an essential connection with the phenomena of chemistry.
CAROLINE.

It is electricity, if I recollect right, which comes next in our list of simple
substances?
MRS. B.

I have placed electricity in that list, rather from the necessity of classing it
somewhere, than from any conviction that it has a right to that situation, for
we are as yet so ignorant of its intimate nature, that we are unable to
determine, not only whether it is simple or compound, but whether it is in
fact a material agent; or, as Sir H. Davy has hinted, whether it may not be
merely a property inherent in matter. As, however, it is necessary to adopt
some hypothesis for the explanation of the discoveries which this agent has
enabled us to make, I have chosen the opinion, at present most prevalent,
which supposes the existence of two kinds of electricity, distinguished by
the names of positive and negative electricity.
CAROLINE.

Well, I must confess, I do not feel nearly so interested in a science in which
so much uncertainty prevails, as in those which rest upon established
principles; I never was fond of electricity, because, however beautiful and
curious the phenomena it exhibits may be, the theories, by which they were

explained, appeared to me so various, so obscure and inadequate, that I
always remained dissatisfied. I was in hopes that the new discoveries in
electricity had thrown so great a light on the subject, that every thing
respecting it would now have been clearly explained.
MRS. B.

That is a point which we are yet far from having attained. But, in spite of
the imperfection of our theories, you will be amply repaid by the
importance and novelty of the subject. The number of new facts which have
already been ascertained, and the immense prospect of discovery which has
lately been opened to us, will, I hope, ultimately lead to a perfect
elucidation of this branch of natural science; but at present you must be
contented with studying the effects, and in some degree explaining the
phenomena, without aspiring to a precise knowledge of the remote cause of
electricity.
You have already obtained some notions of electricity: in our present
conversation, therefore, I shall confine myself to that part of the science
which is of late discovery, and is more particularly connected with
chemistry.
It was a trifling and accidental circumstance which first gave rise to this
new branch of physical science. Galvani, a professor of natural philosophy
at Bologna, being engaged (about twenty years ago) in some experiments
on muscular irritability, observed, that when a piece of metal was laid on
the nerve of a frog, recently dead, whilst the limb supplied by that nerve
rested upon some other metal, the limb suddenly moved, on a
communication being made between the two pieces of metal.
EMILY.

How is this communication made?
MRS. B.

Either by bringing the two metals into contact, or by connecting them by
means of a metallic conductor. But without subjecting a frog to any cruel
experiments, I can easily make you sensible of this kind of electric action.
Here is a piece of zinc, (one of the metals I mentioned in the list of

elementary bodies)—put it under your tongue, and this piece of silver upon
your tongue, and let both the metals project a little beyond the tip of the
tongue—very well—now make the projecting parts of the metals touch
each other, and you will instantly perceive a peculiar sensation.
EMILY.

Indeed I did, a singular taste, and I think a degree of heat: but I can hardly
describe it.
MRS. B.

The action of these two pieces of metal on the tongue is, I believe, precisely
similar to that made on the nerve of a frog. I shall not detain you by a
detailed account of the theory by which Galvani attempted to account for
this fact, as his explanation was soon overturned by subsequent
experiments, which proved that Galvanism (the name this new power had
obtained) was nothing more than electricity. Galvani supposed that the
virtue of this new agent resided in the nerves of the frog, but Volta, who
prosecuted this subject with much greater success, shewed that the
phenomena did not depend on the organs of the frog, but upon the electrical
agency of the metals, which is excited by the moisture of the animal, the
organs of the frog being only a delicate test of the presence of electric
influence.
CAROLINE.

I suppose, then, the saliva of the mouth answers the same purpose as the
moisture of the frog, in exciting the electricity of the pieces of silver and
zinc with which Emily tried the experiment on her tongue.
MRS. B.

Precisely. It does not appear, however, necessary that the fluid used for this
purpose should be of an animal nature. Water, and acids very much diluted
by water, are found to be the most effectual in promoting the developement
of electricity in metals; and, accordingly, the original apparatus which Volta
first constructed for this purpose, consisted of a pile or succession of plates
of zinc and copper, each pair of which was connected by pieces of cloth or
paper impregnated with water; and this instrument, from its original

inconvenient structure and limited strength, has gradually arrived at its
present state of power and improvement, such as is exhibited in the Voltaic
battery. In this apparatus, a specimen of which you see before you (Plate
VI. fig. 1.), the plates of zinc and copper are soldered together in pairs, each
pair being placed at regular distances in wooden troughs and the interstices
being filled with fluid.
p. 151. Plate VI.

Fig. 1. 2. & 4. Voltaic Batteries
Larger view (complete Plate)

CAROLINE.

Though you will not allow us to enquire into the precise cause of electricity,
may we not ask in what manner the fluid acts on the metals so as to
produce it?
MRS. B.

The action of the fluid on the metals, whether water or acid be used, is
entirely of a chemical nature. But whether electricity is excited by this
chemical action, or whether it is produced by the contact of the two metals,
is a point upon which philosophers do not yet perfectly agree.

EMILY.

But can the mere contact of two metals, without any intervening fluid,
produce electricity?
MRS. B.

Yes, if they are afterwards separated. It is an established fact, that when two
metals are put in contact, and afterwards separated, that which has the
strongest attraction for oxygen exhibits signs of positive, the other of
negative electricity.
CAROLINE.

It seems then but reasonable to infer that the power of the Voltaic battery
should arise from the contact of the plates of zinc and copper.
MRS. B.

It is upon this principle that Volta and Sir H. Davy explain the phenomena
of the pile; but notwithstanding these two great authorities, many
philosophers entertain doubts on the truth of this theory. The principal
difficulty which occurs in explaining the phenomena of the Voltaic battery
on this principle, is, that two such plates show no signs of different states of
electricity whilst in contact, but only on being separated after contact. Now
in the Voltaic battery, those plates that are in contact always continue so,
being soldered together: and they cannot therefore receive a succession of
charges. Besides, if we consider the mere disturbance of the balance of
electricity by the contact of the plates, as the sole cause of the production of
Voltaic electricity, it remains to be explained how this disturbed balance
becomes an inexhaustible source of electrical energy, capable of pouring
forth a constant and copious supply of electrical fluid, though without any
means of replenishing itself from other sources. This subject, it must be
owned, is involved in too much obscurity to enable us to speak very
decidedly in favour of any theory. But, in order to avoid perplexing you
with different explanations, I shall confine myself to one which appears to
me to be least encumbered with difficulties, and most likely to accord with
truth. *
This theory supposes the electricity to be excited by the chemical action of
the acid on the zinc; but you are yet such novices in chemistry, that I think it

will be necessary to give you some previous explanation of the nature of
this action.
All metals have a strong attraction for oxygen, and this element is found in
great abundance both in water and in acids. The action of the diluted acid
on the zinc consists therefore in its oxygen combining with it, and
dissolving its surface.
CAROLINE.

In the same manner I suppose as we saw an acid dissolve copper?
MRS. B.

Yes; but in the Voltaic battery the diluted acid is not strong enough to
produce so complete an effect; it acts only on the surface of the zinc, to
which it yields its oxygen, forming upon it a film or crust, which is a
compound of the oxygen and the metal.
EMILY.

Since there is so strong a chemical attraction between oxygen and metals,
I suppose they are naturally in different states of electricity?
MRS. B.

Yes; it appears that all metals are united with the positive, and that oxygen
is the grand source of the negative electricity.
CAROLINE.

Does not then the acid act on the plates of copper, as well as on those of
zinc?
MRS. B.

No; for though copper has an affinity for oxygen, it is less strong than that
of zinc; and therefore the energy of the acid is only exerted upon the zinc.
It will be best, I believe, in order to render the action of the Voltaic battery
more intelligible, to confine our attention at first to the effect produced on
two plates only. (Plate VI. fig. 2.)
If a plate of zinc be placed opposite to one of copper, or any other metal less
attractive of oxygen, and the space between them (suppose of half an inch

in thickness), be filled with an acid or any fluid capable of oxydating the
zinc, the oxydated surface will have its capacity for electricity diminished,
so that a quantity of electricity will be evolved from that surface. This
electricity will be received by the contiguous fluid, by which it will be
transmitted to the opposite metallic surface, the copper, which is not
oxydated, and is therefore disposed to receive it; so that the copper plate
will thus become positive, whilst the zinc plate will be in the negative state.
This evolution of electrical fluid however will be very limited; for as these
two plates admit of but very little accumulation of electricity, and are
supposed to have no communication with other bodies, the action of the
acid, and further developement of electricity, will be immediately stopped.
EMILY.

This action, I suppose, can no more continue to go on, than that of a
common electrical machine, which is not allowed to communicate with
other bodies?
MRS. B.

Precisely; the common electrical machine, when excited by the friction of
the rubber, gives out both the positive and negative electricities.—(Plate
VI. Fig. 3.) The positive, by the rotation of the glass cylinder, is conveyed
into the conductor, whilst the negative goes into the rubber. But unless there
is a communication made between the rubber and the ground, but a very
inconsiderable quantity of electricity can be excited; for the rubber, like the
plates of the battery, has too small a capacity to admit of an accumulation of
electricity. Unless therefore the electricity can pass out of the rubber, it will
not continue to go into it, and consequently no additional accumulation will
take place. Now as one kind of electricity cannot be given out without the
other, the developement of the positive electricity is stopped as well as that
of the negative, and the conductor therefore cannot receive a succession of
charges.
p. 151. Plate VI.

Fig. 3. A the Cylinder. B the Conductor. R the Rubber. C the Chain.
Larger view (complete Plate)

CAROLINE.

But does not the conductor, as well as the rubber, require a communication
with the earth, in order to get rid of its electricity?
MRS. B.

No; for it is susceptible of receiving and containing a considerable quantity
of electricity, as it is much larger than the rubber, and therefore has a greater
capacity; and this continued accumulation of electricity in the conductor is
what is called a charge.
EMILY.

But when an electrical machine is furnished with two conductors to receive
the two electricities, I suppose no communication with the earth is
required?
MRS. B.

Certainly not, until the two are fully charged; for the two conductors will
receive equal quantities of electricity.
CAROLINE.

I thought the use of the chain had been to convey the electricity from the
ground into the machine?

MRS. B.

That was the idea of Dr. Franklin, who supposed that there was but one kind
of electricity, and who, by the terms positive and negative (which he first
introduced), meant only different quantities of the same kind of electricity.
The chain was in that case supposed to convey electricity from the ground
through the rubber into the conductor. But as we have adopted the
hypothesis of two electricities, we must consider the chain as a vehicle to
conduct the negative electricity into the earth.
EMILY.

And are both kinds of electricity produced whenever electricity is excited?
MRS. B.

Yes, invariably. If you rub a tube of glass with a woollen cloth, the glass
becomes positive, and the cloth negative. If, on the contrary, you excite a
stick of sealing-wax by the same means, it is the rubber which becomes
positive, and the wax negative.
But with regard to the Voltaic battery, in order that the acid may act freely
on the zinc, and the two electricities be given out without interruption, some
method must be devised, by which the plates may part with their
electricities as fast as they receive them.—Can you think of any means by
which this might be effected?
EMILY.

Would not two chains or wires, suspended from either plate to the ground,
conduct the electricities into the earth, and thus answer the purpose?
MRS. B.

It would answer the purpose of carrying off the electricity, I admit; but
recollect, that though it is necessary to find a vent for the electricity, yet we
must not lose it, since it is the power which we are endeavouring to obtain.
Instead, therefore, of conducting it into the ground, let us make the wires,
from either plate, meet: the two electricities will thus be brought together,
and will combine and neutralize each other; and as long as this
communication continues, the two plates having a vent for their respective
electricities, the action of the acid will go on freely and uninterruptedly.

EMILY.

That is very clear, so far as two plates only are concerned; but I cannot say I
understand how the energy of the succession of plates, or rather pairs of
plates, of which the Galvanic trough is composed, is propagated and
accumulated throughout a battery?
MRS. B.

In order to shew you how the intensity of the electricity is increased by
increasing the number of plates, we will examine the action of four plates;
if you understand these, you will readily comprehend that of any number
whatever. In this figure (Plate VI. Fig. 4.), you will observe that the two
central plates are united; they are soldered together, (as we observed in
describing the Voltaic trough,) so as to form but one plate which offers two
different surfaces, the one of copper, the other of zinc.
Now you recollect that, in explaining the action of two plates, we supposed
that a quantity of electricity was evolved from the surface of the first zinc
plate, in consequence of the action of the acid, and was conveyed by the
interposed fluid to the copper plate, No. 2, which thus became positive. This
copper plate communicates its electricity to the contiguous zinc plate,
No. 3, in which, consequently, some accumulation of electricity takes place.
When, therefore, the fluid in the next cell acts upon the zinc plate,
electricity is extricated from it in larger quantity, and in a more concentrated
form, than before. This concentrated electricity is again conveyed by the
fluid to the next pair of plates, No. 4 and 5, when it is farther increased by
the action of the fluid in the third cell, and so on, to any number of plates of
which the battery may consist; so that the electrical energy will continue to
accumulate in proportion to the number of double plates, the first zinc plate
of the series being the most negative, and the last copper plate the most
positive.
CAROLINE.

But does the battery become more and more strongly charged, merely by
being allowed to stand undisturbed?
MRS. B.

No, for the action will soon stop, as was explained before, unless a vent be
given to the accumulated electricities. This is easily done, however, by
establishing a communication by means of the wires (Fig. 1.), between the
two ends of the battery: these being brought into contact, the two
electricities meet and neutralize each other, producing the shock and other
effects of electricity; and the action goes on with renewed energy, being no
longer obstructed by the accumulation of the two electricities which
impeded its progress.
EMILY.

Is it the union of the two electricities which produces the electric spark?
MRS. B.

Yes; and it is, I believe, this circumstance which gave rise to Sir H. Davy’s
opinion that caloric may be a compound of the two electricities.
CAROLINE.

Yet surely caloric is very different from the electrical spark?
MRS. B.

The difference may consist probably only in intensity: for the heat of the
electric spark is considerably more intense, though confined to a very
minute spot, than any heat we can produce by other means.
EMILY.

Is it quite certain that the electricity of the Voltaic battery is precisely of the
same nature as that of the common electrical machine?
MRS. B.

Undoubtedly; the shock given to the human body, the spark, the
circumstance of the same substances which are conductors of the one being
also conductors of the other, and of those bodies, such as glass and sealing-
wax, which are non-conductors of the one, being also non-conductors of the
other, are striking proofs of it. Besides, Sir H. Davy has shewn in his
Lectures, that a Leyden jar, and a common electric battery, can be charged
with electricity obtained from a Voltaic battery, the effect produced being
perfectly similar to that obtained by a common machine.

Dr. Wollaston has likewise proved that similar chemical decompositions are
effected by the electric machine and by the Voltaic battery; and has made
other experiments which render it highly probable, that the origin of both
electricities is essentially the same, as they show that the rubber of the
common electrical machine, like the zinc in the Voltaic battery, produces the
two electricities by combining with oxygen.
CAROLINE.

But I do not see whence the rubber obtains oxygen, for there is neither acid
nor water used in the common machine, and I always understood that the
electricity was excited by the friction.
MRS. B.

It appears that by friction the rubber obtains oxygen from the atmosphere,
which is partly composed of that element. The oxygen combines with the
amalgam of the rubber, which is of a metallic nature, much in the same way
as the oxygen of the acid combines with the zinc in the Voltaic battery, and
it is thus that the two electricities are disengaged.
CAROLINE.

But, if the electricities of both machines are similar, why not use the
common machine for chemical decompositions?
MRS. B.

Though its effects are similar to those of the Voltaic battery, they are
incomparably weaker. Indeed Dr. Wollaston, in using it for chemical
decompositions, was obliged to act upon the most minute quantities of
matter, and though the result was satisfactory in proving the similarity of its
effects to those of the Voltaic battery, these effects were too small in extent
to be in any considerable degree applicable to chemical decomposition.
CAROLINE.

How terrible, then, the shock must be from a Voltaic battery, since it is so
much more powerful than an electrical machine!
MRS. B.

It is not nearly so formidable as you think; at least it is by no means
proportional to the chemical effect. The great superiority of the Voltaic
battery consists in the large quantity of electricity that passes; but in regard
to the rapidity or intensity of the charge, it is greatly surpassed by the
common electrical machine. It would seem that the shock or sensation
depends chiefly upon the intensity; whilst, on the contrary, for chemical
purposes, it is quantity which is required. In the Voltaic battery, the
electricity, though copious, is so weak as not to be able to force its way
through the fluid which separates the plates, whilst that of a common
machine will pass through any space of water.
CAROLINE.

Would not it be possible to increase the intensity of the Voltaic battery till it
should equal that of the common machine?
MRS. B.

It can actually be increased till it imitates a weak electrical machine, so as
to produce a visible spark when accumulated in a Leyden jar. But it can
never be raised sufficiently to pass through any considerable extent of air,
because of the ready communication through the fluids employed.
By increasing the number of plates of a battery, you increase its intensity,
whilst, by enlarging the dimensions of the plates, you augment its quantity;
and, as the superiority of the battery over the common machine consists
entirely in the quantity of electricity produced, it was at first supposed that
it was the size, rather than the number of plates that was essential to the
augmentation of power. It was, however, found upon trial, that the quantity
of electricity produced by the Voltaic battery, even when of a very moderate
size, was sufficiently copious, and that the chief advantage in this apparatus
was obtained by increasing the intensity, which, however, still falls very
short of that of the common machine.
I should not omit to mention, that a very splendid, and, at the same time,
most powerful battery, was, a few years ago, constructed under the direction
of Sir H. Davy, which he repeatedly exhibited in his course of electro-
chemical lectures. It consists of two thousand double plates of zinc and
copper, of six square inches in dimensions, arranged in troughs of

Wedgwood-ware, each of which contains twenty of these plates. The
troughs are furnished with a contrivance for lifting the plates out of them in
a very convenient and expeditious manner. *
CAROLINE.

