Chapter III: Introduction (1)

The study of natural science, whose rapid development dates from the days of Galileo ([+]1642) and Newton ([+]1727), and its closer application to the external universe[1] led to the separation of Chemistry as a particular branch of natural philosophy, not only owing to the increasing store of observations and experiments relating to the mutual transformations of substances, but also, and more especially, because in addition to gravity, cohesion, heat, light and electricity it became necessary to recognise the existence of particular internal forces in the ultimate parts of all substances, forces which make themselves manifest in the transformations of substances into one another, but remain hidden (latent) under ordinary circumstances, and whose existence cannot therefore be directly apprehended, and so for a long time remained unrecognised. The primary object of chemistry is the study of the homogeneous substances[2] of which all the objects of the universe are made up, with the transformations of these substances into each other, and with the phenomena[3] which accompany such transformations. Every chemical change or reaction,[4] as it is called, can only take place under a condition of most intimate and close contact of the re-acting substances,[5] and is determined by the forces proper to the smallest invisible particles (molecules) of matter. We must distinguish three chief classes of chemical transformations.

[1] The investigation of a substance or a natural phenomenon consists
(_a_) in determining the relation of the object under examination
to that which is already known, either from previous researches,
or from experiment, or from the knowledge of the common
surroundings of life--that is, in determining and expressing the
quality of the unknown by the aid of that which is known; (_b_)
in measuring all that which can be subjected to measurement,
and thereby denoting the quantitative relation of that under
investigation to that already known and its relation to the
categories of time, space, temperature, mass, &c.; (_c_) in
determining the position held by the object under investigation
in the system of known objects guided by both qualitative and
quantitative data; (_d_) in determining, from the quantities which
have been measured, the empirical (visible) dependence (function,
or 'law,' as it is sometimes termed) of variable factors--for
instance, the dependence of the composition of the substance on
its properties, of temperature on time, of time on locality, &c.;
(_e_) in framing hypotheses or propositions as to the actual cause
and true nature of the relation between that studied (measured
or observed) and that which is known or the categories of time,
space, &c.; (_f_) in verifying the logical consequences of the
hypotheses by experiment; and (_g_) in advancing a theory which
shall account for the nature of the properties of that studied in
its relations with things already known and with those conditions
or categories among which it exists. It is certain that it is
only possible to carry out these investigations when we have
taken as a basis some incontestable fact which is self-evident
to our understanding; as, for instance, number, time, space,
motion, or mass. The determination of such primary or fundamental
conceptions, although not excluded from the possibility of
investigation, frequently does not subject itself to our present
mode of scientific generalisation. Hence it follows that in the
investigation of anything, there always remains something which is
accepted without investigation, or admitted as a known factor. The
axioms of geometry may be taken as an example. Thus in the science
of biology it is necessary to admit the faculty of organisms for
multiplying themselves, as a conception whose meaning is as yet
unknown. In the study of chemistry, too, the notion of elements
must be accepted almost without any further analysis. However, by
first investigating that which is visible and subject to direct
observation by the organs of the senses, we may hope that in
the first place hypotheses will be arrived at, and afterwards
theories of that which has now to be placed at the basis of our
investigations. The minds of the ancients strove to seize at once
the very fundamental categories of investigation, whilst all the
successes of recent knowledge are based on the above-cited method
of investigation without the determination of 'the beginning of
all beginnings.' By following this inductive method, the _exact
sciences_ have already succeeded in becoming accurately acquainted
with much of the invisible world, which directly is imperceptible
to the organs of sense (for example, the molecular motion of all
bodies, the composition of the heavenly luminaries, the paths
of their motion, the necessity for the existence of substances
which cannot be subjected to experiment, &c.), and have verified
the knowledge thus obtained, and employed it for increasing the
interests of humanity. It may therefore be safely said that _the
inductive method of investigation_ is a more perfect mode of
acquiring knowledge than the deductive method alone (starting from
a little of the unknown accepted as incontestable to arrive at the
much which is visible and observable) by which the ancients strove
to embrace the universe. By investigating the universe by an
inductive method (endeavouring from the much which is observable
to arrive at a little which may be verified and is indubitable)
the new science refuses to recognise dogma as truth, but through
_reason_, by a slow and laborious method of investigation, strives
for and attains to true deductions.

