THE RECOGNITION OF CHEMICAL CHANGES AS THE INTERACTIONS OF DEFINITE
SUBSTANCES.
The experimental study of combustion made by Lavoisier proved the
correctness of that part of Stahl's phlogistic theory which asserted
that all processes of combustion are very similar, but also proved
that this likeness consists in the combination of a distinct gaseous
substance with the material undergoing combustion, and not in the
escape therefrom of the _Principle of fire_, as asserted by the theory
of Stahl. After about the year 1790, it was necessary to think of
combustions in the air as combinations of a particular gas, or _air_,
with the burning substances, or some portions of them.
This description of processes of burning necessarily led to a
comparison of the gaseous constituent of the atmosphere which played
so important a part in these processes, with the substances which were
burned; it led to the examination of the compositions of many
substances, and made it necessary to devise a language whereby these
compositions could be stated clearly and consistently.
We have seen, in former chapters, the extreme haziness of the
alchemical views of composition, and the connexions between
composition and properties. Although Boyle[7] had stated very lucidly
what he meant by the composition of a definite substance, about a
century before Lavoisier's work on combustion, nevertheless the views
of chemists concerning composition remained very vague and incapable
of definite expression, until the experimental investigations of
Lavoisier enabled him to form a clear mental picture of chemical
changes as interactions between definite quantities of distinct
substances.
[7] Boyle said, in 1689, "I mean by elements ... certain
primitive and simple, or perfectly unmixed bodies; which not
being made of any other bodies, or of one another, are the
ingredients of which all those called perfectly mixt bodies are
immediately compounded, and into which they are ultimately
resolved."
Let us consider some of the work of Lavoisier in this direction. I
select his experimental examination of the interactions of metals and
acids.
Many experimenters had noticed that gases (or airs, as they were
called up till near the end of the 18th century) are generally
produced when metals are dissolving in acids. Most of those who
noticed this said that the gases came from the dissolving metals;
Lavoisier said they were produced by the decomposition of the acids.
In order to study the interaction of nitric acid and mercury,
Lavoisier caused a weighed quantity of the metal to react with a
weighed quantity of the acid, and collected the gas which was
produced; when all the metal had dissolved, he evaporated the liquid
until a white solid was obtained; he heated this solid until it was
changed to the red substance called, at that time, _red precipitate_,
and collected the gas produced. Finally, Lavoisier strongly heated the
red precipitate; it changed to a gas, which he collected, and mercury,
which he weighed.
The weight of the mercury obtained by Lavoisier at the end of this
series of changes was the same, less a few grains, as the weight of
the mercury which he had caused to react with the nitric acid. The gas
obtained during the solution of the metal in the acid, and during the
decomposition of the white solid by heat, was the same as a gas which
had been prepared by Priestley and called by him _nitrous air_; and
the gas obtained by heating the red precipitate was found to be
oxygen. Lavoisier then mixed measured volumes of oxygen and "nitrous
air," standing over water; a red gas was formed, and dissolved in the
water, and Lavoisier proved that the water now contained nitric acid.
The conclusions regarding the composition of nitric acid drawn by
Lavoisier from these experiments was, that "nitric acid is nothing
else than _nitrous air_, combined with almost its own volume of the
purest part of atmospheric air, and a considerable quantity of water."
Lavoisier supposed that the stages in the complete reaction between
mercury and nitric acid were these: the withdrawal of oxygen from the
acid by the mercury, and the union of the compound of mercury and
oxygen thus formed with the constituents of the acid which remained
when part of its oxygen was taken away. The removal of oxygen from
nitric acid by the mercury produced _nitrous air_; when the product of
the union of the oxide of mercury and the nitric acid deprived of part
of its oxygen was heated, more nitrous air was given off, and oxide of
mercury remained, and was decomposed, at a higher temperature, into
mercury and oxygen.
Lavoisier thought of these reactions as the tearing asunder, by
mercury, of nitric acid into definite quantities of its three
components, themselves distinct substances, nitrous air, water, and
oxygen; and the combination of the mercury with a certain measurable
quantity of one of these components, namely, oxygen, followed by the
union of this compound of mercury and oxygen with what remained of the
components of nitric acid.
Lavoisier had formed a clear, consistent, and suggestive mental
picture of chemical changes. He thought of a chemical reaction as
always the same under the same conditions, as an action between a
fixed and measurable quantity of one substance, having definite and
definable properties, with fixed and measurable quantities of other
substances, the properties of each of which were definite and
definable.