Well, now that we understand the nature of the action of the Voltaic battery,
I long to hear an account of the discoveries to which it has given rise.
MRS. B.

You must restrain your impatience, my dear, for I cannot with any propriety
introduce the subject of these discoveries till we come to them in the regular
course of our studies. But, as almost every substance in nature has already
been exposed to the influence of the Voltaic battery, we shall very soon
have occasion to notice its effects.

* This mode of explaining the phenomena of the Voltaic pile is
called the chemical theory of electricity, because it ascribes the
cause of these phenomena to certain chemical changes which take
place during their appearance. In the preceding edition of this
work, the same theory was presented in a more elaborate, but less
easy form than it is in this. The mode of viewing the subject
which is here sketched was long since suggested by Dr. Bostock,
of whose theory, however, this is by no means to be considered as
a complete statement.
* A model of this mode of construction is exhibited in Plate XIII.
Fig. 1.

Vol. II. page 16.
Plate XIII.

Fig. 1. Voltaic Battery of improved construction with the Plates out of the Cells.
Larger view (complete Plate)

CONVERSATION VI.
ON OXYGEN AND NITROGEN.

MRS. B.

To-day we shall examine the chemical properties of the atmosphere.
CAROLINE.

I thought that we were first to learn the nature of Oxygen, which come next
in our table of simple bodies?
MRS. B.

And so you shall; the atmosphere being composed of two principles,
Oxygen and Nitrogen, we shall proceed to analyse it, and consider its
component parts separately.
EMILY.

I always thought that the atmosphere had been a very complicated fluid,
composed of all the variety of exhalations from the earth.
MRS. B.

Such substances may be considered rather as heterogeneous and accidental,
than as forming any of its component parts; and the proportion they bear to
the whole mass is quite inconsiderable.
Atmospherical air is composed of two gasses, known by the names of
oxygen gas and nitrogen or azotic gas.

EMILY.

Pray what is a gas?
MRS. B.

The name of gas is given to any fluid capable of existing constantly in an
aeriform state, under the pressure and at the temperature of the atmosphere.
CAROLINE.

Is not water, or any other substance, when evaporated by heat, called gas?
MRS. B.

No, my dear; vapour is, indeed, an elastic fluid, and bears a strong
resemblance to a gas; there are, however, several points in which they
essentially differ, and by which you may always distinguish them. Steam, or
vapour, owes its elasticity merely to a high temperature, which is equal to
that of boiling water. And it differs from boiling water only by being united
with more caloric, which, as we before explained, is in a latent state. When
steam is cooled, it instantly returns to the form of water; but air, or gas, has
never yet been rendered liquid or solid by any degree of cold.
EMILY.

But does not gas, as well as vapour, owe its elasticity to caloric?
MRS. B.

It was the prevailing opinion; and the difference of gas or vapour was
thought to depend on the different manner in which caloric was united with
the basis of these two kinds of elastic fluids. In vapour, it was considered as

in a latent state; in gas, it was said to be chemically combined. But the late
researches of Sir H. Davy have given rise to a new theory respecting gasses;
and there is now reason to believe that these bodies owe their permanently
elastic state, not solely to caloric, but likewise to the prevalence of either
the one or the other of the two electricities.
EMILY.

When you speak, then, of the simple bodies oxygen and nitrogen, you mean
to express those substances which are the basis of the two gasses?
MRS. B.

Yes, in strict propriety, for they can properly be called gasses only when
brought to an aeriform state.
CAROLINE.

In what proportions are they combined in the atmosphere?
MRS. B.

The oxygen gas constitutes a little more than one-fifth, and the nitrogen gas
a little less than four-fifths. When separated, they are found to possess
qualities totally different from each other. For oxygen gas is essential both
to respiration and combustion, while neither of these processes can be
performed in nitrogen gas.
CAROLINE.

But if nitrogen gas is unfit for respiration, how does it happen that the large
proportion of it which enters into the composition of the atmosphere is not a
great impediment to breathing?
MRS. B.

We should breathe more freely than our lungs could bear, if we respired
oxygen gas alone. The nitrogen is no impediment to respiration, and
probably, on the contrary, answers some useful purpose, though we do not
know in what manner it acts in that process.
EMILY.

And by what means can the two gasses, which compose the atmospheric air,
be separated?
MRS. B.

There are many ways of analysing the atmosphere: the two gasses may be
separated first by combustion.
EMILY.

You surprise me! how is it possible that combustion should separate them?
MRS. B.

I should previously remind you that oxygen is supposed to be the only
simple body naturally combined with negative electricity. In all the other
elements the positive electricity prevails, and they have consequently, all of
them, an attraction for oxygen. *
CAROLINE.

Oxygen the only negatively electrified body! that surprises me extremely;
how then are the combinations of the other bodies performed, if, according
to your explanation of chemical attraction, bodies are supposed only to
combine in virtue of their opposite states of electricity?
MRS. B.

Observe that I said, that oxygen was the only simple body, naturally
negative. Compound bodies, in which oxygen prevails over the other
component parts, are also negative, but their negative energy is greater or
less in proportion as the oxygen predominates. Those compounds into
which oxygen enters in less proportion than the other constituents, are
positive, but their positive energy is diminished in proportion to the quantity
of oxygen which enters into their composition.
All bodies, therefore, that are not already combined with oxygen, will
attract it, and, under certain circumstances, will absorb it from the
atmosphere, in which case the nitrogen gas will remain alone, and may thus
be obtained in its separate state.
CAROLINE.

I do not understand how a gas can be absorbed?
MRS. B.

It is only the oxygen, or basis of the gas, which is absorbed; and the two
electricities escaping, that is to say, the negative from the oxygen, the
positive from the burning body, unite and produce caloric.
EMILY.

And what becomes of this caloric?
MRS. B.

We shall make this piece of dry wood attract oxygen from the atmosphere,
and you will see what becomes of the caloric.
CAROLINE.

You are joking, Mrs. B—; you do not mean to decompose the atmosphere
with a piece of dry stick?
MRS. B.

Not the whole body of the atmosphere, certainly; but if we can make this
piece of wood attract any quantity of oxygen from it, a proportional
quantity of atmospherical air will be decomposed.
CAROLINE.

If wood has so strong an attraction for oxygen, why does it not decompose
the atmosphere spontaneously?
MRS. B.

It is found by experience, that an elevation of temperature is required for
the commencement of the union of the oxygen and the wood.
This elevation of temperature was formerly thought to be necessary, in
order to diminish the cohesive attraction of the wood, and enable the
oxygen to penetrate and combine with it more readily. But since the
introduction of the new theory of chemical combination, another cause has
been assigned, and it is now supposed that the high temperature, by exalting
the electrical energies of bodies, and consequently their force of attraction,
facilitates their combination.

EMILY.

If it is true, that caloric is composed of the two electricities, an elevation of
temperature must necessarily augment the electric energies of bodies.
MRS. B.

I doubt whether that would be a necessary consequence; for, admitting this
composition of caloric, it is only by its being decomposed that electricity
can be produced. Sir H. Davy, however, in his numerous experiments, has
found it to be an almost invariable rule that the electrical energies of bodies
are increased by elevation of temperature.
What means then shall we employ to raise the temperature of the wood, so
as to enable it to attract oxygen from the atmosphere?
CAROLINE.

Holding it near the fire, I should think, would answer the purpose.
MRS. B.

It may, provided you hold it sufficiently close to the fire; for a very
considerable elevation of temperature is required.
CAROLINE.

It has actually taken fire, and yet I did not let it touch the coals, but I held it
so very close that I suppose it caught fire merely from the intensity of the
heat.
MRS. B.

Or you might say, in other words, that the caloric which the wood imbibed,
so much elevated its temperature, and exalted its electric energy, as to
enable it to attract oxygen very rapidly from the atmosphere.
EMILY.

Does the wood absorb oxygen while it is burning?
MRS. B.

Yes, and the heat and light are produced by the union of the two electricities
which are set at liberty, in consequence of the oxygen combining with the

wood.
CAROLINE.

You astonish me! the heat of a burning body proceeds then as much from
the atmosphere as from the body itself?
MRS. B.

It was supposed that the caloric, given out during combustion, proceeded
entirely, or nearly so, from the decomposition of the oxygen gas; but,
according to Sir H. Davy’s new view of the subject, both the oxygen gas,
and the combustible body, concur in supplying the heat and light, by the
union of their opposite electricities.
EMILY.

I have not yet met with any thing in chemistry that has surprised or
delighted me so much as this explanation of combustion. I was at first
wondering what connection there could be between the affinity of a body
for oxygen and its combustibility; but I think I understand it now perfectly.
MRS. B.

Combustion then, you see, is nothing more than the rapid combination of a
body with oxygen, attended by the disengagement of light and heat.
EMILY.

But are there no combustible bodies whose attraction for oxygen is so
strong, that they will combine with it, without the application of heat?
CAROLINE.

That cannot be; otherwise we should see bodies burning spontaneously.
MRS. B.

But there are some instances of this kind, such as phosphorus, potassium,
and some compound bodies, which I shall hereafter make you acquainted
with. These bodies, however, are prepared by art, for in general, all the
combustions that could occur spontaneously, at the temperature of the
atmosphere, have already taken place; therefore new combustions cannot

happen without the temperature of the body being raised. Some bodies,
however, will burn at a much lower temperature than others.
CAROLINE.

But the common way of burning a body is not merely to approach it to one
already on fire, but rather to put the one in actual contact with the other, as
when I burn this piece of paper by holding it in the flame of the fire.
MRS. B.

The closer it is in contact with the source of caloric, the sooner will its
temperature be raised to the degree necessary for it to burn. If you hold it
near the fire, the same effect will be produced; but more time will be
required, as you found to be the case with the piece of stick.
EMILY.

But why is it not necessary to continue applying caloric throughout the
process of combustion, in order to keep up the electric energy of the wood,
which is required to enable it to combine with the oxygen?
MRS. B.

The caloric which is gradually produced by the two electricities during
combustion, keeps up the temperature of the burning body; so that when
once combustion has begun, no further application of caloric is required.
CAROLINE.

Since I have learnt this wonderful theory of combustion, I cannot take my
eyes from the fire; and I can scarcely conceive that the heat and light, which
I always supposed to proceed entirely from the coals, are really produced as
much by the atmosphere.
EMILY.

When you blow the fire, you increase the combustion, I suppose, by
supplying the coals with a greater quantity of oxygen gas?
MRS. B.

Certainly; but of course no blowing will produce combustion, unless the
temperature of the coals be first raised. A single spark, however, is

sometimes sufficient to produce that effect; for, as I said before, when once
combustion has commenced, the caloric disengaged is sufficient to elevate
the temperature of the rest of the body, provided that there be a free access
of oxygen. It however sometimes happens that if a fire be ill made, it will
be extinguished before all the fuel is consumed, from the very circumstance
of the combustion being so slow that the caloric disengaged is insufficient
to keep up the temperature of the fuel. You must recollect that there are
three things required in order to produce combustion; a combustible body,
oxygen, and a temperature at which the one will combine with the other.
EMILY.

You said that combustion was one method of decomposing the atmosphere,
and obtaining the nitrogen gas in its simple state; but how do you secure
this gas, and prevent it from mixing with the rest of the atmosphere?
MRS. B.

It is necessary for this purpose to burn the body within a close vessel, which
is easily done.—We shall introduce a small lighted taper (Plate VII. Fig. 1.)
under this glass receiver, which stands in a bason over water, to prevent all
communication with the external air.
Vol. I. p. 181. Plate VII.

Fig. 1. Combustion of a taper under a receiver.
Fig. 2. A Retort on a stand.
Fig. 3. A Furnace. B Earthen Retort in the furnace. C Water bath. D Receiver.
E.E Tube conveying the gas from the Retort through the water into the Receiver.

F.F.F Shelf perforated on which the Receiver stands.
Fig. 4. Combustion of iron wire in oxygen gas.
Larger view

CAROLINE.

How dim the light burns already!—It is now extinguished.
MRS. B.

Can you tell us why it is extinguished?
CAROLINE.

Let me consider.—The receiver was full of atmospherical air; the taper, in
burning within it, must have combined with the oxygen contained in that
air, and the caloric that was disengaged produced the light of the taper. But
when the whole of the oxygen was absorbed, the whole of its electricity was
disengaged; consequently no more caloric could be produced, the taper
ceased to burn, and the flame was extinguished.
MRS. B.

Your explanation is perfectly correct.
EMILY.

The two constituents of the oxygen gas being thus disposed of, what
remains under the receiver must be pure nitrogen gas?
MRS. B.

There are some circumstances which prevent the nitrogen gas, thus
obtained, from being perfectly pure; but we may easily try whether the
oxygen has disappeared, by putting another lighted taper under it.—You see
how instantaneously the flame is extinguished, for want of oxygen to
supply the negative electricity required for the formation of caloric; and
were you to put an animal under the receiver, it would immediately be
suffocated. But that is an experiment which I do not think your curiosity
will tempt you to try.
EMILY.

Certainly not.—But look, Mrs. B., the receiver is full of a thick white
smoke. Is that nitrogen gas?
MRS. B.

No, my dear; nitrogen gas is perfectly transparent and invisible, like
common air. This cloudiness proceeds from a variety of exhalations, which
arise from the burning taper, and the nature of which you cannot yet
understand.
CAROLINE.

The water within the receiver has now risen a little above its level in the
bason. What is the reason of this?
MRS. B.

With a moment’s reflection, I dare say, you would have explained it
yourself. The water rises in consequence of the oxygen gas within it having
been destroyed, or rather decomposed, by the combustion of the taper.
CAROLINE.

Then why did not the water rise immediately when the oxygen gas was
destroyed?
MRS. B.

Because the heat of the taper, whilst burning, produced a dilatation of the
air in the vessel, which at first counteracted this effect.
Another means of decomposing the atmosphere is the oxygenation of
certain metals. This process is very analogous to combustion; it is, indeed,
only a more general term to express the combination of a body with
oxygen.
CAROLINE.

In what respect, then, does it differ from combustion?
MRS. B.

The combination of oxygen in combustion is always accompanied by a
disengagement of light and heat; whilst this circumstance is not a necessary
consequence of simple oxygenation.

CAROLINE.

But how can a body absorb oxygen without the combination of the two
electricities which produce caloric?
MRS. B.

Oxygen does not always present itself in a gaseous state; it is a constituent
part of a vast number of bodies, both solid and liquid, in which it exists in a
much denser state than in the atmosphere; and from these bodies it may be
obtained without much disengagement of caloric. It may likewise, in some
cases, be absorbed from the atmosphere without any sensible production of
light and heat; for, if the process be slow, the caloric is disengaged in such
small quantities, and so gradually, that it is not capable of producing either
light or heat. In this case the absorption of oxygen is called oxygenation or
oxydation, instead of combustion, as the production of sensible light and
heat is essential to the latter.
EMILY.

I wonder that metals can unite with oxygen; for, as they are so dense, their
attraction of aggregation must be very great; and I should have thought that
oxygen could never have penetrated such bodies.
MRS. B.

Their strong attraction for oxygen counterbalances this obstacle. Most
metals, however, require to be made red-hot before they are capable of
attracting oxygen in any considerable quantity. By this combination they
lose most of their metallic properties, and fall into a kind of powder,
formerly called calx, but now much more properly termed an oxyd; thus we
have oxyd of lead, oxyd of iron, &c.
EMILY.

And in the Voltaic battery, it is, I suppose, an oxyd of zinc, that is formed by
the union of the oxygen with that metal?
MRS. B.

Yes, it is.
CAROLINE.

The word oxyd, then, simply means a metal combined with oxygen?
MRS. B.

Yes; but the term is not confined to metals, though chiefly applied to them.
Any body whatever, that has combined with a certain quantity of oxygen,
either by means of oxydation or combustion, is called an oxyd, and is said
to be oxydated or oxygenated.
EMILY.

Metals, when converted into oxyds, become, I suppose, negative?
MRS. B.

Not in general; because in most oxyds the positive energy of the metal more
than counterbalances the native energy of the oxygen with which it
combines.
This black powder is an oxyd of manganese, a metal which has so strong an
affinity for oxygen, that it attracts that substance from the atmosphere at
any known temperature: it is therefore never found in its metallic form, but
always in that of an oxyd, in which state, you see, it has very little of the
appearance of a metal. It is now heavier than it was before oxydation, in
consequence of the additional weight of the oxygen with which it has
combined.
CAROLINE.

I am very glad to hear that; for I confess I could not help having some
doubts whether oxygen was really a substance, as it is not to be obtained in
a simple and palpable state; but its weight is, I think, a decisive proof of its
being a real body.
MRS. B.

It is easy to estimate its weight, by separating it from the manganese, and
finding how much the latter has lost.
EMILY.

But if you can take the oxygen from the metal, shall we not then have it in
its palpable simple state?