[2] A substance or material is that which occupies space and has
weight; that is, which presents a mass attracted by the earth and
by other masses of material, and of which the _objects_ of nature
are composed, and by means of which the motions and _phenomena_
of nature are accomplished. It is easy to discover by examining
and investigating, by various methods, the objects met with
in nature and in the arts, that some of them are homogeneous,
whilst others are composed of a mixture of several homogeneous
substances. This is most clearly apparent in solid substances.
The metals used in the arts (for example, gold, iron, copper)
must be homogeneous, otherwise they are brittle and unfit for
many purposes. Homogeneous matter exhibits similar properties in
all its parts. By breaking up a homogeneous substance we obtain
parts which, although different in form, resemble each other in
their properties. Glass, pure sugar, marble, &c., are examples of
homogeneous substances. Examples of non-homogeneous substances
are, however, much more frequent in nature and the arts. Thus
the majority of the rocks are not homogeneous. In porphyries
bright pieces of a mineral called 'orthoclase' are often seen
interspersed amongst the dark mass of the rock. In ordinary red
granite it is easy to distinguish large pieces of orthoclase mixed
with dark semi-transparent quartz and flexible laminæ of mica.
Similarly, plants and animals are non-homogeneous. Thus, leaves
are composed of a skin, fibre, pulp, sap, and a green colouring
matter. As an example of those non-homogeneous substances which
are produced artificially, gunpowder may be cited, which is
prepared by mixing together known proportions of sulphur, nitre,
and charcoal. Many liquids, also, are not homogeneous, as may be
observed by the aid of the microscope, when drops of blood are
seen to consist of a colourless liquid in which red corpuscles,
invisible to the naked eye owing to their small size, are floating
about. It is these corpuscles which give blood its peculiar
colour. Milk is also a transparent liquid, in which microscopical
drops of fat are floating, which rise to the top when milk is
left at rest, forming cream. It is possible to extract from every
non-homogeneous substance those homogeneous substances of which
it is made up. Thus orthoclase may he separated from porphyry by
breaking it off. So also gold is extracted from auriferous sand by
washing away the mixture of clay and sand. Chemistry deals only
with the homogeneous substances met with in nature, or extracted
from natural or artificial non-homogeneous substances. The various
mixtures found in nature form the subjects of other natural
sciences--as geognosy, botany, zoology, anatomy, &c.

[3] All those events which are accomplished by substances in time are
termed 'phenomena.' Phenomena in themselves form the fundamental
subject of the study of physics. Motion is the primary and
most generally understood form of phenomenon, and therefore we
endeavour to reason about other phenomena as clearly as when
dealing with motion. For this reason mechanics, which treats of
motion, forms the fundamental science of natural philosophy, and
all other sciences endeavour to reduce the phenomena with which
they are concerned to mechanical principles. Astronomy was the
first to take to this path of reasoning, and succeeded in many
cases in reducing astronomical to purely mechanical phenomena.
Chemistry and physics, physiology and biology are proceeding in
the same direction. One of the most important questions of all
natural science, and one which has been handed down from the
philosophers of classic times, is, whether the comprehension of
all that is visible can be reduced to motion? Its participation
in all, from the 'fixed' stars to the most minute parts of the
coldest bodies (Dewar, in 1894 showed that many substances cooled
to -180° fluoresce more strongly than at the ordinary temperature;
_i.e._ that there is a motion in them which produces light) must
now be recognised as undoubtable from direct experiment and
observation, but it does not follow from this that by motion alone
can all be explained. This follows, however, from the fact that
we cannot apprehend motion otherwise than by recognising matter
in a state of motion. If light and electricity be understood as
particular forms of motion, then we must inevitably recognise
the existence of a peculiar luminiferous (universal) ether as
a material, transmitting this form of motion. And so, under
the present state of knowledge, it is inevitably necessary to
recognise the particular categories, motion and matter, and as
chemistry is more closely concerned with the various forms of the
latter, it should, together with mechanics or the study of motion,
lie at the basis of natural science.

[4] The verb 'to react' means to act or change chemically.

[5] If a phenomenon proceeds at visible or measurable distances (as,
for instance, magnetic attraction or gravity), it cannot be
described as chemical, since these phenomena only take place at
distances immeasurably small and undistinguishable to the eye or
the microscope; that is to say, they are purely molecular.

1. _Combination_ is a reaction in which the union of two substances yields a new one, or in general terms, from a given number of substances, a lesser number is obtained. Thus, by heating a mixture of iron and sulphur[6] a single new substance is produced, iron sulphide, in which the constituent substances cannot be distinguished even by the highest magnifying power. Before the reaction, the iron could be separated from the mixture by a magnet, and the sulphur by dissolving it in certain oily liquids;[7] in general, before combination they might be mechanically separated from each other, but after combination both substances penetrate into each other, and are then neither mechanically separable nor individually distinguishable. As a rule, reactions of direct combination are accompanied by an evolution of heat, and the common case of combustion, evolving heat, consists in the combination of combustible substances with a portion (oxygen) of the atmosphere, the gases and vapours contained in the smoke being the products of combination.

2. Reactions of _decomposition_ are cases the reverse of those of combination, that is, in which one substance gives two--or, in general, a given number of substances a greater number. Thus, by heating wood (and also coal and many animal or vegetable substances) without access to air, a combustible gas, a watery liquid, tar, and carbon are obtained. It is in this way that tar, illuminating gas, and charcoal are prepared on a large scale.[8] All limestones, for example, flagstones, chalk, or marble, are decomposed by heating to redness into lime and a peculiar gas called carbonic anhydride. A similar decomposition, taking place, however, at a much lower temperature, proceeds with the green copper carbonate which is contained in natural malachite. This example will be studied more in detail presently. Whilst heat is evolved in the ordinary reactions of combination, it is, on the contrary, absorbed in the reactions of decomposition.