Lavoisier also recognised that certain definite substances could be
divided into things simpler than themselves, but that other substances
refused to undergo simplification by division into two or more unlike
portions. He spoke of the object of chemistry as follows:--[8] "In
submitting to experiments the different substances found in nature,
chemistry seeks to decompose these substances, and to get them into
such conditions that their various components may be examined
separately. Chemistry advances to its end by dividing, sub-dividing,
and again sub-dividing, and we do not know what will be the limits of
such operations. We cannot be certain that what we regard as simple
to-day is indeed simple; all we can say is, that such a substance is
the actual term whereat chemical analysis has arrived, and that with
our present knowledge we cannot sub-divide it."
[8] I have given a free rendering of Lavoisier's words.
In these words Lavoisier defines the chemical conception of
_elements_; since his time an element is "the actual term whereat
chemical analysis has arrived," it is that which "with our present
knowledge we cannot sub-divide"; and, as a working hypothesis, the
notion of _element_ has no wider meaning than this. I have already
quoted Boyle's statement that by _elements_ he meant "certain
primitive and simple bodies ... not made of any other bodies, or of
one another." Boyle was still slightly restrained by the alchemical
atmosphere around him; he was still inclined to say, "this _must_ be
the way nature works, she _must_ begin with certain substances which
are absolutely simple." Lavoisier had thrown off all the trammels
which hindered the alchemists from making rigorous experimental
investigations. If one may judge from his writings, he had not
struggled to free himself from these trammels, he had not slowly
emerged from the quagmires of alchemy, and painfully gained firmer
ground; the extraordinary clearness and directness of his mental
vision had led him straight to the very heart of the problems of
chemistry, and enabled him not only calmly to ignore all the machinery
of Elements, Principles, Essences, and the like, which the alchemists
had constructed so laboriously, but also to construct, in place of
that mechanism which hindered inquiry, genuine scientific hypotheses
which directed inquiry, and were themselves altered by the results of
the experiments they had suggested.
Lavoisier made these great advances by applying himself to the minute
and exhaustive examination of a few cases of chemical change, and
endeavouring to account for everything which took part in the
processes he studied, by weighing or measuring each distinct substance
which was present when the change began, and each which was present
when the change was finished. He did not make haphazard experiments;
he had a method, a system; he used hypotheses, and he used them
rightly. "Systems in physics," Lavoisier writes, "are but the proper
instruments for helping the feebleness of our senses. Properly
speaking, they are methods of approximation which put us on the track
of solving problems; they are the hypotheses which, successively
modified, corrected, and changed, by experience, ought to conduct us,
some day, by the method of exclusions and eliminations, to the
knowledge of the true laws of nature."
In a memoir wherein he is considering the production of carbonic acid
and alcohol by the fermentation of fruit-juice, Lavoisier says, "It is
evident that we must know the nature and composition of the
substances which can be fermented and the products of fermentation;
for nothing is created, either in the operations of art or in those of
nature; and it may be laid down that the quantity of material present
at the beginning of every operation is the same as the quantity
present at the end, that the quality and quantity of the principles[9]
are the same, and that nothing happens save certain changes, certain
modifications. On this principle is based the whole art of
experimenting in chemistry; in all chemical experiments we must
suppose that there is a true equality between the principles[10] of
the substances which are examined and those which are obtained from
them by analysis."
[9, 10] Lavoisier uses the word _principle_, here and
elsewhere, to mean a definite homogeneous substance; he uses it
as synonymous with the more modern terms element and compound.
If Lavoisier's memoirs are examined closely, it is seen that at the
very beginning of his chemical inquiries he assumed the accuracy, and
the universal application, of the generalisation "nothing is created,
either in the operations of art or in those of nature." Naturalists
had been feeling their way for centuries towards such a generalisation
as this; it had been in the air for many generations; sometimes it was
almost realised by this or that investigator, then it escaped for long
periods. Lavoisier seems to have realised, by what we call intuition,
that however great and astonishing may be the changes in the
properties of the substances which mutually react, there is no change
in the total quantity of material.
Not only did Lavoisier realise and act on this principle, he also
measured quantities of substances by the one practical method, namely,
by weighing; and by doing this he showed chemists the only road along
which they could advance towards a genuine knowledge of material
changes.
The generalisation expressed by Lavoisier in the words I have quoted
is now known as the _law of the conservation of mass_; it is generally
stated in some such form as this:--the sum of the masses of all the
homogeneous substances which take part in a chemical (or physical)
change does not itself change. The science of chemistry rests on this
law; every quantitative analysis assumes the accuracy, and is a proof
of the validity, of it.[11]
[11] I have considered the law of the conservation of mass in some
detail in Chapter IV. of _The Story of the Chemical Elements_.
By accepting the accuracy of this generalisation, and using it in
every experiment, Lavoisier was able to form a clear mental picture of
a chemical change as the separation and combination of homogeneous
substances; for, by using the balance, he was able to follow each
substance through the maze of changes, to determine when it united
with other substances, and when it separated into substances simpler
than itself.