MRS. B.

No; for I can only separate the oxygen from the manganese, by presenting
to it some other body, for which it has a greater affinity than for the
manganese. Caloric affording the two electricities is decomposed, and one
of them uniting with the oxygen, restores it to the aëriform state.
EMILY.

But you said just now, that manganese would attract oxygen from the
atmosphere in which it is combined with the negative electricity; how,
therefore, can the oxygen have a superior affinity for that electricity, since it
abandons it to combine with the manganese?
MRS. B.

I give you credit for this objection, Emily; and the only answer I can make
to it is, that the mutual affinities of metals for oxygen, and of oxygen for
electricity, vary at different temperatures; a certain degree of heat will,
therefore, dispose a metal to combine with oxygen, whilst, on the contrary,
the former will be compelled to part with the latter, when the temperature is
further increased. I have put some oxyd of manganese into a retort, which is
an earthen vessel with a bent neck, such as you see here. (Plate VII. Fig. 2.)
—The retort containing the manganese you cannot see, as I have enclosed it
in this furnace, where it is now red-hot. But, in order to make you sensible
of the escape of the gas, which is itself invisible, I have connected the neck
of the retort with this bent tube, the extremity of which is immersed in this
vessel of water. (Plate VII. Fig. 3.)—Do you see the bubbles of air rise
through the water?
CAROLINE.

Perfectly. This, then, is pure oxygen gas; what a pity it should be lost!
Could you not preserve it?
MRS. B.

We shall collect it in this receiver.—For this purpose, you observe, I first fill
it with water, in order to exclude the atmospherical air; and then place it
over the bubbles that issue from the retort, so as to make them rise through
the water to the upper part of the receiver.

EMILY.

The bubbles of oxygen gas rise, I suppose, from their specific levity?
MRS. B.

Yes; for though oxygen forms rather a heavy gas, it is light compared to
water. You see how it gradually displaces the water from the receiver. It is
now full of gas, and I may leave it inverted in water on this shelf, where I
can keep the gas as long as I choose, for future experiments. This apparatus
(which is indispensable in all experiments in which gases are concerned) is
called a water-bath.
CAROLINE.

It is a very clever contrivance, indeed; equally simple and useful. How
convenient the shelf is for the receiver to rest upon under water, and the
holes in it for the gas to pass into the receiver! I long to make some
experiments with this apparatus.
MRS. B.

I shall try your skill that way, when you have a little more experience. I am
now going to show you an experiment, which proves, in a very striking
manner, how essential oxygen is to combustion. You will see that iron itself
will burn in this gas, in the most rapid and brilliant manner.
CAROLINE.

Really! I did not know that it was possible to burn iron.
EMILY.

Iron is a simple body, and you know, Caroline, that all simple bodies are
naturally positive, and therefore must have an affinity for oxygen.
MRS. B.

Iron will, however, not burn in atmospherical air without a very great
elevation of temperature; but it is eminently combustible in pure oxygen
gas; and what will surprise you still more, it can be set on fire without any
considerable rise of temperature. You see this spiral iron wire—I fasten it at
one end to this cork, which is made to fit an opening at the top of the glass-
receiver. (Plate VII. Fig. 4.)

EMILY.

I see the opening in the receiver; but it is carefully closed by a ground glass-
stopper.
MRS. B.

That is in order to prevent the gas from escaping; but I shall take out the
stopper, and put in the cork, to which the wire hangs.—Now I mean to burn
this wire in the oxygen gas, but I must fix a small piece of lighted tinder to
the extremity of it, in order to give the first impulse to combustion; for,
however powerful oxygen is in promoting combustion, you must recollect
that it cannot take place without some elevation of temperature. I shall now
introduce the wire into the receiver, by quickly changing the stoppers.
CAROLINE.

Is there no danger of the gas escaping while you change the stoppers?
MRS. B.

Oxygen gas is a little heavier than atmospherical air, therefore it will not
mix with it very rapidly; and, if I do not leave the opening uncovered, we
shall not lose any——
CAROLINE.

Oh, what a brilliant and beautiful flame!
EMILY.

It is as white and dazzling as the sun!—Now a piece of the melted wire
drops to the bottom: I fear it is extinguished; but no, it burns again as bright
as ever.
MRS. B.

It will burn till the wire is entirely consumed, provided the oxygen is not
first expended: for you know it can burn only while there is oxygen to
combine with it.
CAROLINE.

I never saw a more beautiful light. My eyes can hardly bear it! How
astonishing to think that all this caloric was contained in the small quantity

of gas and iron that was enclosed in the receiver; and that, without
producing any sensible heat!
CAROLINE.

How wonderfully quick combustion goes on in pure oxygen gas! But pray,
are these drops of burnt iron as heavy as the wire was before?
MRS. B.

They are even heavier; for the iron, in burning, has acquired exactly the
weight of the oxygen which has disappeared, and is now combined with it.
It has become an oxyd of iron.
CAROLINE.

I do not know what you mean by saying that the oxygen has disappeared,
Mrs. B., for it was always invisible.
MRS. B.

True, my dear; the expression was incorrect. But though you could not see
the oxygen gas, I believe you had no doubt of its presence, as the effect it
produced on the wire was sufficiently evident.
CAROLINE.

Yes, indeed; yet you know it was the caloric, and not the oxygen gas itself,
that dazzled us so much.
MRS. B.

You are not quite correct in your turn, in saying the caloric dazzled you; for
caloric is invisible; it affects only the sense of feeling; it was the light which
dazzled you.
CAROLINE.

True; but light and caloric are such constant companions, that it is difficult
to separate them, even in idea.
MRS. B.

The easier it is to confound them, the more careful you should be in making
the distinction.

CAROLINE.

But why has the water now risen, and filled part of the receiver?
MRS. B.

Indeed, Caroline, I did not suppose you would have asked such a question!
I dare say, Emily, you can answer it.
EMILY.

Let me reflect . . . . . . The oxygen has combined with the wire; the caloric
has escaped; consequently nothing can remain in the receiver, and the water
will rise to fill the vacuum.
CAROLINE.

I wonder that I did not think of that. I wish that we had weighed the wire
and the oxygen gas before combustion; we might then have found whether
the weight of the oxyd was equal to that of both.
MRS. B.

You might try the experiment if you particularly wished it; but I can assure
you, that, if accurately performed, it never fails to show that the additional
weight of the oxyd is precisely equal to that of the oxygen absorbed,
whether the process has been a real combustion, or a simple oxygenation.
CAROLINE.

But this cannot be the case with combustions in general; for when any
substance is burnt in the common air, so far from increasing in weight, it is
evidently diminished, and sometimes entirely consumed.
MRS. B.

But what do you mean by the expression consumed? You cannot suppose
that the smallest particle of any substance in nature can be actually
destroyed. A compound body is decomposed by combustion; some of its
constituent parts fly off in a gaseous form, while others remain in a concrete
state; the former are called the volatile, the latter the fixed products of
combustion. But if we collect the whole of them, we shall always find that
they exceed the weight of the combustible body, by that of the oxygen
which has combined with them during combustion.

EMILY.

In the combustion of a coal fire, then, I suppose that the ashes are what
would be called the fixed product, and the smoke the volatile product?
MRS. B.

Yet when the fire burns best, and the quantity of volatile products should be
the greatest, there is no smoke; how can you account for that?
EMILY.

Indeed I cannot; therefore I suppose that I was not right in my conjecture.
MRS. B.

Not quite: ashes, as you supposed, are a fixed product of combustion; but
smoke, properly speaking, is not one of the volatile products, as it consists
of some minute undecomposed particles of the coals that are carried off by
the heated air without being burnt, and are either deposited in the form of
soot, or dispersed by the wind. Smoke, therefore, ultimately, becomes one
of the fixed products of combustion. And you may easily conceive that the
stronger the fire is, the less smoke is produced, because the fewer particles
escape combustion. On this principle depends the invention of Argand’s
Patent Lamps; a current of air is made to pass through the cylindrical wick
of the lamp, by which means it is so plentifully supplied with oxygen, that
scarcely a particle of oil escapes combustion, nor is there any smoke
produced.
EMILY.

But what then are the volatile products of combustion?
MRS. B.

Various new compounds, with which you are not yet acquainted, and which
being converted by caloric either into vapour or gas, are invisible; but they
can be collected, and we shall examine them at some future period.
CAROLINE.

There are then other gases, besides the oxygen and nitrogen gases.
MRS. B.

Yes, several: any substance that can assume and maintain the form of an
elastic fluid at the temperature of the atmosphere, is called a gas. We shall
examine the several gases in their respective places; but we must now
confine our attention to those that compose the atmosphere.
I shall show you another method of decomposing the atmosphere, which is
very simple. In breathing, we retain a portion of the oxygen, and expire the
nitrogen gas; so that if we breathe in a closed vessel, for a certain length of
time, the air within it will be deprived of its oxygen gas. Which of you will
make the experiment?
CAROLINE.

I should be very glad to try it.
MRS. B.

Very well; breathe several times through this glass tube into the receiver
with which it is connected, until you feel that your breath is exhausted.
CAROLINE.

I am quite out of breath already!
MRS. B.

Now let us try the gas with a lighted taper.
EMILY.

It is very pure nitrogen gas, for the taper is immediately extinguished.
MRS. B.

That is not a proof of its being pure, but only of the absence of oxygen, as it
is that principle alone which can produce combustion, every other gas being
absolutely incapable of it.
EMILY.

In the methods which you have shown us, for decomposing the atmosphere,
the oxygen always abandons the nitrogen; but is there no way of taking the
nitrogen from the oxygen, so as to obtain the latter pure from the
atmosphere?

MRS. B.

You must observe, that whenever oxygen is taken from the atmosphere, it is
by decomposing the oxygen gas; we cannot do the same with the nitrogen
gas, because nitrogen has a stronger affinity for caloric than for any other
known principle: it appears impossible therefore to separate it from the
atmosphere by the power of affinities. But if we cannot obtain the oxygen
gas, by this means, in its separate state, we have no difficulty (as you have
seen) to procure it in its gaseous form, by taking it from those substances
that have absorbed it from the atmosphere, as we did with the oxyd of
manganese.
EMILY.

Can atmospherical air be recomposed, by mixing due proportions of oxygen
and nitrogen gases?
MRS. B.

Yes: if about one part of oxygen gas be mixed with about four parts of
nitrogen gas, atmospherical air is produced. *
EMILY.

The air, then, must be an oxyd of nitrogen?
MRS. B.

No, my dear; for there must be a chemical combination between oxygen
and nitrogen in order to produce an oxyd; whilst in the atmosphere these
two substances are separately combined with caloric, forming two distinct
gases, which are simply mixed in the formation of the atmosphere.
I shall say nothing more of oxygen and nitrogen at present, as we shall
continually have occasion to refer to them in our future conversations. They
are both very abundant in nature; nitrogen is the most plentiful in the
atmosphere, and exists also in all animal substances; oxygen forms a
constituent part, both of the animal and vegetable kingdoms, from which it
may be obtained by a variety of chemical means. But it is now time to
conclude our lesson. I am afraid you have learnt more to-day than you will
be able to remember.

CAROLINE.

I assure you that I have been too much interested in it, ever to forget it. In
regard to nitrogen there seems to be but little to remember; it makes a very
insignificant figure in comparison to oxygen, although it composes a much
larger portion of the atmosphere.
MRS. B.

Perhaps this insignificance you complain of may arise from the compound
nature of nitrogen, for though I have hitherto considered it as a simple body,
because it is not known in any natural process to be decomposed, yet from
some experiments of Sir H. Davy, there appears to be reason for suspecting
that nitrogen is a compound body, as we shall see afterwards. But even in
its simple state, it will not appear so insignificant when you are better
acquainted with it; for though it seems to perform but a passive part in the
atmosphere, and has no very striking properties, when considered in its
separate state, yet you will see by-and-bye what a very important agent it
becomes, when combined with other bodies. But no more of this at present;
we must reserve it for its proper place.

* If chlorine or oxymuriatic gas be a simple body, according to
Sir H. Davy’s view of the subject, it must be considered as an
exception to this statement; but this subject cannot be discussed
till the properties and nature of chlorine come under examination.
* The proportion of oxygen in the atmosphere varies from 21 to
22 per cent.

CONVERSATION VII.
ON HYDROGEN.

CAROLINE.

The next simple bodies we come to are chlorine and iodine. Pray what
kinds of substances are these; are they also invisible?
MRS. B.

No; for chlorine, in the state of gas, has a distinct greenish colour, and is
therefore visible; and iodine, in the same state, has a beautiful claret-red
colour. The knowledge of these two bodies, however, and the explanation of
their properties, imply various considerations, which you would not yet be
able to understand; we shall therefore defer their examination to some
future conversation, and we shall pass on to the next simple substance,
Hydrogen, which we cannot, any more than oxygen, obtain in a visible or
palpable form. We are acquainted with it only in its gaseous state, as we are
with oxygen and nitrogen.
CAROLINE.

But in its gaseous state it cannot be called a simple substance, since it is
combined with heat and electricity?
MRS. B.

True, my dear; but as we do not know in nature of any substance which is
not more or less combined with caloric and electricity, we are apt to say that
a substance is in its pure state when combined with those agents only.
Hydrogen was formerly called inflammable air, as it is extremely
combustible, and burns with a great flame. Since the invention of the new
nomenclature, it has obtained the name of hydrogen, which is derived from
two Greek words, the meaning of which is, to produce water.
EMILY.

And how does hydrogen produce water?
MRS. B.

By its combustion. Water is composed of eighty-five parts, by weight, of
oxygen, combined with fifteen parts of hydrogen; or of two parts, by bulk
of hydrogen gas, to one part of oxygen gas.
CAROLINE.

Really! is it possible that water should be a combination of two gases, and
that one of these should be inflammable air! Hydrogen must be a most
extraordinary gas that will produce both fire and water.
EMILY.

But I thought you said that combustion could take place in no gas but
oxygen?
MRS. B.

Do you recollect what the process of combustion consists in?
EMILY.

In the combination of a body with oxygen, with disengagement of light and
heat.
MRS. B.

Therefore when I say that hydrogen is combustible, I mean that it has an
affinity for oxygen; but, like all other combustible substances, it cannot
burn unless supplied with oxygen, and also heated to a proper temperature.
CAROLINE.

The simply mixing fifteen parts of hydrogen, with eighty-five parts of
oxygen gas, will not, therefore, produce water?
MRS. B.

No; water being a much denser fluid than gases, in order to reduce these
gases to a liquid, it is necessary to diminish the quantity of caloric or
electricity which maintains them in an elastic form.
EMILY.

That I should think might be done by combining the oxygen and hydrogen
together; for in combining they would give out their respective electricities
in the form of caloric, and by this means would be condensed.
CAROLINE.

But you forget, Emily, that in order to make the oxygen and hydrogen
combine, you must begin by elevating their temperature, which increases,
instead of diminishing, their electric energies.
MRS. B.

Emily is, however, right; for though it is necessary to raise their
temperature, in order to make them combine, as that combination affords
them the means of parting with their electricities, it is eventually the cause
of the diminution of electric energy.
CAROLINE.

You love to deal in paradoxes to-day, Mrs. B.—Fire, then, produces water?
MRS. B.

The combustion of hydrogen gas certainly does; but you do not seem to
have remembered the theory of combustion so well as you thought you
would. Can you tell me what happens in the combustion of hydrogen gas?
CAROLINE.

The hydrogen combines with the oxygen, and their opposite electricities are
disengaged in the form of caloric.—Yes, I think I understand it now—by
the loss of this caloric, the gases are condensed into a liquid.
EMILY.

Water, then, I suppose, when it evaporates and incorporates with the
atmosphere, is decomposed and converted into hydrogen and oxygen gases?
MRS. B.

No, my dear—there you are quite mistaken: the decomposition of water is
totally different from its evaporation; for in the latter case (as you should
recollect) water is only in a state of very minute division; and is merely
suspended in the atmosphere, without any chemical combination, and
without any separation of its constituent parts. As long as these remain
combined, they form water, whether in a state of liquidity, or in that of an
elastic fluid, as vapour, or under the solid form of ice.
In our experiments on latent heat, you may recollect that we caused water
successively to pass through these three forms, merely by an increase or
diminution of caloric, without employing any power of attraction, or
effecting any decomposition.
CAROLINE.

But are there no means of decomposing water?
MRS. B.

Yes, several: charcoal, and metals, when heated red hot, will attract the
oxygen from water, in the same manner as they will from the atmosphere.
CAROLINE.

Hydrogen, I see, is like nitrogen, a poor dependant friend of oxygen, which
is continually forsaken for greater favourites.
MRS. B.

The connection, or friendship, as you choose to call it, is much more
intimate between oxygen and hydrogen, in the state of water, than between
oxygen and nitrogen, in the atmosphere; for, in the first case, there is a

chemical union and condensation of the two substances; in the latter, they
are simply mixed together in their gaseous state. You will find, however,
that, in some cases, nitrogen is quite as intimately connected with oxygen,
as hydrogen is.—But this is foreign to our present subject.
EMILY.

Water, then, is an oxyd, though the atmospherical air is not?
MRS. B.

It is not commonly called an oxyd, though, according to our definition, it
may, no doubt, be referred to that class of bodies.
CAROLINE.

I should like extremely to see water decomposed.
MRS. B.