3. The third class of chemical reactions--where the number of re-acting substances is equal to the number of substances formed--may be considered as a simultaneous decomposition and combination. If, for instance, two compounds A and B are taken and they react on each other to form the substances C and D, then supposing that A is decomposed into D and E, and that E combines with B to form C, we have a reaction in which two substances A, or D E, and B were taken and two others C, or E B, and D were produced. Such reactions ought to be placed under the general term of reactions of '_rearrangement_,' and the particular case where two substances give two fresh ones, reactions of '_substitution_.'[9] Thus, if a piece of iron be immersed in a solution of blue vitriol (copper sulphate), copper is formed--or, rather, separated out, and green vitriol (iron sulphate, which only differs from the blue vitriol in that the iron has replaced the copper) is obtained in solution. In this manner iron may be coated with copper, so also copper with silver; such reactions are frequently made use of in practice.

[6] For this purpose a piece of iron may be made red hot in a forge,
and then placed in contact with a lump of sulphur, when iron
sulphide will be obtained as a molten liquid, the combination
being accompanied by a visible increase in the glow of the iron.
Or else iron filings are mixed with powdered sulphur in the
proportion of 5 parts of iron to 3 parts of sulphur, and the
mixture placed in a glass tube, which is then heated in one place.
Combination does not commence without the aid of external heat,
but when once started in any portion of the mixture it extends
throughout the entire mass, because the portion first heated
evolves sufficient heat in forming iron sulphide to raise the
adjacent parts of the mixture to the temperature required for
starting the reaction. The rise in temperature thus produced is so
high as to soften the glass tube.

[7] Sulphur is slightly soluble in many thin oils; it is very soluble
in carbon bisulphide and in some other liquids. Iron is insoluble
in carbon bisulphide, and the sulphur therefore can be dissolved
away from the iron.

[8] Decomposition of this kind is termed 'dry distillation,' because,
as in distillation, the substance is heated and vapours are
given off which, on cooling, condense into liquids. In general,
decomposition, in absorbing heat, presents much in common to a
physical change of state--such as, for example, that of a liquid
into a gas. Deville likened complete decomposition to boiling, and
compared partial decomposition, when a portion of a substance is
not decomposed in the presence of its products of decomposition
(or dissociation), to evaporation.

[9] A reaction of rearrangement may in certain cases take place with
one substance only; that is to say, a substance may by itself
change into a new isomeric form. Thus, for example, if hard yellow
sulphur be heated to a temperature of 250° and then poured into
cold water it gives, on cooling, a soft, brown variety. Ordinary
phosphorus, which is transparent, poisonous, and phosphorescent
in the dark (in the air), gives, after being heated at 270° (in
an atmosphere incapable of supporting combustion, such as steam),
an opaque, red, and non-poisonous isomeric variety, which is not
phosphorescent. Cases of isomerism point out the possibility of an
internal rearrangement in a substance, and are the result of an
alteration in the grouping of the same elements, just as a certain
number of balls may be grouped in figures and forms of different
shapes.

The majority of the chemical changes which occur in nature and are made use of technically are very complicated, as they consist of an association of many separate and simultaneous combinations, decompositions, and replacements. It is chiefly due to this natural complexity of chemical phenomena that for so many centuries chemistry did not exist as an exact science; that is so say, that although many chemical changes were known and made use of,[10] yet their real nature was unknown, nor could they be predicted or directed at will. Another reason for the tardy progress of chemical knowledge is the participation of gaseous substances, especially air, in many reactions. The true comprehension of air as a ponderable substance, and of gases in general as peculiar elastic and dispersive states of matter, was only arrived at in the sixteenth and seventeenth centuries, and it was only after this that the transformations of substances could form a science. Up to that time, without understanding the invisible and yet ponderable gaseous and vaporous states of substances, it was impossible to obtain any fundamental chemical evidence, because gases escaped from notice between the reacting and resultant substances. It is easy from the impression conveyed to us by the phenomena we observe to form the opinion that matter is created and destroyed: a whole mass of trees burn, and there only remains a little charcoal and ash, whilst from one small seed there grows little by little a majestic tree. In one case matter seems to be destroyed, and in the other to be created. This conclusion is arrived at because the formation or consumption of gases, being under the circumstances invisible to the eye, is not observed. When wood burns it undergoes a chemical change into gaseous products, which escape as smoke. A very simple experiment will prove this. By collecting the smoke it may be observed that it contains gases which differ entirely from air, being incapable of supporting combustion or respiration. These gases may be weighed, and it will then be seen that their weight exceeds that of the wood taken. This increase in weight arises from the fact that, in burning, the component parts of the wood combine with a portion of the air; in like manner iron increases in weight by rusting. In burning gunpowder its substance is not destroyed, but only converted into gases and smoke. So also in the growth of a tree; the seed does not increase in mass of itself and from itself, but grows because it absorbs gases from the atmosphere and sucks water and substances dissolved therein from the earth through its roots. The sap and solid substances which give plants their form are produced from these absorbed gases and liquids by complicated chemical processes. The gases and liquids are converted into solid substances by the plants themselves. Plants not only do not increase in size, but die, in a gas which does not contain the constituents of air. When moist substances dry they decrease in weight; when water evaporates we know that it does not disappear, but will return from the atmosphere as rain, dew, and snow. When water is absorbed by the earth, it does not disappear there for ever, but accumulates somewhere underground, from whence it afterwards flows forth as a spring. Thus matter does not disappear and is not created, but only undergoes various physical and chemical transformations--that is to say, changes its locality and form. Matter remains on the earth in the same quantity as before; in a word it is, so far as we are concerned, everlasting. It was difficult to submit this simple and primary truth of chemistry to investigation, but when once made clear it rapidly spread, and now seems as natural and simple as many truths which have been acknowledged for ages. Mariotte and other savants of the seventeenth century already suspected the existence of the law of the indestructibility of matter, but they made no efforts to express it or to apply it to the requirements of science. The experiments by means of which this simple law was arrived at were made during the latter half of the last century by the founder of modern chemistry, LAVOISIER, the French Academician and tax farmer. The numerous experiments of this savant were conducted with the aid of the balance, which is the only means of directly and accurately determining the quantity of matter.