I can gratify your curiosity by a much more easy process than the oxydation
of charcoal or metals: the decomposition of water by these latter means
takes up a great deal of time, and is attended with much trouble; for it is
necessary that the charcoal or metal should be made red hot in a furnace,
that the water should pass over them in a state of vapour, that the gas
formed should be collected over the water-bath, &c. In short, it is a very
complicated affair. But the same effect may be produced with the greatest
facility, by the action of the Voltaic battery, which this will give me an
opportunity of exhibiting.
CAROLINE.

I am very glad of that, for I longed to see the power of this apparatus in
decomposing bodies.
MRS. B.

For this purpose I fill this piece of glass-tube (Plate VIII. fig. 1.) with
water, and cork it up at both ends; through one of the corks I introduce that
wire of the battery which conveys the positive electricity; and the wire
which conveys the negative electricity is made to pass through the other
cork, so that the two wires approach each other sufficiently near to give out
their respective electricities.

Vol. I. p. 206
Plate VIII.

Fig. 1. Apparatus for the decomposition of water by the Voltaic Battery.
Larger view (complete Plate)

CAROLINE.

It does not appear to me that you approach the wires so near as you did
when you made the battery act by itself.
MRS. B.

Water being a better conductor of electricity than air, the two wires will act
on each other at a greater distance in the former than in the latter.
EMILY.

Now the electrical effect appears: I see small bubbles of air emitted from
each wire.
MRS. B.

Each wire decomposes the water, the positive by combining with its oxygen
which is negative, the negative by combining with its hydrogen which is
positive.
CAROLINE.

That is wonderfully curious! But what are the small bubbles of air?
MRS. B.

Those that appear to proceed from the positive wire, are the result of the
decomposition of the water by that wire. That is to say, the positive
electricity having combined with some of the oxygen of the water, the
particles of hydrogen which were combined with that portion of oxygen are
set at liberty, and appear in the form of small bubbles of gas or air.
EMILY.

And I suppose the negative fluid having in the same manner combined with
some of the hydrogen of the water, the particles of oxygen that were
combined with it, are set free, and emitted in a gaseous form.
MRS. B.

Precisely so. But I should not forget to observe, that the wires used in this
experiment are made of platina, a metal which is not capable of combining
with oxygen; for otherwise the wire would combine with the oxygen, and
the hydrogen alone would be disengaged.
CAROLINE.

But could not water be decomposed without the electric circle being
completed? If, for instance, you immersed only the positive wire in the
water, would it not combine with the oxygen, and the hydrogen gas be
given out?
MRS. B.

No; for as you may recollect, the battery cannot act unless the circle be
completed; since the positive wire will not give out its electricity, unless
attracted by that of the negative wire.
CAROLINE.

I understand it now.—But look, Mrs. B., the decomposition of the water
which has now been going on for some time, does not sensibly diminish its
quantity—what is the reason of that?
MRS. B.

Because the quantity decomposed is so extremely small. If you compare the
density of water with that of the gases into which it is resolved, you must be
aware that a single drop of water is sufficient to produce thousands of such
small bubbles as those you now perceive.
CAROLINE.

But in this experiment, we obtain the oxygen and hydrogen gases mixed
together. Is there any means of procuring the two gases separately?
MRS. B.

They can be collected separately with great ease, by modifying a little the
experiment. Thus if instead of one tube, we employ two, as you see here,
(c, d, Plate VIII. fig. 2.) both tubes being closed at one end, and open at the
other; and if after filling these tubes with water, we place them standing in a
glass of water (e), with their open end downwards, you will see that the
moment we connect the wires (a, b) which proceed upwards from the
interior of each tube, the one with one end of the battery, and the other with
the other end, the water in the tubes will be decomposed; hydrogen will be
given out round the wire in the tube connected with the positive end of the
battery, and oxygen in the other; and these gases will be evolved, exactly in
the proportions which I have before mentioned, namely, two measures of
hydrogen for one of oxygen. We shall now begin the experiment, but it will
be some time before any sensible quantity of the gases can be collected.
Vol. I. p. 206 Plate VIII.

Fig. 2. Apparatus for decomposing water by Voltaic Electricity & obtaining the gasses
separate.
Larger view (complete Plate)

EMILY.

The decomposition of water in this way, slow as it is, is certainly very
striking; but I confess that I should be still more gratified, if you could shew
it us on a larger scale, and by a quicker process. I am sorry that the
decomposition of water by charcoal or metals is attended with so much
inconvenience.
MRS. B.

Water may be decomposed by means of metals without any difficulty; but
for this purpose the intervention of an acid is required. Thus, if we add

some sulphuric acid (a substance with the nature of which you are not yet
acquainted) to the water which the metal is to decompose, the acid disposes
the metal to combine with the oxygen of the water so readily and
abundantly, that no heat is required to hasten the process. Of this I am going
to shew you an instance. I put into this bottle the water that is to be
decomposed, as also the metal that is to effect that decomposition by
combining with the oxygen, and the acid which is to facilitate the
combination of the metal and the oxygen. You will see with what violence
these will act on each other.
CAROLINE.

But what metal is it that you employ for this purpose?
MRS. B.

It is iron; and it is used in the state of filings, as these present a greater
surface to the acid than a solid piece of metal. For as it is the surface of the
metal which is acted upon by the acid, and is disposed to receive the oxygen
produced by the decomposition of the water, it necessarily follows that the
greater is the surface, the more considerable is the effect. The bubbles
which are now rising are hydrogen gas——
CAROLINE.

How disagreeably it smells!
MRS. B.

It is indeed unpleasant, though, I believe, not particularly hurtful. We shall
not, however, suffer any more to escape, as it will be wanted for
experiments. I shall, therefore, collect it in a glass-receiver, by making it
pass through this bent tube, which will conduct it into the water-bath.
(Plate VIII. fig. 3.)
Vol. I. p. 206 Plate VIII.

Fig. 3. Apparatus for preparing & collecting hydrogen gas.
Fig. 4. Receiver full of hydrogen gas inverted over water.
Fig. 5. Slow combustion of hydrogen gas.
Fig. 6. Apparatus for illustrating the formation of water by the combustion of
hydrogen gas.
Fig. 7. Apparatus for producing harmonic sounds by the combustion of hydrogen gas.
Larger view (complete Plate)

EMILY.

How very rapidly the gas escapes! it is perfectly transparent, and without
any colour whatever.—Now the receiver is full——
MRS. B.

We shall, therefore, remove it, and substitute another in its place. But you
must observe, that when the receiver is full, it is necessary to keep it
inverted with the mouth under water, otherwise the gas would escape. And
in order that it may not be in the way, I introduce within the bath, under the
water, a saucer, into which I slide the receiver, so that it can be taken out of
the bath and conveyed any where, the water in the saucer being equally
effectual in preventing its escape as that in the bath. (Plate VIII. fig. 4.)
EMILY.

I am quite surprised to see what a large quantity of hydrogen gas can be
produced by such a small quantity of water, especially as oxygen is the
principal constituent of water.

MRS. B.

In weight it is; but not in volume. For though the proportion, by weight, is
nearly six parts of oxygen to one of hydrogen, yet the proportion of the
volume of the gases, is about one part of oxygen to two of hydrogen; so
much heavier is the former than the latter.
CAROLINE.

But why is the vessel in which the water is decomposed so hot? As the
water changes from a liquid to a gaseous form, cold should be produced
instead of heat.
MRS. B.

No; for if one of the constituents of water is converted into a gas, the other
becomes solid in combining with the metal.
EMILY.

In this case, then, neither heat nor cold should be produced?
MRS. B.

True: but observe that the sensible heat which is disengaged in this
operation, is not owing to the decomposition of the water, but to an
extrication of heat produced by the mixture of water and sulphuric acid.
I will mix some water and sulphuric acid together in this glass, that you
may feel the surprising quantity of heat that is disengaged by their union—
now take hold of the glass——
CAROLINE.

Indeed I cannot; it feels as hot as boiling water. I should have imagined
there would have been heat enough disengaged to have rendered the liquid
solid.
MRS. B.

As, however, it does not produce that effect, we cannot refer this heat to the
modification called latent heat. We may, however, I think, consider it as
heat of capacity, as the liquid is condensed by its loss; and if you were to
repeat the experiment, in a graduated tube, you would find that the two
liquids, when mixed, occupy considerably less space than they did

separately.—But we will reserve this to another opportunity, and attend at
present to the hydrogen gas which we have been producing.
If I now set the hydrogen gas, which is contained in this receiver, at liberty
all at once, and kindle it as soon as it comes in contact with the atmosphere,
by presenting it to a candle, it will so suddenly and rapidly decompose the
oxygen gas, by combining with its basis, that an explosion, or a detonation
(as chemists commonly call it), will be produced. For this purpose, I need
only take up the receiver, and quickly present its open mouth to the candle
——so . . . .
CAROLINE.

It produced only a sort of hissing noise, with a vivid flash of light. I had
expected a much greater report.
MRS. B.

And so it would have been, had the gases been closely confined at the
moment they were made to explode. If, for instance, we were to put in this
bottle a mixture of hydrogen gas and atmospheric air; and if, after corking
the bottle, we should kindle the mixture by a very small orifice, from the
sudden dilatation of the gases at the moment of their combination, the bottle
must either fly to pieces, or the cork be blown out with considerable
violence.
CAROLINE.

But in the experiment which we have just seen, if you did not kindle the
hydrogen gas, would it not equally combine with the oxygen?
MRS. B.

Certainly not; for, as I have just explained to you, it is necessary that the
oxygen and hydrogen gases be burnt together, in order to combine
chemically and produce water.
CAROLINE.

That is true; but I thought this was a different combination, for I see no
water produced.
MRS. B.

The water resulting from this detonation was so small in quantity, and in
such a state of minute division, as to be invisible. But water certainly was
produced; for oxygen is incapable of combining with hydrogen in any other
proportions than those that form water; therefore water must always be the
result of their combination.
If, instead of bringing the hydrogen gas into sudden contact with the
atmosphere (as we did just now) so as to make the whole of it explode the
moment it is kindled, we allow but a very small surface of gas to burn in
contact with the atmosphere, the combustion goes on quietly and gradually
at the point of contact, without any detonation, because the surfaces brought
together are too small for the immediate union of gases. The experiment is a
very easy one. This phial, with a narrow neck, (Plate VIII. fig. 5.) is full of
hydrogen gas, and is carefully corked. If I take out the cork without moving
the phial, and quickly approach the candle to the orifice, you will see how
different the result will be——
EMILY.

How prettily it burns, with a blue flame! The flame is gradually sinking
within the phial—now it has entirely disappeared. But does not this
combustion likewise produce water?
MRS. B.

Undoubtedly. In order to make the formation of the water sensible to you,
I shall procure a fresh supply of hydrogen gas, by putting into this bottle
(Plate VIII. fig. 6.) iron filings, water, and sulphuric acid, materials similar
to those which we have just used for the same purpose. I shall then cork up
the bottle, leaving only a small orifice in the cork, with a piece of glass-tube
fixed to it, through which the gas will issue in a continued rapid stream.
CAROLINE.

I hear already the hissing of the gas through the tube, and I can feel a strong
current against my hand.
MRS. B.

This current I am going to kindle with the candle—see how vividly it burns
——

EMILY.

It burns like a candle with a long flame. But why does this combustion last
so much longer than in the former experiment?
MRS. B.

The combustion goes on uninterruptedly as long as the new gas continues to
be produced. Now if I invert this receiver over the flame, you will soon
perceive its internal surface covered with a very fine dew, which is pure
water——
CAROLINE.

Yes, indeed; the glass is now quite dim with moisture! How glad I am that
we can see the water produced by this combustion.
EMILY.

It is exactly what I was anxious to see; for I confess I was a little
incredulous.
MRS. B.

If I had not held the glass-bell over the flame, the water would have escaped
in the state of vapour, as it did in the former experiment. We have here, of
course, obtained but a very small quantity of water; but the difficulty of
procuring a proper apparatus, with sufficient quantities of gases, prevents
my showing it you on a larger scale.
The composition of water was discovered about the same period, both by
Mr. Cavendish, in this country, and by the celebrated French chemist
Lavoisier. The latter invented a very perfect and ingenious apparatus to
perform, with great accuracy, and upon a large scale, the formation of water
by the combination of oxygen and hydrogen gases. Two tubes, conveying
due proportions, the one of oxygen, the other of hydrogen gas, are inserted
at opposite sides of a large globe of glass, previously exhausted of air; the
two streams of gas are kindled within the globe, by the electrical spark, at
the point where they come in contact; they burn together, that is to say, the
hydrogen combines with the oxygen, the caloric is set at liberty, and a
quantity of water is produced exactly equal, in weight, to that of the two
gases introduced into the globe.

CAROLINE.

And what was the greatest quantity of water ever formed in this apparatus?
MRS. B.

Several ounces; indeed, very nearly a pound, if I recollect right; but the
operation lasted many days.
EMILY.

This experiment must have convinced all the world of the truth of the
discovery. Pray, if improper proportions of the gases were mixed and set
fire to, what would be the result?
MRS. B.

Water would equally be formed, but there would be a residue of either one
or other of the gases, because, as I have already told you, hydrogen and
oxygen will combine only in the proportions requisite for the formation of
water.
EMILY.

Look, Mrs. B., our experiment with the Voltaic battery (Plate VIII. fig. 2.)
has made great progress; a quantity of gas has been formed in each tube, but
in one of them there is twice as much gas as in the other.
MRS. B.

Yes; because, as I said before, water is composed of two volumes of
hydrogen to one of oxygen—and if we should now mix these gases together
and set fire to them by an electrical spark, both gases would entirely
disappear, and a small quantity of water would be formed.
There is another curious effect produced by the combustion of hydrogen
gas, which I shall show you, though I must acquaint you first, that I cannot
well explain the cause of it. For this purpose, I must put some materials into
our apparatus, in order to obtain a stream of hydrogen gas, just as we have
done before. The process is already going on, and the gas is rushing through
the tube—I shall now kindle it with the taper——
EMILY.

It burns exactly as it did before——What is the curious effect which you
were mentioning?
MRS. B.

Instead of the receiver, by means of which we have just seen the drops of
water form, we shall invert over the flame this piece of tube, which is about
two feet in length, and one inch in diameter (Plate VIII. fig. 7.); but you
must observe that it is open at both ends.
EMILY.

What a strange noise it makes! something like the Æolian harp, but not so
sweet.
CAROLINE.

It is very singular, indeed; but I think rather too powerful to be pleasing.
And is not this sound accounted for?
MRS. B.

That the percussion of glass, by a rapid stream of gas, should produce a
sound, is not extraordinary: but the sound here is so peculiar, that no other
gas has a similar effect. Perhaps it is owing to a brisk vibratory motion of
the glass, occasioned by the successive formation and condensation of small
drops of water on the sides of the glass tube, and the air rushing in to
replace the vacuum formed. *
CAROLINE.

How very much this flame resembles the burning of a candle.
MRS. B.

The burning of a candle is produced by much the same means. A great deal
of hydrogen is contained in candles, whether of tallow or wax. This
hydrogen being converted into gas by the heat of the candle, combines with
the oxygen of the atmosphere, and flame and water result from this
combination. So that, in fact, the flame of a candle is owing to the
combustion of hydrogen gas. An elevation of temperature, such as is
produced by a lighted match or taper, is required to give the first impulse to
the combustion; but afterwards it goes on of itself, because the candle finds

a supply of caloric in the successive quantities of heat which results from
the union of the two electricities given out by the gases during their
combustion. But there are other circumstances connected with the
combustion of candles and lamps, which I cannot explain to you till you are
acquainted with carbon, which is one of their constituent parts. In general,
however, whenever you see flame, you may infer that it is owing to the
formation and burning of hydrogen gas *; for flame is the peculiar mode of
burning hydrogen gas, which, with only one or two apparent exceptions,
does not belong to any other combustible.
EMILY.

You astonish me! I understood that flame was the caloric produced by the
union of the two electricities, in all combustions whatever?
MRS. B.

Your error proceeded from your vague and incorrect idea of flame; you
have confounded it with light and caloric in general. Flame always implies
caloric, since it is produced by the combustion of hydrogen gas; but all
caloric does not imply flame. Many bodies burn with intense heat without
producing flame. Coals, for instance, burn with flame until all the hydrogen
which they contain is evaporated; but when they afterwards become red hot,
much more caloric is disengaged than when they produce flame.
CAROLINE.

But the iron wire, which you burnt in oxygen gas, appeared to me to emit
flame; yet, as it was a simple metal, it could contain no hydrogen?
MRS. B.

It produced a sparkling dazzling blaze of light, but no real flame.
EMILY.

And what is the cause of the regular shape of the flame of a candle?
MRS. B.

The regular stream of hydrogen gas which exhales from its combustible
matter.

CAROLINE.

But the hydrogen gas must, from its great levity, ascend into the upper
regions of the atmosphere; why therefore does not the flame continue to
accompany it?
MRS. B.

The combustion of the hydrogen gas is completed at the point where the
flame terminates; it then ceases to be hydrogen gas, as it is converted by its
combination with oxygen into watery vapour; but in a state of such minute
division as to be invisible.
CAROLINE.

I do not understand what is the use of the wick of a candle, since the
hydrogen gas burns so well without it?
MRS. B.

The combustible matter of the candle must be decomposed in order to emit
the hydrogen gas, and the wick is instrumental in effecting this
decomposition. Its combustion first melts the combustible matter, and . . . .
CAROLINE.

But in lamps the combustible matter is already fluid, and yet they also
require wicks?
MRS. B.

I am going to add that, afterwards, the burning wick (by the power of
capillary attraction) gradually draws up the fluid to the point where
combustion takes place; for you must have observed that the wick does not
burn quite to the bottom.
CAROLINE.

Yes; but I do not understand why it does not.
MRS. B.

Because the air has not so free an access to that part of the wick which is
immediately in contact with the candle, as to the part just above, so that the

heat there is not sufficient to produce its decomposition; the combustion
therefore begins a little above this point.
CAROLINE.

But, Mrs. B., in those beautiful lights, called gas-lights, which are now seen
in many streets, and will, I hope, be soon adopted every where, I can
perceive no wick at all. How are these lights managed?
MRS. B.

I am glad you have put me in mind of saying a few words on this very
useful and interesting improvement. In this mode of lighting, the gas is
conveyed to the extremity of a tube, where it is kindled, and burns as long
as the supply continues. There is, therefore, no occasion for a wick, or any
other fuel whatever.
EMILY.

But how is all this gas procured in such large quantities?
MRS. B.

It is obtained from coal, by distillation.—Coal, when exposed to heat in a
close vessel, is decomposed; and hydrogen, which is one of its constituents,
rises in the state of gas, combined with another of its component parts,
carbon, forming a compound gas, called Hydrocarbonat, the nature of
which we shall again have an opportunity of noticing when we treat of
carbon. This gas, like hydrogen, is perfectly transparent, invisible, and
highly inflammable; and in burning it emits that vivid light which you have
so often observed.
CAROLINE.

And does the process for procuring it require nothing but heating the coals,
and conveying the gas through tubes?
MRS. B.

Nothing else; except that the gas must be made to pass, immediately at its
formation, through two or three large vessels of water, in which it deposits
some other ingredients, and especially water, tar, and oil, which also arise
from the distillation of coals. The gas-light apparatus, therefore, consists

simply in a large iron vessel, in which the coals are exposed to the heat of a
furnace,—some reservoirs of water, in which the gas deposits its impurities,
—and tubes that convey it to the desired spot, being propelled with uniform
velocity through the tubes by means of a certain degree of pressure which is
made upon the reservoir.
EMILY.

What an admirable contrivance! Do you not think, Mrs. B., that it will soon
get into universal use?
MRS. B.

Most probably, as to the lighting of streets, offices, and public places, as it
far surpasses any former invention for that purpose; but as to the interior of
private houses, this mode of lighting has not yet been sufficiently tried to
know whether it will be found generally desirable, either in regard to
economy or convenience. It may, however, be considered as one of the
happiest applications of chemistry to the comforts of life; and there is every
reason to suppose that it will answer the full extent of public, expectation.
I have another experiment to show you with hydrogen gas, which I think
will entertain you. Have you ever blown bubbles with soap and water?
EMILY.

Yes, often, when I was a child; and I used to make them float in the air by
blowing them upwards.
MRS. B.

We shall fill some such bubbles with hydrogen gas, instead of atmospheric
air, and you will see with what ease and rapidity they will ascend, without
the assistance of blowing, from the lightness of the gas.—Will you mix
some soap and water whilst I fill this bladder with the gas contained in the
receiver which stands on the shelf in the water-bath?
CAROLINE.

What is the use of the brass-stopper and turn-cock at the top of the receiver?
MRS. B.

It is to afford a passage to the gas when required. There is, you see,
a similar stop-cock fastened to this bladder, which is made to fit that on the
receiver. I screw them one on the other, and now turn the two cocks, to open
a communication between the receiver and the bladder; then, by sliding the
receiver off the shelf, and gently sinking it into the bath, the water rises in
the receiver and forces the gas into the bladder. (Plate IX. fig. 1.)
Vol. I. p. 228 Plate IX.

Fig. 1. Apparatus for transferring gases from a Receiver into a bladder.
Fig. 2. Apparatus for blowing Soap bubbles.
Larger view

CAROLINE.

Yes, I see the bladder swell as the water rises in the receiver.
MRS. B.

I think that we have already a sufficient quantity in the bladder for our
purpose; we must be careful to stop both the cocks before we separate the
bladder from the receiver, lest the gas should escape.—Now I must fix a
pipe to the stopper of the bladder, and by dipping its mouth into the soap
and water, take up a few drops—then I again turn the cock, and squeeze the
bladder in order to force the gas into the soap and water at the mouth of the
pipe. (Plate IX. fig. 2.)

EMILY.

There is a bubble—but it bursts before it leaves the mouth of the pipe.
MRS. B.

We must have patience and try again; it is not so easy to blow bubbles by
means of a bladder, as simply with the breath.
CAROLINE.

Perhaps there is not soap enough in the water; I should have had warm
water, it would have dissolved the soap better.
EMILY.

Does not some of the gas escape between the bladder and the pipe?
MRS. B.

No, they are perfectly air tight; we shall succeed presently, I dare say.
CAROLINE.

Now a bubble ascends; it moves with the rapidity of a balloon. How
beautifully it refracts the light!
EMILY.

It has burst against the ceiling—you succeed now wonderfully; but why do
they all ascend and burst against the ceiling?
MRS. B.

Hydrogen gas is so much lighter than atmospherical air, that it ascends
rapidly with its very light envelope, which is burst by the force with which
it strikes the ceiling.
Air-balloons are filled with this gas, and if they carried no other weight than
their covering, would ascend as rapidly as these bubbles.
CAROLINE.

Yet their covering must be much heavier than that of these bubbles?
MRS. B.

Not in proportion to the quantity of gas they contain. I do not know whether
you have ever been present at the filling of a large balloon. The apparatus
for that purpose is very simple. It consists of a number of vessels, either jars
or barrels, in which the materials for the formation of the gas are mixed,
each of these being furnished with a tube, and communicating with a long
flexible pipe, which conveys the gas into the balloon.
EMILY.

But the fire-balloons which were first invented, and have been since
abandoned, on account of their being so dangerous, were constructed,
I suppose, on a different principle.
MRS. B.

They were filled simply with atmospherical air, considerably rarefied by
heat; and the necessity of having a fire underneath the balloon, in order to
preserve the rarefaction of the air within it, was the circumstance productive
of so much danger.
If you are not yet tired of experiments, I have another to show you. It
consists in filling soap-bubbles with a mixture of hydrogen and oxygen
gases, in the proportions that form water; and afterwards setting fire to
them.
EMILY.

They will detonate, I suppose?
MRS. B.

Yes, they will. As you have seen the method of transferring the gas from the
receiver into the bladder, it is not necessary to repeat it. I have therefore
provided a bladder which contains a due proportion of oxygen and
hydrogen gases, and we have only to blow bubbles with it.
CAROLINE.

Here is a fine large bubble rising—shall I set fire to it with the candle?
MRS. B.

If you please . . . .

CAROLINE.

Heavens, what an explosion!—It was like the report of a gun: I confess it
frightened me much. I never should have imagined it could be so loud.
EMILY.

And the flash was as vivid as lightning.
MRS. B.

The combination of the two gases takes place during that instant of time
that you see the flash, and hear the detonation.
EMILY.

This has a strong resemblance to thunder and lightning.
MRS. B.

These phenomena, however, are generally of an electrical nature. Yet
various meteorological effects may be attributed to accidental detonations
of hydrogen gas in the atmosphere; for nature abounds with hydrogen: it
constitutes a very considerable portion of the whole mass of water
belonging to our globe, and from that source almost every other body
obtains it. It enters into the composition of all animal substances, and of a
great number of minerals; but it is most abundant in vegetables. From this
immense variety of bodies, it is often spontaneously disengaged; its great
levity makes it rise into the superior regions of the atmosphere; and when,
either by an electrical spark, or any casual elevation of temperature, it takes
fire, it may produce such meteors or luminous appearances as are
occasionally seen in the atmosphere. Of this kind are probably those broad
flashes which we often see on a summer-evening, without hearing any
detonation.
EMILY.

Every flash, I suppose, must produce a quantity of water?
CAROLINE.

And this water, naturally, descends in the form of rain?
MRS. B.

That probably is often the case, though it is not a necessary consequence;
for the water may be dissolved by the atmosphere, as it descends towards
the lower regions, and remain there in the form of clouds.
The application of electrical attraction to chemical phenomena is likely to
lead to many very interesting discoveries in meteorology; for electricity
evidently acts a most important part in the atmosphere. This subject
however, is, as yet, not sufficiently developed for me to venture enlarging
upon it. The phenomena of the atmosphere are far from being well
understood; and even with the little that is known, I am but imperfectly
acquainted.

But before we take leave of hydrogen, I must not omit to mention to you a
most interesting discovery of Sir H. Davy, which is connected with this
subject.
CAROLINE.

You allude, I suppose, to the new miner’s lamp, which has of late been so
much talked of? I have long been desirous of knowing what that discovery
was, and what purpose it was intended to answer.
MRS. B.

It often happens in coal-mines, that quantities of the gas, called by chemists
hydro-carbonat, or by the miners fire-damp, (the same from which the gas-
lights are obtained,) ooze out from fissures in the beds of coal, and fill the
cavities in which the men are at work; and this gas being inflammable, the
consequence is, that when the men approach those places with a lighted
candle, the gas takes fire, and explosions happen which destroy the men and
horses employed in that part of the colliery, sometimes in great numbers.
EMILY.

What tremendous accidents these must be! But whence does that gas
originate?
MRS. B.

Being the chief product of the combustion of coal, no wonder that
inflammable gas should occasionally appear in situations in which this

mineral abounds, since there can be no doubt that processes of combustion
are frequently taking place at a great depth under the surface of the earth;
and therefore those accumulations of gas may arise either from combustions
actually going on, or from former combustions, the gas having perhaps been
confined there for ages.
CAROLINE.

And how does Sir H. Davy’s lamp prevent those dreadful explosions?
MRS. B.

By a contrivance equally simple and ingenious; and one which does no less
credit to the philosophical views from which it was deduced, than to the
philanthropic motives from which the enquiry sprung. The principle of the
lamp is shortly this: It was ascertained, two or three years ago, both by Mr.
Tennant and by Sir Humphry himself, that the combustion of inflammable
gas could not be propagated through small tubes; so that if a jet of an
inflammable gaseous mixture, issuing from a bladder or any other vessel,
through a small tube, be set fire to, it burns at the orifice of the tube, but the
flame never penetrates into the vessel. It is upon this fact that Sir
Humphry’s safety-lamp is founded.
EMILY.

But why does not the flame ever penetrate through the tube into the vessel
from which the gas issues, so as to explode at once the whole of the gas?
MRS. B.

Because, no doubt, the inflamed gas is so much cooled in its passage
through a small tube as to cease to burn before the combustion reaches the
reservoir.
CAROLINE.

And how can this principle be applied to the construction of a lamp?
MRS. B.

Nothing easier. You need only suppose a lamp enclosed all round in glass or
horn, but having a number of small open tubes at the bottom, and others at
the top, to let the air in and out. Now, if such a lamp or lanthorn be carried

into an atmosphere capable of exploding, an explosion or combustion of the
gas will take place within the lamp; and although the vent afforded by the
tubes will save the lamp from bursting, yet, from the principle just
explained, the combustion will not be propagated to the external air through
the tubes, so that no farther consequence will ensue.
EMILY.

And is that all the mystery of that valuable lamp?
MRS. B.

No; in the early part of the enquiry a lamp of this kind was actually
proposed; but it was but a rude sketch compared to its present state of
improvement. Sir H. Davy, after a succession of trials, by which he brought
his lamp nearer and nearer to perfection, at last conceived the happy idea
that if the lamp were surrounded with a wire-work or wire-gauze, of a close
texture, instead of glass or horn, the tubular contrivance I have just
described would be entirely superseded, since each of the interstices of the
gauze would act as a tube in preventing the propagation of explosions; so
that this pervious metallic covering would answer the various purposes of
transparency, of permeability to air, and of protection against explosion.
This idea, Sir Humphry immediately submitted to the test of experiment,
and the result has answered his most sanguine expectations, both in his
laboratory and in the collieries, where it has already been extensively tried.
And he has now the happiness of thinking that his invention will probably
be the means of saving every year a number of lives, which would have
been lost in digging out of the bowels of the earth one of the most valuable
necessaries of life. Here is one of these lamps, every part of which you will
at once comprehend. (See Plate X. fig. 1.)
Plate X.

Fig. 1. A. the cistern containing the Oil B. the rim or screw by which the gauze cage is
fixed to the cistern. C. apperture for supplying Oil. E. a wire for trimming the wick.
D. F. the wire gauze cylinder. G. a double top.
Larger view (complete Plate)

CAROLINE.

How very simple and ingenious! But I do not yet well see why an explosion
taking place within the lamp should not communicate to the external air
around it, through the interstices of the wire?
MRS. B.

This has been and is still a subject of wonder, even to philosophers; and the
only mode they have of explaining it is, that flame or ignition cannot pass

through a fine wire-work, because the metallic wire cools the flame
sufficiently to extinguish it in passing through the gauze. This property of
the wire-gauze is quite similar to that of the tubes which I mentioned on
introducing the subject; for you may consider each interstice of the gauze as
an extremely short tube of a very small diameter.
EMILY.

But I should expect the wire would often become red-hot, by the burning of
the gas within the lamp?
MRS. B.

And this is actually the case, for the top of the lamp is very apt to become
red-hot. But, fortunately, inflammable gaseous mixtures cannot be exploded
by red-hot wire, the intervention of actual flame being required for that
purpose; so that the wire does not set fire to the explosive gas around it.
EMILY.

I can understand that; but if the wire be red-hot, how can it cool the flame
within, and prevent its passing through the gauze?
MRS. B.

The gauze, though red-hot, is not so hot as the flame by which it has been
heated; and as metallic wire is a good conductor, the heat does not much
accumulate in it, as it passes off quickly to the other parts of the lamp, as
well as to any contiguous bodies.
CAROLINE.

This is indeed a most interesting discovery, and one which shows at once
the immense utility with which science may be practically applied to some
of the most important purposes.

* This ingenious explanation was first suggested by Dr. Delarive.
—See Journals of the Royal Institution, vol. i. p. 259.
* Or rather, hydro-carbonat, a gas composed of hydrogen and
carbon, which will be noticed under the head Carbon.

CONVERSATION VIII.
ON SULPHUR AND PHOSPHORUS.

MRS. B.

Sulphur is the next substance that comes under our consideration. It differs
in one essential point from the preceding, as it exists in a solid form at the
temperature of the atmosphere.
CAROLINE.

I am glad that we have at last a solid body to examine; one that we can see
and touch. Pray, is it not with sulphur that the points of matches are
covered, to make them easily kindle?
MRS. B.

Yes, it is; and you therefore already know that sulphur is a very combustible
substance. It is seldom discovered in nature in a pure unmixed state; so
great is its affinity for other substances, that it is almost constantly found
combined with some of them. It is most commonly united with metals,
under various forms, and is separated from them by a very simple process.
It exists likewise in many mineral waters, and some vegetables yield it in
various proportions, especially those of the cruciform tribe. It is also found
in animal matter; in short, it may be discovered in greater or less quantity,
in the mineral, vegetable, and animal kingdoms.
EMILY.

I have heard of flowers of sulphur, are they the produce of any plant?
MRS. B.

By no means: they consist of nothing more than common sulphur, reduced
to a very fine powder by a process called sublimation.—You see some of it
in this phial; it is exactly the same substance as this lump of sulphur, only
its colour is a paler yellow, owing to its state of very minute division.
EMILY.

Pray what is sublimation?
MRS. B.

It is the evaporation, or, more properly speaking, the volatilisation of solid
substances, which, in cooling, condense again in a concrete form. The
process, in this instance, must be performed in a closed vessel, both to
prevent combustion, which would take place if the access of air were not
carefully precluded, and likewise in order to collect the substance after the
operation. As it is rather a slow process, we shall not try the experiment
now; but you will understand it perfectly if I show you the apparatus used
for the purpose. (Plate XI. fig. 1.) Some lumps of sulphur are put into a
receiver of this kind, which is called a cucurbit. Its shape, you see,
somewhat resembles that of a pear, and is open at the top, so as to adapt
itself exactly to a kind of conical receiver of this sort, called the head. The
cucurbit, thus covered with its head, is placed over a sand-bath; this is
nothing more than a vessel full of sand, which is kept heated by a furnace,
such as you see here, so as to preserve the apparatus in a moderate and
uniform temperature. The sulphur then soon begins to melt, and
immediately after this, a thick white smoke rises, which is gradually
deposited within the head, or upper part of the apparatus, where it
condenses against the sides, somewhat in the form of a vegetation, whence
it has obtained the name of flowers of sulphur. This apparatus, which is
called an alembic, is highly useful in all kinds of distillations, as you will
see when we come to treat of those operations. Alembics are not commonly
made of glass, like this, which is applicable only to distillations upon a very
small scale. Those used in manufactures are generally made of copper, and
are, of course, considerably larger. The principal construction, however, is
always the same, although their shape admits of some variation.
Vol. I. p. 237. Plate XI.

Fig. 1. A Alembic. B Sand-bath. C Furnace.
Fig. 2. Eudiometer.
Fig. 3. A Retort containing water. B Lamp to heat the water. C.C Porcelain tube
containing Carbone. D Furnace through which the tube passes. E Receiver for the
gas produced. F Water bath.
Larger view

CAROLINE.