[10] Thus the ancients knew how to convert the juice of grapes
containing the saccharine principle (glucose) into wine or
vinegar, how to extract metals from the ores which are found
in the earth's crust, and how to prepare glass from earthy
substances.

Lavoisier found, by weighing all the substances, and even the apparatus, used in every experiment, and then weighing the substances obtained after the chemical change, that the sum of the weights of the substances formed was always equal to the sum of the weights of the substances taken; or, in other words: MATTER IS NOT CREATED AND DOES NOT DISAPPEAR, or that, _matter is everlasting_. This expression naturally includes a hypothesis, but our only aim in using it is to concisely express the following lengthy period--That in all experiments, and in all the investigated phenomena of nature, it has never been observed that the weight of the substances formed was less or greater (as far as accuracy of weighing permits[11]) than the weight of the substances originally taken, and as weight is proportional to mass[11 bis] or quantity of matter, it follows that no one has ever succeeded in observing a disappearance of matter or its appearance in fresh quantities. The law of the indestructibility of matter endows all chemical investigations with exactitude, as, on its basis, an equation may be formed for every chemical reaction. If in any reaction the weights of the substances taken be designated by the letters A, B, C, &c., and the weights of the substances formed by the letters M, N, O, &c., then

A + B + C + ... ... ... = M + N + O + ... ... ...

Therefore, should the weight of one of the re-acting or resultant substances be unknown, it may be determined by solving the equation. The chemist, in applying the law of the indestructibility of matter, and in making use of the chemical balance, must never lose sight of any one of the re-acting or resultant substances. Should such an over-sight be made, it will at once be remarked that the sum of the weights of the substances taken is unequal to the sum of the weights of the substances formed. All the progress made by chemistry during the end of the last, and in the present, century is entirely and immovably founded on the law of the indestructibility of matter. It is absolutely necessary in beginning the study of chemistry to become familiar with the simple truth which is expressed by this law, and for this purpose several examples elucidating its application will now be cited.

[11] The experiments conducted by Staas (described in detail in Chap.
XXIV. on Silver) form some of the accurate researches, proving
that the weight of matter is not altered in chemical reactions,
because he accurately weighed (introducing all the necessary
corrections) the reacting and resultant substances. Landolt
(1893) carried on various reactions in inverted and sealed glass
U-tubes, and on weighing the tubes before reaction (when the
reacting solutions were separated in each of the branches of
the tubes), and after (when the solutions had been well mixed
together by shaking), found that either the weight remained
perfectly constant or that the variation was so small (for
instance, 0·2 milligram in a total weight of about a million
milligrams) as to be ascribed to the inevitable errors of
weighing.

[11 bis] The idea of the mass of matter was first shaped into an exact
form by Galileo (died 1642), and more especially by Newton (born
1643, died 1727), in the glorious epoch of the development of
the principles of inductive reasoning enunciated by Bacon and
Descartes in their philosophical treatises. Shortly after the
death of Newton, Lavoisier, whose fame in natural philosophy
should rank with that of Galileo and Newton, was born on August
26, 1743. The death of Lavoisier occurred during the Reign
of Terror of the French Revolution, when he, together with
twenty-six other chief farmers of the revenue, was guillotined
on May 8, 1794, at Paris; but his works and ideas have made him
immortal.

1. It is well known that iron rusts in damp air,[12] and that when heated to redness in air it becomes coated with scoria (oxide), having, like rust, the appearance of an earthy substance resembling some of the iron ores from which metallic iron is extracted. If the iron is weighed before and after the formation of the scoria or rust, it will be found that the metal has increased in weight during the operation.[13] It can easily be proved that this increase in weight is accomplished at the expense of the atmosphere, and mainly, as Lavoisier proved, at the expense of that portion which is called oxygen. In fact, in a vacuum, or in gases which do not contain oxygen, for instance, in hydrogen or nitrogen, the iron neither rusts nor becomes coated with scoria. Had the iron not been weighed, the participation of the oxygen of the atmosphere in its transformation into an earthy substance might have easily passed unnoticed, as was formerly the case, when phenomena like the above were, for this reason, misunderstood. It is evident from the law of the indestructibility of matter that as the iron increases in weight in its conversion into rust, the latter must be a more complex substance than the iron itself, and its formation is due to a reaction of combination. We might form an entirely wrong opinion about it, and might, for instance, consider rust to be a simpler substance than iron, and explain the formation of rust as the removal of something from the iron. Such, indeed, was the general opinion prior to Lavoisier, when it was held that iron contained a certain unknown substance called 'phlogiston,' and that rust was iron deprived of this supposed substance.

[12] By covering iron with an enamel, or varnish, or with unrustable
metals (such as nickel), or a coating of paraffin, or other
similar substances, it is protected from the air and moisture,
and so kept from rusting.

[13] Such an experiment may easily be made by taking the finest
(unrusted) iron filings (ordinary filings must be first washed
in ether, dried, and passed through a very fine sieve). The
filings thus obtained are capable of burning directly in air
(by oxidising or forming rust), especially when they hang (are
attracted) on a magnet. A compact piece of iron does not burn in
air, but spongy iron glows and smoulders like tinder. In making
the experiment, a horse-shoe magnet is fixed, with the poles
downwards, on one arm of a rather sensitive balance, and the iron
filings are applied to the magnet (on a sheet of paper) so as to
form a beard about the poles. The balance pan should be exactly
under the filings on the magnet, in order that any which might
fall from it should not alter the weight. The filings, having
been weighed, are set light to by applying the flame of a candle;
they easily take fire, and go on burning by themselves, forming
rust. When the combustion is ended, it will be clear that the
iron has increased in weight; from 5-1/2 parts by weight of iron
filings taken, there are obtained, by complete combustion, 7-1/2
parts by weight of rust.

2. Copper carbonate (in the form of a powder, or as the well-known green mineral called 'malachite,' which is used for making ornaments, or as an ore for the extraction of copper) changes into a black substance called 'copper oxide' when heated to redness.[14] This black substance is also obtained by heating copper to redness in air--that is, it is the scoria or oxidation product of copper. The weight of the black oxide of copper left is less than that of the copper carbonate originally taken, and therefore we consider the reaction which occurred to have been one of decomposition, and that by it something was separated from the green copper carbonate, and, in fact, by closing the orifice of the vessel in which the copper carbonate is heated with a well-fitting cork, through which a gas delivery tube[15] passes whose end is immersed under water, it will be observed that on heating, a gas is formed which bubbles through the water. This gas can be easily collected, as will presently be described, and it will be found to essentially differ from air in many respects; for instance, a burning taper is extinguished in it as if it had been plunged into water. If weighing had not proved to us that some substance had been separated, the formation of the gas might easily have escaped our notice, for it is colourless and transparent like air, and is therefore evolved without any striking feature. The carbonic anhydride evolved may be weighed,[16] and it will be seen that the sum of the weights of the black copper oxide and carbonic anhydride is equal to the weight of the copper carbonate[17] originally taken, and thus by carefully following out the various stages of all chemical reactions we arrive at a confirmation of the law of the indestructibility of matter.

[14] For the purpose of experiment, it is most convenient to take
copper carbonate, which may be prepared by the experimenter
himself, by adding a solution of sodium carbonate to a solution
of copper sulphate. The precipitate (deposit) so formed is
collected on a filter, washed, and dried. The decomposition of
copper carbonate into copper oxide is effected by so moderate
a heat that it may be performed in a glass vessel heated by a
lamp. For this purpose a thin glass tube, closed at one end, and
called a 'test tube,' may be employed, or else a vessel called a
'retort.' The experiment is carried on, as described in example
three on p. 11, by collecting the carbonic anhydride over water,
as will be afterwards explained.

[15] Gas delivery tubes are usually made of glass tubing of various
diameters and thicknesses. If of small diameter and thickness, a
glass tube is easily bent by heating in a gas jet or the flame
of a spirit lamp, and it may also be easily divided at a given
point by making a deep scratch with a file and then breaking the
tube at this point with a sharp jerk. These properties, together
with their impermeability, transparency, hardness, and regularity
of bore, render glass tubes most useful in experiments with
gases. Naturally they might be replaced by straws, india-rubber,
metallic, or other tubes, but these are more difficult to fix on
to a vessel, and are not entirely impervious to gases. A glass
gas delivery tube may be hermetically fixed into a vessel by
fitting it into a perforated cork, which should be soft and free
from flaws, and fixing the cork into the orifice of the vessel.
To protect the cork from the action of gases it is sometimes
previously soaked in paraffin, or it may be replaced by an
india-rubber cork.