What is the use of that neck, or tube, which bends down from the upper
piece of the apparatus?
MRS. B.

It is of no use in sublimations; but in distillations (the general object of
which is to evaporate, by heat, in closed vessels, the volatile parts of a
compound body, and to condense them again into a liquid,) it serves to
carry off the condensed fluid, which otherwise would fall back into the
cucurbit. But this is rather foreign to our present subject. Let us return to the
sulphur. You now perfectly understand, I suppose, what is meant by
sublimation?
EMILY.

I believe I do. Sublimation appears to consist in destroying, by means of
heat, the attraction of aggregation of the particles of a solid body, which are
thus volatilised; and as soon as they lose the caloric which produced that
effect, they are deposited in the form of a fine powder.

CAROLINE.

It seems to me to be somewhat similar to the transformation of water into
vapour, which returns to its liquid state when deprived of caloric.
EMILY.

There is this difference, however, that the sulphur does not return to its
former state, since, instead of lumps, it changes to a fine powder.
MRS. B.

Chemically speaking, it is exactly the same substance, whether in the form
of lump or powder. For if this powder be melted again by heat, it will, in
cooling, be restored to the same solid state in which it was before its
sublimation.
CAROLINE.

But if there be no real change, produced by the sublimation of the sulphur,
what is the use of that operation?
MRS. B.

It divides the sulphur into very minute parts, and thus disposes it to enter
more readily into combination with other bodies. It is used also as a means
of purification.
CAROLINE.

Sublimation appears to me like the beginning of combustion, for the
completion of which one circumstance only is wanting, the absorption of
oxygen.
MRS. B.

But that circumstance is every thing. No essential alteration is produced in
sulphur by sublimation; whilst in combustion it combines with the oxygen,
and forms a new compound totally different in every respect from sulphur
in its pure state.—We shall now burn some sulphur, and you will see how
very different the result will be. For this purpose I put a small quantity of
flowers of sulphur into this cup, and place it in a dish, into which I have
poured a little water: I now set fire to the sulphur with the point of this hot
wire; for its combustion will not begin unless its temperature be

considerably raised.—You see that it burns with a faint blueish flame; and
as I invert over it this receiver, white fumes arise from the sulphur, and fill
the vessel.—You will soon perceive that the water is rising within the
receiver, a little above its level in the plate.—Well, Emily, can you account
for this?
EMILY.

I suppose that the sulphur has absorbed the oxygen from the atmospherical
air within the receiver, and that we shall find some oxygenated sulphur in
the cup. As for the white smoke, I am quite at a loss to guess what it
may be.
MRS. B.

Your first conjecture is very right: but you are mistaken in the last; for
nothing will be left in the cup. The white vapour is the oxygenated sulphur,
which assumes the form of an elastic fluid of a pungent and offensive smell,
and is a powerful acid. Here you see a chemical combination of oxygen and
sulphur, producing a true gas, which would continue such under the
pressure and at the temperature of the atmosphere, if it did not unite with
the water in the plate, to which it imparts its acid taste, and all its acid
properties.—You see, now, with what curious effects the combustion of
sulphur is attended.
CAROLINE.

This is something quite new; and I confess that I do not perfectly
understand why the sulphur turns acid.
MRS. B.

It is because it unites with oxygen, which is the acidifying principle. And,
indeed, the word oxygen is derived from two Greek words signifying to
produce an acid.
CAROLINE.

Why, then, is not water, which contains such a quantity of oxygen, acid?
MRS. B.

Because hydrogen, which is the other constituent of water, is not
susceptible of acidification.—I believe it will be necessary, before we
proceed further, to say a few words of the general nature of acids, though it
is rather a deviation from our plan of examining the simple bodies
separately, before we consider them in a state of combination.
Acids may be considered as a peculiar class of burnt bodies, which during
their combustion, or combination with oxygen, have acquired very
characteristic properties. They are chiefly discernible by their sour taste,
and by turning red most of the blue vegetable colours. These two properties
are common to the whole class of acids; but each of them is distinguished
by other peculiar qualities. Every acid consists of some particular
substance, (which constitutes its basis, and is different in each,) and of
oxygen, which is common to them all.
EMILY.

But I do not clearly see the difference between acids and oxyds.
MRS. B.

Acids were, in fact, oxyds, which, by the addition of a sufficient quantity of
oxygen, have been converted into acids. For acidification, you must
observe, always implies previous oxydation, as a body must have combined
with the quantity of oxygen requisite to constitute it an oxyd, before it can
combine with the greater quantity that is necessary to render it an acid.
CAROLINE.

Are all oxyds capable of being converted into acids?
MRS. B.

Very far from it; it is only certain substances which will enter into that
peculiar kind of union with oxygen that produces acids, and the number of
these is proportionally very small; but all burnt bodies may be considered as
belonging either to the class of oxyds, or to that of acids. At a future period,
we shall enter more at large into this subject. At present, I have but one
circumstance further to point out to your observation respecting acids: it is,
that most of them are susceptible of two degrees of acidification, according
to the different quantities of oxygen with which their basis combines.

EMILY.

And how are these two degrees of acidification distinguished?
MRS. B.

By the peculiar properties which result from them. The acid we have just
made is the first or weakest degree of acidification, and is called
sulphureous acid; if it were fully saturated with oxygen, it would be called
sulphuric acid. You must therefore remember, that in this, as in all acids, the
first degree of acidification is expressed by the termination in ous; the
stronger, by the termination in ic.
CAROLINE.

And how is the sulphuric acid made?
MRS. B.

By burning sulphur in pure oxygen gas, and thus rendering its combustion
much more complete. I have provided some oxygen gas for this purpose; it
is in that bottle, but we must first decant the gas into the glass receiver
which stands on the shelf in the bath, and is full of water.
CAROLINE.

Pray, let me try to do it, Mrs. B.
MRS. B.

It requires some little dexterity—hold the bottle completely under water,
and do not turn the mouth upwards, till it is immediately under the aperture
in the shelf, through which the gas is to pass into the receiver, and then turn
it up gradually.—Very well, you have only let a few bubbles escape, and
that must be expected at a first trial.—Now I shall put this piece of sulphur
into the receiver, through the opening at the top, and introduce along with it
a small piece of lighted tinder to set fire to it.—This requires being done
very quickly, lest the atmospherical air should get in, and mix with the pure
oxygen gas.
EMILY.

How beautifully it burns!

CAROLINE.

But it is already buried in the thick vapour. This, I suppose, is sulphuric
acid?
EMILY.

Are these acids always in a gaseous state?
MRS. B.

Sulphureous acid, as we have already observed, is a permanent gas, and can
be obtained in a liquid form only by condensing it in water. In its pure state,
the sulphureous acid is invisible, and it now appears in the form of a white
smoke, from its combining with the moisture. But the vapour of sulphuric
acid, which you have just seen to rise during the combustion, is not a gas,
but only a vapour, which condenses into liquid sulphuric acid, by losing its
caloric. But it appears from Sir H. Davy’s experiments, that this formation
and condensation of sulphuric acid requires the presence of water, for which
purpose the vapour is received into cold water, which may afterwards be
separated from the acid by evaporation.
Sulphur has hitherto been considered as a simple substance; but Sir H. Davy
has suspected that it contains a small portion of hydrogen, and perhaps also
of oxygen.
On submitting sulphur to the action of the Voltaic battery, he observed that
the negative wire gave out hydrogen; and the existence of hydrogen in
sulphur was rendered still more probable by his observing that a small
quantity of water was produced during the combustion of sulphur.
EMILY.

And pray of what nature is sulphur when perfectly pure?
MRS. B.

Sulphur has probably never been obtained perfectly free from combination,
so that its radical may possibly possess properties very different from those
of common sulphur. It has been suspected to be of a metallic nature; but this
is mere conjecture.

Before we quit the subject of sulphur, I must tell you that it is susceptible of
combining with a great variety of substances, and especially with hydrogen,
with which you are already acquainted. Hydrogen gas can dissolve a small
portion of it.
EMILY.

What! can a gas dissolve a solid substance?
MRS. B.

Yes; a solid substance may be so minutely divided by heat, as to become
soluble in a gas: and there are several instances of it. But you must observe,
that, in this case, a chemical union or combination of the sulphur with the
hydrogen gas is produced. In order to effect this, the sulphur must be
strongly heated in contact with the gas; the heat reduces the sulphur to such
a state of extreme division, and diffuses it so thoroughly through the gas,
that they combine and incorporate together. And as a proof that there must
be a chemical union between the sulphur and the gas, it is sufficient to
remark that they are not separated when the sulphur loses the caloric by
which it was volatilized. Besides, it is evident, from the peculiar fetid smell
of this gas, that it is a new compound totally different from either of its
constituents; it is called sulphuretted hydrogen gas, and is contained in great
abundance in sulphureous mineral waters.
CAROLINE.

Are not the Harrogate waters of this nature?
MRS. B.

Yes; they are naturally impregnated with sulphuretted hydrogen gas, and
there are many other springs of the same kind, which shows that this gas
must often be formed in the bowels of the earth by spontaneous processes
of nature.
CAROLINE.

And could not such waters be made artificially by impregnating common
water with this gas?
MRS. B.

Yes; they can be so well imitated, as perfectly to resemble the Harrogate
waters.
Sulphur combines likewise with phosphorus, and with the alkalies, and
alkaline earths, substances with which you are yet unacquainted. We cannot,
therefore, enter into these combinations at present. In our next lesson we
shall treat of phosphorus.
EMILY.

May we not begin that subject to-day; this lesson has been so short?
MRS. B.

I have no objection, if you are not tired. What do you say, Caroline?
CAROLINE.

I am as desirous as Emily of prolonging the lesson to-day, especially as we
are to enter on a new subject; for I confess that sulphur has not appeared to
me so interesting as the other simple bodies.
MRS. B.

Perhaps you may find phosphorus more entertaining. You must not,
however, be discouraged when you meet with some parts of a study less
amusing than others; it would answer no good purpose to select the most
pleasing parts, since, if we did not proceed with some method, in order to
acquire a general idea of the whole, we could scarcely expect to take
interest in any particular subjects.

PHOSPHORUS.

Phosphorus is considered as a simple body; though, like sulphur, it has been
suspected of containing hydrogen. It was not known by the earlier chemists.
It was first discovered by Brandt, a chemist of Hamburgh, whilst employed
in researches after the philosopher’s stone; but the method of obtaining it
remained a secret till it was a second time discovered both by Kunckel and
Boyle, in the year 1680. You see a specimen of phosphorus in this phial; it

is generally moulded into small sticks of a yellowish colour, as you find it
here.
CAROLINE.

I do not understand in what the discovery consisted; there may be a secret
method of making an artificial composition, but how can you talk of making
a substance which naturally exists?
MRS. B.

A body may exist in nature so closely combined with other substances, as to
elude the observation of chemists, or render it extremely difficult to obtain
it in its separate state. This is the case with phosphorus, which is always so
intimately combined with other substances, that its existence remained
unnoticed till Brandt discovered the means of obtaining it free from other
combinations. It is found in all animal substances, and is now chiefly
extracted from bones, by a chemical process. It exists also in some plants,
that bear a strong analogy to animal matter in their chemical composition.
EMILY.

But is it never found in its pure separate state?
MRS. B.

Never, and this is the reason that it has remained so long undiscovered.
Phosphorus is eminently combustible; it melts and takes fire at the
temperature of one hundred degrees, and absorbs in its combustion nearly
once and a half its own weight of oxygen.
CAROLINE.

What! will a pound of phosphorus consume a pound and half of oxygen?
MRS. B.

So it appears from accurate experiments. I can show you with what violence
it combines with oxygen, by burning some of it in that gas. We must
manage the experiment in the same manner as we did the combustion of
sulphur. You see I am obliged to cut this little bit of phosphorus under

water, otherwise there would be danger of its taking fire by the heat of my
fingers. I now put into the receiver, and kindle it by means of a hot wire.
EMILY.

What a blaze! I can hardly look at it. I never saw any thing so brilliant.
Does it not hurt your eyes, Caroline?
CAROLINE.

Yes; but still I cannot help looking at it. A prodigious quantity of oxygen
must indeed be absorbed, when so much light and caloric are disengaged!
MRS. B.

In the combustion of a pound of phosphorus, a sufficient quantity of caloric
is set free to melt upwards of a hundred pounds of ice; this has been
computed by direct experiments with the calorimeter.
EMILY.

And is the result of this combustion, like that of sulphur, an acid?
MRS. B.

Yes; phosphoric acid. And had we duly proportioned the phosphorus and
the oxygen, they would have been completely converted into phosphoric
acid, weighing together, in this new state, exactly the sum of their weights
separately. The water would have ascended into the receiver, on account of
the vacuum formed, and would have filled it entirely. In this case, as in the
combustion of sulphur, the acid vapour formed is absorbed and condensed
in the water of the receiver. But when this combustion is performed without
any water or moisture being present, the acid then appears in the form of
concrete whitish flakes, which are, however, extremely ready to melt upon
the least admission of moisture.
EMILY.

Does phosphorus, in burning in atmospherical air, produce, like sulphur,
a weaker sort of the same acid?
MRS. B.

No: for it burns in atmospherical air, nearly at the same temperature as in
pure oxygen gas; and it is in both cases so strongly disposed to combine
with the oxygen, that the combustion is perfect, and the product similar;
only in atmospherical air, being less rapidly supplied with oxygen, the
process is performed in a slower manner.
CAROLINE.

But is there no method of acidifying phosphorus in a slighter manner, so as
to form phosphorus acid?
MRS. B.

Yes, there is. When simply exposed to the atmosphere, phosphorus
undergoes a kind of slow combustion at any temperature above zero.
EMILY.

But is not the process in this case rather an oxydation than a combustion?
For if the oxygen is too slowly absorbed for a sensible quantity of light and
heat to be disengaged, it is not a true combustion.
MRS. B.

The case is not as you suppose: a faint light is emitted which is very
discernible in the dark; but the heat evolved is not sufficiently strong to be
sensible: a whitish vapour arises from this combustion, which, uniting with
water, condenses into liquid phosphorus acid.
CAROLINE.

Is it not very singular that phosphorus should burn at so low a temperature
in atmospherical air, whilst it does not burn in pure oxygen without the
application of heat?
MRS. B.

So it at first appears. But this circumstance seems to be owing to the
nitrogen gas of the atmosphere. This gas dissolves small particles of
phosphorus, which being thus minutely divided and diffused in the
atmospherical air, combines with the oxygen, and undergoes this slow
combustion. But the same effect does not take place in oxygen gas, because
it is not capable of dissolving phosphorus; it is therefore necessary, in this

case, that heat should be applied to effect that division of particles, which,
in the former instance, is produced by the nitrogen.
EMILY.

I have seen letters written with phosphorus, which are invisible by day-
light, but may be read in the dark by their own light. They look as if they
were written with fire; yet they do not seem to burn.
MRS. B.

But they do really burn; for it is by their slow combustion that the light is
emitted; and phosphorus acid is the result of this combustion.
Phosphorus is sometimes used as a test to estimate the purity of
atmospherical air. For this purpose, it is burnt in a graduated tube, called an
Eudiometer (Plate XI. fig. 2.), and from the quantity of air which the
phosphorus absorbs, the proportion of oxygen in the air examined is
deduced; for the phosphorus will absorb all the oxygen, and the nitrogen
alone will remain.
EMILY.

And the more oxygen is contained in the atmosphere, the purer, I suppose, it
is esteemed?
MRS. B.

Certainly. Phosphorus, when melted, combines with a great variety of
substances. With sulphur it forms a compound so extremely combustible,
that it immediately takes fire on coming in contact with the air. It is with
this composition that phosphoric matches are prepared, which kindle as
soon as they are taken out of their case and are exposed to the air.
EMILY.

I have a box of these curious matches; but I have observed, that in very cold
weather, they will not take fire without being previously rubbed.
MRS. B.

By rubbing them you raise their temperature; for, you know, friction is one
of the means of extricating heat.

EMILY.

Will phosphorus combine with hydrogen gas, as sulphur does?
MRS. B.

Yes; and the compound gas which results from this combination has a smell
still more fetid than the sulphuretted hydrogen; it resembles that of garlic.
The phosphoretted hydrogen gas has this remarkable peculiarity, that it
takes fire spontaneously in the atmosphere, at any temperature. It is thus,
probably, that are produced those transient flames, or flashes of light, called
by the vulgar Will-of-the Whisp, or more properly Ignes-fatui, which are
often seen in church-yards, and places where the putrefactions of animal
matter exhale phosphorus and hydrogen gas.
CAROLINE.

Country people, who are so much frightened by those appearances, would
soon be reconciled to them, if they knew from what a simple cause they
proceed.
MRS. B.

There are other combinations of phosphorus that have also very singular
properties, particularly that which results from its union with lime.
EMILY.

Is there any name to distinguish the combination of two substances, like
phosphorus and lime, neither of which are oxygen, and which cannot
therefore produce either an oxyd or an acid?
MRS. B.

The names of such combinations are composed from those of their
ingredients, merely by a slight change in their termination. Thus the
combination of sulphur with lime is called a sulphuret, and that of
phosphorus, a phosphuret of lime. This latter compound, I was going to say,
has the singular property of decomposing water, merely by being thrown
into it. It effects this by absorbing the oxygen of water, in consequence of
which bubbles of hydrogen gas ascend, holding in solution a small quantity
of phosphorus.