[16] Gases, like all other substances, may be weighed, but, owing to
their extreme lightness and the difficulty of dealing with them
in large masses, they can only be weighed by very sensitive
balances; that is, in such as, with a considerable load, indicate
a very small difference in weight--for example, a centigram or a
milligram with a load of 1,000 grams. In order to weigh a gas,
a glass globe furnished with a tight-fitting stop-cock is first
of all exhausted of air by an air-pump (a Sprengel pump is the
best). The stop-cock is then closed, and the exhausted globe
weighed. If the gas to be weighed is then let into the globe,
its weight can be determined from the increase in the weight of
the globe. It is necessary, however, that the temperature and
pressure of the air about the balance should remain constant for
both weighings, as the weight of the globe in air will (according
to the laws of hydrostatics) vary with its density. The volume of
the air displaced, and its weight, must therefore be determined
by observing the temperature, density, and moisture of the
atmosphere during the time of experiment. This will be partly
explained later, but may be studied more in detail by physics.
Owing to the complexity of all these operations, the mass of a
gas is usually determined from its volume and density, or from
the weight of a known volume.

[17] The copper carbonate should be dried before weighing, as
otherwise--besides copper oxide and carbonic anhydride--water
will be obtained in the decomposition. Water forms a part of the
composition of malachite, and has therefore to be taken into
consideration. The water produced in the decomposition may be all
collected by absorbing it in sulphuric acid or calcium chloride,
as will be described further on. In order to dry a salt it must
be heated at about 100° until its weight remains constant, or
be placed under an air pump over sulphuric acid, as will also
be presently described. As water is met with almost everywhere,
and as it is absorbed by many substances, the possibility of its
presence should never be lost sight of.

3. Red mercury oxide (which is formed as mercury rust by heating mercury in air) is decomposed like copper carbonate (only by heating more slowly and at a somewhat higher temperature), with the formation of the peculiar gas, oxygen. For this purpose the mercury oxide is placed in a glass tube or retort,[18] to which a gas delivery tube is attached by means of a cork. This tube is bent downwards, as shown in the drawing (Fig. 1). The open end of the gas delivery tube is immersed in a vessel filled with water, called a pneumatic trough.[19] When the gas begins to be evolved in the retort it is obliged, having no other outlet, to escape through the gas delivery tube into the water in the pneumatic trough, and therefore its evolution will be rendered visible by the bubbles coming from this tube. In heating the retort containing the mercury oxide, the air contained in the apparatus is first partly expelled, owing to its expansion by heat, and then the peculiar gas called 'oxygen' is evolved, and may be easily collected as it comes off. For this purpose a vessel (an ordinary cylinder, as in the drawing) is filled quite full with water and its mouth closed; it is then inverted and placed in this position under the water in the trough; the mouth is then opened. The cylinder will remain full of water--that is, the water will remain at a higher level in it than in the surrounding vessel, owing to the atmospheric pressure. The atmosphere presses on the surface of the water in the trough, and prevents the water from flowing out of the cylinder. The mouth of the cylinder is placed over the end of the gas delivery tube,[20] and the bubbles issuing from it will rise into the cylinder and displace the water contained in it. Gases are generally collected in this manner. When a sufficient quantity of gas has accumulated in the cylinder it can be clearly shown that it is not air, but another gas which is distinguished by its capacity for vigorously supporting combustion. In order to show this, the cylinder is closed, under water, and removed from the bath; its mouth is then turned upwards, and a smouldering taper plunged into it. As is well known, a smouldering taper will be extinguished in air, but in the gas which is given off from red mercury oxide it burns clearly and vigorously, showing the property possessed by this gas for supporting combustion more energetically than air, and thus enabling it to be distinguished from the latter. It may be observed in this experiment that, besides the formation of oxygen, metallic mercury is formed, which, volatilising at the high temperature required for the reaction, condenses on the cooler parts of the retort as a mirror or in globules. Thus two substances, mercury and oxygen, are obtained by heating red mercury oxide. In this reaction, from one substance, two new substances are produced--that is, a decomposition has taken place. The means of collecting and investigating gases were known before Lavoisier's time, but he first showed the real part they played in the processes of many chemical changes which before his era were either wrongly understood (as will be afterwards explained) or were not explained at all, but only observed in their superficial aspects. This experiment on red mercury oxide has a special significance in the history of chemistry contemporary with Lavoisier, because the oxygen gas which is here evolved is contained in the atmosphere, and plays a most important part in nature, especially in the respiration of animals, in combustion in air, and in the formation of rusts or scoriæ (earths, as they were then called) from metals--that is, of earthy substances, like the ores from which metals are extracted.

[18] As the decomposition of red oxide of mercury requires so high a
temperature, near redness, as to soften ordinary glass, it is
necessary for this experiment to take a retort (or test tube)
made of hard glass, which is able to stand high temperatures
without softening. For the same reason, the lamp used must give
a strong heat and a large flame, capable of embracing the whole
bottom of the retort, which should be as small as possible for
the convenience of the experiment.

[19]

The pneumatic trough may naturally be made of any material
(china, earthenware, or metal, &c.), but usually a glass one,
as shown in the drawing, is used, as it allows the progress
of the experiment to be better observed. For this reason, as
well as the ease with which they are kept clean, and from the
fact also that glass is not acted on by many substances which
affect other materials (for instance, metals), glass vessels
of all kinds--such as retorts, test tubes, cylinders, beakers,
flasks, globes, &c.--are preferred to any other for chemical
experiments. Glass vessels may be heated without any danger if
the following precautions be observed: 1st, they should be made
of thin glass, as otherwise they are liable to crack from the bad
heat-conducting power of glass; 2nd, they should be surrounded
by a liquid or with sand (Fig. 2), or sand bath as it is called;
or else should stand in a current of hot gases without touching
the fuel from which they proceed, or in the flame of a smokeless
lamp. A common candle or lamp forms a deposit of soot on a
cold object placed in their flames. The soot interferes with
the transmission of heat, and so a glass vessel when covered
with soot often cracks. And for this reason spirit lamps, which
burn with a smokeless flame, or gas burners of a peculiar
construction, are used. In the Bunsen burner the gas is mixed
with air, and burns with a non-luminous and smokeless flame.
On the other hand, if an ordinary lamp (petroleum or benzine)
does not smoke it may be used for heating a glass vessel without
danger, provided the glass is placed well above the flame in the
current of hot gases. In all cases, the heating should be begun
very carefully by raising the temperature by degrees.

[20] In order to avoid the necessity of holding the cylinder, its open
end is widened (and also ground so that it may be closely covered
with a ground-glass plate when necessary), and placed on a stand
below the level of the water in the bath. This stand is called
'the bridge.' It has several circular openings cut through it,
and the gas delivery tube is placed under one of these, and the
cylinder for collecting the gas over it.

4. In order to illustrate by experiment one more example of chemical change and the application of the law of the indestructibility of matter, we will consider the reaction between common table salt and lunar caustic, which is well known from its use in cauterising wounds. By taking a clear solution of each and mixing them together, it will at once be observed that a solid white substance is formed, which settles to the bottom of the vessel, and is insoluble in water. This substance may be separated from the solution by filtering; it is then found to be an entirely different substance from either of those taken originally in the solutions. This is at once evident from the fact that it does not dissolve in water. On evaporating the liquid which passed through the filter, it will be found to contain a new substance unlike either table salt or lunar caustic, but, like them, soluble in water. Thus table salt and lunar caustic, two substances soluble in water, produced, by their mutual chemical action, two new substances, one insoluble in water, and the other remaining in solution. Here, from two substances, two others are obtained, consequently there occurred a reaction of substitution. The water served only to convert the re-acting substances into a liquid and mobile state. If the lunar caustic and salt be dried[21] and weighed, and if about 58-1/2 grams[22] of salt and 170 grams of lunar caustic be taken, then 143-1/2 grams of insoluble silver chloride and 85 grams of sodium nitrate will be obtained. The sum of the weights of the re-acting and resultant substances are seen to be similar and equal to 228-1/2 grams, which necessarily follows from the law of the indestructibility of matter.

[21] Drying is necessary in order to remove any water which may be held
in the salts (_see_ Note 17, and Chapter I., Notes 13 and 14).

[22] The exact weights of the re-acting and resulting substances are
determined with the greatest difficulty, not only from the
possible inexactitude of the balance (every weighing is only
correct within the limits of the sensitiveness of the balance)
and weights used in weighing, not only from the difficulty in
making corrections for the weight of air displaced by the vessels
holding the substances weighed and by the weights themselves, but
also from the hygroscopic nature of many substances (and vessels)
causing absorption of moisture from the atmosphere, and from the
difficulty in not losing any of the substance to be weighed in
the several operations (filtering, evaporating, and drying, &c.)
which have to be performed before arriving at a final result. All
these circumstances have to be taken into consideration in exact
researches, and their elimination requires very many special
precautions which are impracticable in preliminary experiments.