EMILY.

These bubbles then are phosphoretted hydrogen gas?
MRS. B.

Yes; and they produce the singular appearance of a flash of fire issuing from
water, as the bubbles kindle and detonate on the surface of the water, at the
instant that they come in contact with the atmosphere.
CAROLINE.

Is not this effect nearly similar to that produced by the combination of
phosphorus and sulphur, or, more properly speaking, the phosphuret of
sulphur?
MRS. B.

Yes; but the phenomenon appears more extraordinary in this case, from the
presence of water, and from the gaseous form of the combustible
compound. Besides, the experiment surprises by its great simplicity. You
only throw a piece of phosphoret of lime into a glass of water, and bubbles
of fire will immediately issue from it.
CAROLINE.

Cannot we try the experiment?
MRS. B.

Very easily: but we must do it in the open air; for the smell of the
phosphorated hydrogen gas is so extremely fetid, that it would be
intolerable in the house. But before we leave the room, we may produce, by
another process, some bubbles of the same gas, which are much less
offensive.
There is in this little glass retort a solution of potash in water; I add to it a
small piece of phosphorus. We must now heat the retort over the lamp, after
having engaged its neck under water—you see it begins to boil; in a few
minutes bubbles will appear, which take fire and detonate as they issue from
the water.
CAROLINE.

There is one—and another. How curious it is!—But I do not understand
how this is produced.
MRS. B.

It is the consequence of a display of affinities too complicated, I fear, to be
made perfectly intelligible to you at present.
In a few words, the reciprocal action of the potash, phosphorus, caloric, and
water are such, that some of the water is decomposed, and the hydrogen gas
thereby formed carries off some minute particles of phosphorus, with which
it forms phosphoretted hydrogen gas, a compound which spontaneously
takes fire at almost any temperature.
EMILY.

What is that circular ring of smoke which slowly rises from each bubble
after its detonation?
MRS. B.

It consists of water and phosphoric acid in vapour, which are produced by
the combustion of hydrogen and phosphorus.

CONVERSATION IX.
ON CARBON.

CAROLINE.

To-day, Mrs. B., I believe we are to learn the nature and properties of
carbon. This substance is quite new to me; I never heard it mentioned
before.
MRS. B.

Not so new as you imagine; for carbon is nothing more than charcoal in a
state of purity, that is to say, unmixed with any foreign ingredients.
CAROLINE.

But charcoal is made by art, Mrs. B., and a body consisting of one simple
substance cannot be fabricated?
MRS. B.

You again confound the idea, of making a simple body, with that of
separating it from a compound. The chemical processes by which a simple
body is obtained in a state of purity, consist in unmaking the compound in
which it is contained, in order to separate from it the simple substance in
question. The method by which charcoal is usually obtained, is, indeed,
commonly called making it; but, upon examination, you will find this
process to consist simply in separating it from other substances with which
it is found combined in nature.
Carbon forms a considerable part of the solid matter of all organised bodies;
but it is most abundant in the vegetable creation, and it is chiefly obtained
from wood. When the oil and water (which are other constituents of
vegetable matter) are evaporated, the black, porous, brittle substance that
remains, is charcoal.
CAROLINE.

But if heat be applied to the wood in order to evaporate the oil and water,
will not the temperature of the charcoal be raised so as to make it burn; and
if it combines with oxygen, can we any longer call it pure?
MRS. B.

I was going to say, that, in this operation, the air must be excluded.
CAROLINE.

How then can the vapour of the oil and water fly off?
MRS. B.

In order to produce charcoal in its purest state (which is, even then, but a
less imperfect sort of carbon), the operation should be performed in an

earthen retort. Heat being applied to the body of the retort, the evaporable
part of the wood will escape through its neck, into which no air can
penetrate as long as the heated vapour continues to fill it. And if it be
wished to collect these volatile products of the wood, this can easily be
done by introducing the neck of the retort into the water-bath apparatus,
with which you are acquainted. But the preparation of common charcoal,
such as is used in kitchens and manufactures, is performed on a much larger
scale, and by an easier and less expensive process.
EMILY.

I have seen the process of making common charcoal. The wood is ranged
on the ground in a pile of a pyramidical form, with a fire underneath; the
whole is then covered with clay, a few holes only being left for the
circulation of air.
MRS. B.

These holes are closed as soon as the wood is fairly lighted, so that the
combustion is checked, or at least continues but in a very imperfect manner;
but the heat produced by it is sufficient to force out and volatilize, through
the earthy cover, most part of the oily and watery principles of the wood,
although it cannot reduce it to ashes.
EMILY.

Is pure carbon as black as charcoal?
MRS. B.

The purest charcoal we can prepare is so; but chemists have never yet been
able to separate it entirely from hydrogen. Sir H. Davy says, that the most
perfect carbon that is prepared by art contains about five per cent. of
hydrogen; he is of opinion, that if we could obtain it quite free from foreign
ingredients, it would be metallic, in common with other simple substances.
But there is a form in which charcoal appears, that I dare say will surprise
you.—This ring, which I wear on my finger, owes its brilliancy to a small
piece of carbon.
CAROLINE.

Surely, you are jesting, Mrs. B.?
EMILY.

I thought your ring was diamond?
MRS. B.

It is so. But diamond is nothing more than carbon in a crystallized state.
EMILY.

That is astonishing! Is it possible to see two things apparently more
different than diamond and charcoal?
CAROLINE.

It is, indeed, curious to think that we adorn ourselves with jewels of
charcoal!
MRS. B.

There are many other substances, consisting chiefly of carbon, that are
remarkably white. Cotton, for instance, is almost wholly carbon.
CAROLINE.

That, I own, I could never have imagined!—But pray, Mrs. B., since it is
known of what substance diamond and cotton are composed, why should
they not be manufactured, or imitated, by some chemical process, which
would render them much cheaper, and more plentiful than the present mode
of obtaining them?
MRS. B.

You might as well, my dear, propose that we should make flowers and fruit,
nay, perhaps even animals, by a chemical process; for it is known of what
these bodies consist, since every thing which we are acquainted with in
nature is formed from the various simple substances that we have
enumerated. But you must not suppose that a knowledge of the component
parts of a body will in every case enable us to imitate it. It is much less
difficult to decompose bodies, and discover of what materials they are
made, than it is to recompose them. The first of these processes is called
analysis, the last synthesis. When we are able to ascertain the nature of a

substance by both these methods, so that the result of one confirms that of
the other, we obtain the most complete knowledge of it that we are capable
of acquiring. This is the case with water, with the atmosphere, with most of
the oxyds, acids, and neutral salts, and with many other compounds. But the
more complicated combinations of nature, even in the mineral kingdom, are
in general beyond our reach, and any attempt to imitate organised bodies
must ever prove fruitless; their formation is a secret that rests in the bosom
of the Creator. You see, therefore, how vain it would be to attempt to make
cotton by chemical means. But, surely, we have no reason to regret our
inability in this instance, when nature has so clearly pointed out a method of
obtaining it in perfection and abundance.
CAROLINE.

I did not imagine that the principle of life could be imitated by the aid of
chemistry; but it did not appear to me ridiculous to suppose that chemists
might attain a perfect imitation of inanimate nature.
MRS. B.

They have succeeded in this point in a variety of instances; but, as you
justly observe, the principle of life, or even the minute and intimate
organisation of the vegetable kingdom, are secrets that have almost entirely
eluded the researches of philosophers; nor do I imagine that human art will
ever be capable of investigating them with complete success.
EMILY.

But diamond, since it consists of one simple unorganised substance, might
be, one would think, perfectly imitable by art?
MRS. B.

It is sometimes as much beyond our power to obtain a simple body in a
state of perfect purity, as it is to imitate a complicated combination; for the
operations by which nature separates bodies are frequently as inimitable as
those which she uses for their combination. This is the case with carbon; all
the efforts of chemists to separate it entirely from other substances have
been fruitless, and in the purest state in which it can be obtained by art, it
still retains a portion of hydrogen, and probably of some other foreign

ingredients. We are ignorant of the means which nature employs to
crystallize it. It may probably be the work of ages, to purify, arrange, and
unite the particles of carbon in the form of diamond. Here is some charcoal
in the purest state we can procure it: you see that it is a very black, brittle,
light, porous substance, entirely destitute of either taste or smell. Heat,
without air, produces no alteration in it, as it is not volatile; but, on the
contrary, it invariably remains at the bottom of the vessel after all the other
parts of the vegetable are evaporated.
EMILY.

Yet carbon is, no doubt, combustible, since you say that charcoal would
absorb oxygen if air were admitted during its preparation?
CAROLINE.

Unquestionably. Besides, you know, Emily, how much it is used in cooking.
But pray what is the reason that charcoal burns without smoke, whilst a
wood fire smokes so much?
MRS. B.

Because, in the conversion of wood into charcoal, the volatile particles of
the former have been evaporated.
CAROLINE.

Yet I have frequently seen charcoal burn with flame; therefore it must, in
that case, contain some hydrogen.
MRS. B.

Very true; but you must recollect that charcoal, especially that which is used
for common purposes, is not perfectly pure. It generally retains some
remains of the various other component parts of vegetables, and hydrogen
particularly, which accounts for the flame in question.
CAROLINE.

But what becomes of the carbon itself during its combustion?
MRS. B.

It gradually combines with the oxygen of the atmosphere, in the same way
as sulphur and phosphorus, and, like those substances, it is converted into a
peculiar acid, which flies off in a gaseous form. There is this difference,
however, that the acid is not, in this instance, as in the two cases just
mentioned, a mere condensable vapour, but a permanent elastic fluid, which
always remains in the state of gas, under any pressure and at any
temperature. The nature of this acid was first ascertained by Dr. Black, of
Edinburgh; and, before the introduction of the new nomenclature, it was
called fixed air. It is now distinguished by the more appropriate name of
carbonic acid gas.
EMILY.

Carbon, then, can be volatilized by burning, though, by heat alone, no such
effect is produced?
MRS. B.

Yes; but then it is no longer simple carbon, but an acid of which carbon
forms the basis. In this state, carbon retains no more appearance of solidity
or corporeal form, than the basis of any other gas. And you may, I think,
from this instance, derive a more clear idea of the basis of the oxygen,
hydrogen, and nitrogen gases, the existence of which, as real bodies, you
seemed to doubt, because they were not to be obtained simply in a solid
form.
EMILY.

That is true; we may conceive the basis of the oxygen, and of the other
gases, to be solid, heavy substances, like carbon; but so much expanded by
caloric as to become invisible.
CAROLINE.

But does not the carbonic acid gas partake of the blackness of charcoal?
MRS. B.

Not in the least. Blackness, you know, does not appear to be essential to
carbon, and it is pure carbon, and not charcoal, that we must consider as the
basis of carbonic acid. We shall make some carbonic acid, and, in order to
hasten the process, we shall burn the carbon in oxygen gas.

EMILY.

But do you mean then to burn diamond?
MRS. B.

Charcoal will answer the purpose still better, being softer and more easy to
inflame; besides the experiments on diamond are rather expensive.
CAROLINE.

But is it possible to burn diamond?
MRS. B.

Yes, it is; and in order to effect this combustion, nothing more is required
than to apply a sufficient degree of heat by means of the blow-pipe, and of a
stream of oxygen gas. Indeed it is by burning diamond that its chemical
nature has been ascertained. It has long been known as a combustible
substance, but it is within these few years only that the product of its
combustion has been proved to be pure carbonic acid. This remarkable
discovery is due to Mr. Tennant.
Now let us try to make some carbonic acid.—Will you, Emily, decant some
oxygen gas from this large jar into the receiver in which we are to burn the
carbon; and I shall introduce this small piece of charcoal, with a little
lighted tinder, which will be necessary to give the first impulse to the
combustion.
EMILY.

I cannot conceive how so small a piece of tinder, and that but just lighted,
can raise the temperature of the carbon sufficiently to set fire to it; for it can
produce scarcely any sensible heat, and it hardly touches the carbon.
MRS. B.

The tinder thus kindled has only heat enough to begin its own combustion,
which, however, soon becomes so rapid in the oxygen gas, as to raise the
temperature of the charcoal sufficiently for this to burn likewise, as you see
is now the case.
EMILY.

I am surprised that the combustion of carbon is not more brilliant; it does
not give out near so much light or caloric as phosphorus, or sulphur. Yet
since it combines with so much oxygen, why is not a proportional quantity
of light and heat disengaged from the decomposition of the oxygen gas, and
the union of its electricity with that of the charcoal?
MRS. B.

It is not surprising that less light and heat should be liberated in this than in
almost any other combustion, since the oxygen, instead of entering into a
solid or liquid combination, as it does in the phosphoric and sulphuric acids,
is employed in forming another elastic fluid; it therefore parts with less of
its caloric.
EMILY.

True; and, on second consideration, it appears, on the contrary, surprising
that the oxygen should, in its combination with carbon, retain a sufficient
portion of caloric to maintain both substances in a gaseous state.
CAROLINE.

We may then judge of the degree of solidity in which oxygen is combined
in a burnt body, by the quantity of caloric liberated during its combustion?
MRS. B.

Yes; provided that you take into the account the quantity of oxygen
absorbed by the combustible body, and observe the proportion which the
caloric bears to it.
CAROLINE.

But why should the water, after the combustion of carbon, rise in the
receiver, since the gas within it retains an aëriform state?
MRS. B.

Because the carbonic acid gas is gradually absorbed by the water; and this
effect would be promoted by shaking the receiver.
EMILY.

The charcoal is now extinguished, though it is not nearly consumed; it has
such an extraordinary avidity for oxygen, I suppose, that the receiver did
not contain enough to satisfy the whole.
MRS. B.

That is certainly the case; for if the combustion were performed in the exact
proportions of 28 parts of carbon to 72 of oxygen, both these ingredients
would disappear, and 100 parts of carbonic acid would be produced.
CAROLINE.

Carbonic acid must be a very strong acid, since it contains so great a
proportion of oxygen?
MRS. B.

That is a very natural inference; yet it is erroneous. For the carbonic is the
weakest of all the acids. The strength of an acid seems to depend upon the
nature of its basis, and its mode of combination, as well as upon the
proportion of the acidifying principle. The same quantity of oxygen that
will convert some bodies into strong acids, will only be sufficient simply to
oxydate others.
CAROLINE.

Since this acid is so weak, I think chemists should have called it the
carbonous, instead of the carbonic acid.
EMILY.

But, I suppose, the carbonous acid is still weaker, and is formed by burning
carbon in atmospherical air.
MRS. B.

It has been lately discovered, that carbon may be converted into a gas, by
uniting with a smaller proportion of oxygen; but as this gas does not possess
any acid properties, it is no more than an oxyd; it is called gaseous oxyd of
carbon.
CAROLINE.

Pray is not carbonic acid a very wholesome gas to breathe, as it contains so
much oxygen?
MRS. B.

On the contrary, it is extremely pernicious. Oxygen, when in a state of
combination with other substances, loses, in almost every instance, its
respirable properties, and the salubrious effects which it has on the animal
economy when in its unconfined state. Carbonic acid is not only unfit for
respiration, but extremely deleterious if taken into the lungs.
EMILY.

You know, Caroline, how very unwholesome the fumes of burning charcoal
are reckoned.
CAROLINE.

Yes; but, to confess the truth, I did not consider that a charcoal fire
produced carbonic acid gas.—Can this gas be condensed into a liquid?
MRS. B.

No: for, as I told you before, it is a permanent elastic fluid. But water can
absorb a certain quantity of this gas, and can even be impregnated with it, in
a very strong degree, by the assistance of agitation and pressure, as I am
going to show you. I shall decant some carbonic acid gas into this bottle,
which I fill first with water, in order to exclude the atmospherical air; the
gas is then introduced through the water, which you see it displaces, for it
will not mix with it in any quantity, unless strongly agitated, or allowed to
stand over it for some time. The bottle is now about half full of carbonic
acid gas, and the other half is still occupied by the water. By corking the
bottle, and then violently shaking it, in this way, I can mix the gas and water
together.—Now will you taste it?
EMILY.

It has a distinct acid taste.
CAROLINE.

Yes, it is sensibly sour, and appears full of little bubbles.

MRS. B.

It possesses likewise all the other properties of acids, but, of course, in a
less degree than the pure carbonic acid gas, as it is so much diluted by
water.
This is a kind of artificial Seltzer water. By analysing that which is
produced by nature, it was found to contain scarcely any thing more than
common water impregnated with a certain proportion of carbonic acid gas.
We are, therefore, able to imitate it, by mixing those proportions of water
and carbonic acid. Here, my dear, is an instance, in which, by a chemical
process, we can exactly copy the operations of nature; for the artificial
Seltzer waters can be made in every respect similar to those of nature; in
one point, indeed, the former have an advantage, since they may be
prepared stronger, or weaker, as occasion requires.
CAROLINE.

I thought I had tasted such water before. But what renders it so brisk and
sparkling?
MRS. B.

This sparkling, or effervescence, as it is called, is always occasioned by the
action of an elastic fluid escaping from a liquid; in the artificial Seltzer
water, it is produced by the carbonic acid, which being lighter than the
water in which it was strongly condensed, flies off with great rapidity the
instant the bottle is uncorked; this makes it necessary to drink it
immediately. The bubbling that took place in this bottle was but trifling, as
the water was but very slightly impregnated with carbonic acid. It requires a
particular apparatus to prepare the gaseous artificial mineral waters.
EMILY.