Accepting the truth of the above law, the question naturally arises as to whether there is any limit to the various chemical transformations, or are they unrestricted in number--that is to say, is it possible from a given substance to obtain an equivalent quantity of any other substance? In other words, does there exist a perpetual and infinite change of one kind of material into every other kind, or is the cycle of these transformations limited? This is the second essential problem of Chemistry, a question of quality of matter, and one, it is evident, which is more complicated than the question of quantity. It cannot be solved by a mere superficial glance at the subject. Indeed, on seeing how all the varied forms and colours of plants are built up from air and the elements of the soil, and how metallic iron can be transformed into colours such as inks and Prussian blue, we might be led to think that there is no end to the qualitative changes to which matter is susceptible. But, on the other hand, the experiences of everyday life compel us to acknowledge that food cannot be made out of a stone, or gold out of copper. Thus a definite answer can only be looked for in a close and diligent study of the subject, and the problem has been resolved in different way at different times. In ancient times the opinion most generally held was that everything visible was composed of four elements--Air, Water, Earth, and Fire. The origin of this doctrine can be traced far back into the confines of Asia, whence it was handed down to the Greeks, and most fully expounded by Empedocles, who lived before 460 B.C. This doctrine was not the result of exact research, but apparently owes its origin to the clear division of bodies into gases (like air), liquids (like water), and solids (like the earth). The Arabs appear to have been the first who attempted to solve the question by experimental methods, and they introduced, through Spain, the taste for the study of similar problems into Europe, where from that time there appear many adepts in chemistry, which was considered as an unholy art, and called 'alchemy.' As the alchemists were ignorant of any exact law which could guide them in their researches, they obtained most anomalous results. Their chief service to chemistry was that they made a number of experiments, and discovered many new chemical transformations; but it is well known how they solved the fundamental problem of chemistry. Their view may be taken as a positive acknowledgment of the infinite transmutability of matter, for they aimed at discovering the Philosopher's Stone, capable of converting everything into gold and diamonds, and of making the old young again. This solution of the question was afterwards completely overthrown, but it must not, for this reason, be thought that the hopes held by the alchemists were only the fruit of their imaginations. The first chemical experiments might well lead them to their conclusions. They took, for instance, the bright metallic mineral galena, and extracted metallic lead from it. Thus they saw that from a metallic substance which is unfitted for use they could obtain another metallic substance which is ductile and valuable for many technical purposes. Furthermore, they took this lead and obtained silver, a still more valuable metal, from it. Thus they might easily conclude that it was possible to ennoble metals by means of a whole series of transmutations--that is to say, to obtain from them those which are more and more precious. Having got silver from lead, they assumed that it would be possible to obtain gold from silver. The mistake they made was that they never weighed or measured the substances used or produced in their experiments. Had they done so, they would have learnt that the weight of the lead was much less than that of the galena from which it was obtained, and the weight of the silver infinitesimal compared with that of the lead. Had they looked more closely into the process of the extraction of the silver from lead (and silver at the present time is chiefly obtained from the lead ores) they would have seen that the lead does not change into silver, but that it only contains a certain small quantity of it, and this amount having once been separated from the lead it cannot by any further operation give more. The silver which the alchemists extracted from the lead was in the lead, and was not obtained by a chemical change of the lead itself. This is now well known from experiment, but the first view of the nature of the process was very likely to be an erroneous one.[23] The methods of research adopted by the alchemists could give but little success, for they did not set themselves clear and simple questions whose answers would aid them to make further progress. Thus though they did not arrive at any exact law, they left nevertheless numerous and useful experimental data as an inheritance to chemistry; they investigated, in particular, the transformations proper to metals, and for this reason chemistry was for long afterwards entirely confined to the study of metallic substances.

[23] Besides which, in the majority of cases, the first explanation of
most subjects which do not repeat themselves in everyday
experience under various aspects, but always in one form, or
only at intervals and infrequently, is usually wrong. Thus
the daily evidence of the rising of the sun and stars evokes
the erroneous idea that the heavens move and the earth stands
still. This apparent truth is far from being the real truth,
and, as a matter of fact, is contradictory to it. Similarly, an
ordinary mind and everyday experience concludes that iron is
incombustible, whereas it burns not only as filings, but even as
wire, as we shall afterwards see. With the progress of knowledge
very many primitive prejudices have been obliged to give way to
true ideas which have been verified by experiment. In ordinary
life we often reason at first sight with perfect truth, only
because we are taught a right judgment by our daily experience.
It is a necessary consequence of the nature of our minds to reach
the attainment of truth through elementary and often erroneous
reasoning and through experiment, and it would be very wrong
to expect a knowledge of truth from a simple mental effort.
Naturally, experiment itself cannot give truth, but it gives the
means of destroying erroneous representations whilst confirming
those which are true in all their consequences.

In their researches, the alchemists frequently made use of two chemical processes which are now termed 'reduction' and 'oxidation.' The rusting of metals, and in general their conversion from a metallic into an earthy form, is called 'oxidation,' whilst the extraction of a metal from an earthy substance is called 'reduction.' Many metals--for instance, iron, lead, and tin--are oxidised by heating in air alone, and may be again reduced by heating with carbon. Such oxidised metals are found in the earth, and form the majority of metallic ores. The metals, such as tin, iron, and copper, may be extracted from these ores by heating them together with carbon. All these processes were well studied by the alchemists. It was afterwards shown that all earths and minerals are formed of similar metallic rusts or oxides, or of their combinations. Thus the alchemists knew of two forms of chemical changes: the oxidation of metals and the reduction of the oxides so formed into metals. The explanation of the nature of these two classes of chemical phenomena was the means for the discovery of the most important chemical laws. The first hypothesis on their nature is due to Becker, and more particularly to Stahl, a surgeon to the King of Prussia. Stahl writes in his 'Fundamenta Chymiæ,' 1723, that all substances consist of an imponderable fiery substance called 'phlogiston' (materia aut principium ignis non ipse ignis), and of another element having particular properties for each substance. The greater the capacity of a body for oxidation, or the more combustible it is, the richer it is in phlogiston. Carbon contains it in great abundance. In oxidation or combustion phlogiston is emitted, and in reduction it is consumed or enters into combination. Carbon reduces earthy substances because it is rich in phlogiston, and gives up a portion of its phlogiston to the substance reduced. Thus Stahl supposed metals to be compound substances consisting of phlogiston and an earthy substance or oxide. This hypothesis is distinguished for its very great simplicity, and for this and other reasons it acquired many supporters.[24]