If, then, a bottle of Seltzer water remains for any length of time uncorked,
I suppose it returns to the state of common water?
MRS. B.

The whole of the carbonic acid gas, or very nearly so, will soon disappear;
but there is likewise in Seltzer water a very small quantity of soda, and of a

few other saline or earthy ingredients, which will remain in the water,
though it should be kept uncorked for any length of time.
CAROLINE.

I have often heard of people drinking soda-water. Pray what sort of water is
that?
MRS. B.

It is a kind of artificial Seltzer water, holding in solution, besides the
gaseous acid, a particular saline substance, called soda, which imparts to the
water certain medicinal qualities.
CAROLINE.

But how can these waters be so wholesome, since carbonic acid is so
pernicious?
MRS. B.

A gas, we may conceive, though very prejudicial to breathe, may be
beneficial to the stomach.—But it would be of no use to attempt explaining
this more fully at present.
CAROLINE.

Are waters never impregnated with other gases?
MRS. B.

Yes; there are several kinds of gaseous waters. I forgot to tell you that
waters have, for some years past, been prepared, impregnated both with
oxygen and hydrogen gases. These are not an imitation of nature, but are
altogether obtained by artificial means. They have been lately used
medicinally, particularly on the continent, where, I understand, they have
acquired some reputation.
EMILY.

If I recollect right, Mrs. B., you told us that carbon was capable of
decomposing water; the affinity between oxygen and carbon must,
therefore, be greater than between oxygen and hydrogen?
MRS. B.

Yes; but this is not the case unless their temperature be raised to a certain
degree. It is only when carbon is red-hot, that it is capable of separating the
oxygen from the hydrogen. Thus, if a small quantity of water be thrown on
a red-hot fire, it will increase rather than extinguish the combustion; for the
coals or wood (both of which contain a quantity of carbon) decompose the
water, and thus supply the fire both with oxygen and hydrogen gases. If, on
the contrary, a large mass of water be thrown over the fire, the diminution
of heat thus produced is such, that the combustible matter loses the power
of decomposing the water, and the fire is extinguished.
EMILY.

I have heard that fire-engines sometimes do more harm than good, and that
they actually increase the fire when they cannot throw water enough to
extinguish it. It must be owing, no doubt, to the decomposition of the water
by the carbon during the conflagration.
MRS. B.

Certainly.—The apparatus which you see here (Plate XI. fig. 3.), may be
used to exemplify what we have just said. It consists in a kind of open
furnace, through which a porcelain tube, containing charcoal, passes. To
one end of the tube is adapted a glass retort with water in it; and the other
end communicates with a receiver placed on the water-bath. A lamp being
applied to the retort, and the water made to boil, the vapour is gradually
conveyed through the red-hot charcoal, by which it is decomposed; and the
hydrogen gas which results from this decomposition is collected in the
receiver. But the hydrogen thus obtained is far from being pure; it retains in
solution a minute portion of carbon, and contains also a quantity of carbonic
acid. This renders it heavier than pure hydrogen gas, and gives it some
peculiar properties; it is distinguished by the name of carbonated hydrogen
gas.
CAROLINE.

And whence does it obtain the carbonic acid that is mixed with it?
EMILY.

I believe I can answer that question, Caroline.—From the union of the
oxygen (proceeding from the decomposed water) with the carbon, which,
you know, makes carbonic acid.
CAROLINE.

True; I should have recollected that.—The product of the decomposition of
water by red-hot charcoal, therefore, is carbonated hydrogen gas, and
carbonic acid gas.
MRS. B.

You are perfectly right now.
Carbon is frequently found combined with hydrogen in a state of solidity,
especially in coals, which owe their combustible nature to these two
principles.
EMILY.

Is it the hydrogen, then, that produces the flame of coals?
MRS. B.

It is so; and when all the hydrogen is consumed, the carbon continues to
burn without flame. But again, as I mentioned when speaking of the gas-
lights, the hydrogen gas produced by the burning of coals is not pure; for,
during the combustion, particles of carbon are successively volatilized with
the hydrogen, with which they form what is called a hydro-carbonat, which
is the principal product of this combustion.
Carbon is a very bad conductor of heat; for this reason, it is employed (in
conjunction with other ingredients) for coating furnaces and other chemical
apparatus.
EMILY.

Pray what is the use of coating furnaces?
MRS. B.

In most cases, in which a furnace is used, it is necessary to produce and
preserve a great degree of heat, for which purpose every possible means are
used to prevent the heat from escaping by communicating with other

bodies, and this object is attained by coating over the inside of the furnace
with a kind of plaster, composed of materials that are bad conductors of
heat.
Carbon, combined with a small quantity of iron, forms a compound called
plumbago, or black-lead, of which pencils are made. This substance,
agreeably to the nomenclature, is a carburet of iron.
EMILY.

Why, then, is it called black-lead?
MRS. B.

It is an ancient name given to it by ignorant people, from its shining
metallic appearance; but it is certainly a most improper name for it, as there
is not a particle of lead in the composition. There is only one mine of this
mineral, which is in Cumberland. It is supposed to approach as nearly to
pure carbon as the best prepared charcoal does, as it contains only five parts
of iron, unadulterated by any other foreign ingredients. There is another
carburet of iron, in which the iron, though united only to an extremely small
proportion of carbon, acquires very remarkable properties; this is steel.
CAROLINE.

Really; and yet steel is much harder than iron?
MRS. B.

But carbon is not ductile like iron, and therefore may render the steel more
brittle, and prevent its bending so easily. Whether it is that the carbon, by
introducing itself into the pores of the iron, and, by filling them, makes the
metal both harder and heavier; or whether this change depends upon some
chemical cause, I cannot pretend to decide. But there is a subsequent
operation, by which the hardness of steel is very much increased, which
simply consists in heating the steel till it is red-hot, and then plunging it into
cold water.
Carbon, besides the combination just mentioned, enters into the
composition of a vast number of natural productions, such, for instance, as
all the various kinds of oils, which result from the combination of carbon,
hydrogen, and caloric, in various proportions.

EMILY.

I thought that carbon, hydrogen, and caloric, formed carbonated hydrogen
gas?
MRS. B.

That is the case when a small portion of carbonic acid gas is held in solution
by hydrogen gas. Different proportions of the same principles, together with
the circumstances of their union, produce very different combinations; of
this you will see innumerable examples. Besides, we are not now talking of
gases, but of carbon and hydrogen, combined only with a quantity of caloric
sufficient to bring them to the consistency of oil or fat.
CAROLINE.

But oil and fat are not of the same consistence?
MRS. B.

Fat is only congealed oil; or oil, melted fat. The one requires a little more
heat to maintain it in a fluid state than the other. Have you never observed
the fat of meat turned to oil by the caloric it has imbibed from the fire?
EMILY.

Yet oils in general, as salad-oil, and lamp-oil, do not turn to fat when cold?
MRS. B.

Not at the common temperature of the atmosphere, because they retain too
much caloric to congeal at that temperature; but if exposed to a sufficient
degree of cold, their latent heat is extricated, and they become solid fat
substances. Have you never seen salad oil frozen in winter?
EMILY.

Yes; but it appears to me in that state very different from animal fat.
MRS. B.

The essential constituent parts of either vegetable or animal oils are the
same, carbon and hydrogen; their variety arises from the different
proportions of these substances, and from other accessory ingredients that
may be mixed with them. The oil of a whale, and the oil of roses, are, in

their essential constituent parts, the same; but the one is impregnated with
the offensive particles of animal matter, the other with the delicate perfume
of a flower.
The difference of fixed oils, and volatile or essential oils, consists also in
the various proportions of carbon and hydrogen. Fixed oils are those which
will not evaporate without being decomposed; this is the case with all
common oils, which contain a greater proportion of carbon than the
essential oils. The essential oils (which comprehend the whole class of
essences and perfumes) are lighter; they contain more equal proportions of
carbon and hydrogen, and are volatilized or evaporated without being
decomposed.
EMILY.

When you say that one kind of oil will evaporate, and the other be
decomposed, you mean, I suppose, by the application of heat?
MRS. B.

Not necessarily; for there are oils that will evaporate slowly at the common
temperature of the atmosphere; but for a more rapid volatilization, or for
their decomposition, the assistance of heat is required.
CAROLINE.

I shall now remember, I think, that fat and oil are really the same
substances, both consisting of carbon and hydrogen; that in fixed oils the
carbon preponderates, and heat produces a decomposition; while, in
essential oils, the proportion of hydrogen is greater, and heat produces a
volatilization only.
EMILY.

I suppose the reason why oil burns so well in lamps is because its two
constituents are so combustible?
MRS. B.

Certainly; the combustion of oil is just the same as that of a candle; if
tallow, it is only oil in a concrete state; if wax, or spermaceti, its chief
chemical ingredients are still hydrogen and carbon.

EMILY.

I wonder, then, there should be so great a difference between tallow and
wax?
MRS. B.

I must again repeat, that the same substances, in different proportions,
produce results that have sometimes scarcely any resemblance to each
other. But this is rather a general remark that I wish to impress upon your
minds, than one which is applicable to the present case; for tallow and wax
are far from being very dissimilar; the chief difference consists in the wax
being a purer compound of carbon and hydrogen than the tallow, which
retains more of the gross particles of animal matter. The combustion of a
candle, and that of a lamp, both produce water and carbonic acid gas. Can
you tell me how these are formed?
EMILY.

Let me reflect . . . . Both the candle and lamp burn by means of fixed oil—
this is decomposed as the combustion goes on; and the constituent parts of
the oil being thus separated, the carbon unites to a portion of oxygen from
the atmosphere to form carbonic acid gas, whilst the hydrogen combines
with another portion of oxygen, and forms with it water.—The products,
therefore, of the combustion of oils are water and carbonic acid gas.
CAROLINE.

But we see neither water nor carbonic acid produced by the combustion of a
candle.
MRS. B.

The carbonic acid gas, you know, is invisible, and the water being in a state
of vapour, is so likewise. Emily is perfectly correct in her explanation, and I
am very much pleased with it.
All the vegetable acids consist of various proportions of carbon and
hydrogen, acidified by oxygen. Gums, sugar, and starch, are likewise
composed of these ingredients; but, as the oxygen which they contain is not
sufficient to convert them into acids, they are classed with the oxyds, and
called vegetable oxyds.

CAROLINE.

I am very much delighted with all these new ideas; but, at the same time,
I cannot help being apprehensive that I may forget many of them.
MRS. B.

I would advise you to take notes, or, what would answer better still, to write
down, after every lesson, as much of it as you can recollect. And, in order to
give you a little assistance, I shall lend you the heads or index, which I
occasionally consult for the sake of preserving some method and
arrangement in these conversations. Unless you follow some such plan, you
cannot expect to retain nearly all that you learn, how great soever be the
impression it may make on you at first.
EMILY.

I will certainly follow your advice.—Hitherto I have found that I
recollected pretty well what you have taught us; but the history of carbon is
a more extensive subject than any of the simple bodies we have yet
examined.
MRS. B.

I have little more to say on carbon at present; but hereafter you will see that
it performs a considerable part in most chemical operations.
CAROLINE.

That is, I suppose, owing to its entering into the composition of so great a
variety of substances?
MRS. B.

Certainly; it is the basis, you have seen, of all vegetable matter; and you
will find that it is very essential to the process of animalization. But in the
mineral kingdom also, particularly in its form of carbonic acid, we shall
often discover it combined with a great variety of substances.
In chemical operations, carbon is particularly useful, from its very great
attraction for oxygen, as it will absorb this substance from many
oxygenated or burnt bodies, and thus deoxygenate, or unburn them, and
restore them to their original combustible state.

CAROLINE.

I do not understand how a body can be unburnt, and restored to its original
state. This piece of tinder, for instance, that has been burnt, if by any means
the oxygen were extracted from it, would not be restored to its former state
of linen; for its texture is destroyed by burning, and that must be the case
with all organized or manufactured substances, as you observed in a former
conversation.
MRS. B.

A compound body is decomposed by combustion in a way which generally
precludes the possibility of restoring it to its former state; the oxygen, for
instance, does not become fixed in the tinder, but it combines with its
volatile parts, and flies off in the shape of gas, or watery vapour. You see,
therefore, how vain it would be to attempt the recomposition of such
bodies. But, with regard to simple bodies, or at least bodies whose
component parts are not disturbed by the process of oxygenation or
deoxygenation, it is often possible to restore them, after combustion, to
their original state.—The metals, for instance, undergo no other alteration
by combustion than a combination with oxygen; therefore, when the oxygen
is taken from them, they return to their pure metallic state. But I shall say
nothing further of this at present, as the metals will furnish ample subject
for another morning; and they are the class of simple bodies that come next
under consideration.

CONVERSATION X.
ON METALS.

MRS. B.

The metals, which we are now to examine, are bodies of a very different
nature from those which we have hitherto considered. They do not, like the

bases of gases, elude the immediate observation of our senses; for they are
the most brilliant, the most ponderous, and the most palpable substances in
nature.
CAROLINE.

I doubt, however, whether the metals will appear to us so interesting, and
give us so much entertainment as those mysterious elements which conceal
themselves from our view. Besides, they cannot afford so much novelty;
they are bodies with which we are already so well acquainted.
MRS. B.

You are not aware, my dear, of the interesting discoveries which were a few
years ago made by Sir H. Davy respecting this class of bodies. By the aid of
the Voltaic battery, he has obtained from a variety of substances, metals
before unknown, the properties of which are equally new and curious. We
shall begin, however, by noticing those metals with which you profess to be
so well acquainted. But the acquaintance, you will soon perceive, is but
very superficial; and I trust that you will find both novelty and
entertainment in considering the metals in a chemical point of view. To treat
of this subject fully, would require a whole course of lectures; for metals
form of themselves a most important branch of practical chemistry. We
must, therefore, confine ourselves to a general view of them. These bodies
are seldom found naturally in their metallic form: they are generally more
or less oxygenated or combined with sulphur, earths, or acids, and are often
blended with each other. They are found buried in the bowels of the earth in
most parts of the world, but chiefly in mountainous districts, where the
surface of the globe has suffered from the earthquakes, volcanos, and other
convulsions of nature. They are spread in strata or beds, called veins, and
these veins are composed of a certain quantity of metal, combined with
various earthy substances, with which they form minerals of different
nature and appearance, which are called ores.
CAROLINE.

I now feel quite at home, for my father has a lead-mine in Yorkshire, and I
have heard a great deal about veins of ore, and of the roasting and smelting

of the lead; but, I confess, that I do not understand in what these operations
consist.
MRS. B.

Roasting is the process by which the volatile parts of the ore are evaporated;
smelting, that by which the pure metal is afterwards separated from the
earthy remains of the ore. This is done by throwing the whole into a
furnace, and mixing with it certain substances that will combine with the
earthy parts and other foreign ingredients of the ore; the metal being the
heaviest, falls to the bottom, and runs out by proper openings in its pure
metallic state.
EMILY.

You told us in a preceding lesson that metals had a great affinity for oxygen.
Do they not, therefore, combine with oxygen, when strongly heated in the
furnace, and run out in the state of oxyds?
MRS. B.

No; for the scoriæ, or oxyd, which soon forms on the surface of the fused
metal, when it is oxydable, prevents the air from having any further
influence on the mass; so that neither combustion nor oxygenation can take
place.
CAROLINE.

Are all the metals equally combustible?
MRS. B.

No; their attraction for oxygen varies extremely. There are some that will
combine with it only at a very high temperature, or by the assistance of
acids; whilst there are others that oxydate spontaneously and with great
rapidity, even at the lowest temperature; such is in particular manganese,
which scarcely ever exists in the metallic state, as it immediately absorbs
oxygen on being exposed to the air, and crumbles to an oxyd in the course
of a few hours.
EMILY.

Is not that the oxyd from which you extracted the oxygen gas?

MRS. B.

It is: so that, you see, this metal attracts oxygen at a low temperature, and
parts with it when strongly heated.
EMILY.

Is there any other metal that oxydates at the temperature of the atmosphere?
MRS. B.

They all do, more or less, excepting gold, silver, and platina.
Copper, lead, and iron, oxydate slowly in the air, and cover themselves with
a sort of rust, a process which depends on the gradual conversion of the
surface into an oxyd. This rusty surface preserves the interior metal from
oxydation, as it prevents the air from coming in contact with it. Strictly
speaking, however, the word rust applies only to the oxyd, which forms on
the surface of iron, when exposed to air and moisture, which oxyd appears
to be united with a small portion of carbonic acid.
EMILY.

When metals oxydate from the atmosphere without an elevation of
temperature, some light and heat, I suppose, must be disengaged, though
not in sufficient quantities to be sensible.
MRS. B.

Undoubtedly; and, indeed, it is not surprising that in this case the light and
heat should not be sensible, when you consider how extremely slow, and,
indeed, how imperfectly, most metals oxydate by mere exposure to the
atmosphere. For the quantity of oxygen with which metals are capable of
combining, generally depends upon their temperature; and the absorption
stops at various points of oxydation, according to the degree to which their
temperature is raised.
EMILY.

That seems very natural; for the greater the quantity of caloric introduced
into a metal, the more will its positive electricity be exalted, and
consequently the stronger will be its affinity for oxygen.
MRS. B.