ESTABLISHMENT OF CHEMISTRY AS A SCIENCE--PERIOD OF BLACK, PRIESTLEY AND
LAVOISIER.
_Joseph Black_, 1728-1799. _Joseph, Priestley_,
1733-1804. _Antoine Laurent Lavoisier_, 1743-1794.
During this period of advance, which may be broadly stated as comprising
the last half of the eighteenth century, the aim and scope of chemical
science were clearly indicated by the labours of Black, Priestley and
Lavoisier. The work of these men dealt chiefly with the process of
combustion. Black and Priestley finally proved the existence of airs or
gases different from common air, and Lavoisier applied these discoveries to
give a clear explanation of what happens when a substance burns.
* * * * *
JOSEPH BLACK was born near Bordeaux in the year 1728. His father was of
Scottish family, but a native of Belfast; his mother was the daughter of
Mr. Gordon, of Hilhead in Aberdeenshire. We are told by Dr. Robison, in
his preface to Black's Lectures, that John Black, the father of Joseph, was
a man "of most amiable manners, candid and liberal in his sentiments, and
of no common information."
At the age of twelve Black was sent home to a school at Belfast; after
spending six years there he went to the University of Glasgow in the year
1746. Little is known of his progress at school or at the university, but
judging from his father's letters, which his son preserved, he seems to
have devoted himself to study. While at Glasgow he was attracted to the
pursuit of physical science, and chose medicine as a profession. Becoming a
pupil of Dr. Cullen, he was much impressed with the importance of chemical
knowledge to the student of medicine. Dr. Cullen appears to have been one
of the first to take large and philosophical views of the scope of chemical
science, and to attempt to raise chemistry from the rank of a useful art to
that of a branch of natural philosophy. Such a man must have been attracted
by the young student, whose work was already at once accurate in detail and
wide in general scope.
In the notes of work kept by Black at this time are displayed those
qualities of methodical arrangement, perseverance and thoroughness which
are so prominent in his published investigations and lectures. In one place
we find, says his biographer, many disjointed facts and records of diverse
observations, but the next time he refers to the same subjects we
generally have analogous facts noted and some conclusions drawn--we have
the beginnings of knowledge. Having once entered on an investigation Black
works it out steadily until he gets definite results.
His earlier notes are concerned chiefly with heat and cold; about 1752 he
begins to make references to the subject of "fixed air."
About 1750 Black went to Edinburgh University to complete his medical
studies, and here he was again fortunate in finding a really scientific
student occupying the chair of natural philosophy.
The attention of medical men was directed at this time to the action of
limewater as a remedy for stone in the bladder. All the medicines which
were of any avail in mitigating the pain attendant on this disease more or
less resembled the "caustic ley of the soap-boilers" (or as we should now
call it caustic potash or soda). These caustic medicines were mostly
prepared by the action of quicklime on some other substance, and quicklime
was generally supposed to derive its caustic, or corrosive properties from
the fire which was used in changing ordinary limestone into quicklime.
When quicklime was heated with "fixed alkalis" (_i.e._ with potassium or
sodium carbonate), it changed these substances into caustic bodies which
had a corrosive action on animal matter; hence it was concluded that the
quicklime had derived a "power"--or some said had derived "igneous
matter"--from the fire, and had communicated this to the fixed alkalis,
which thereby acquired the property of corroding animal matter.
Black thought that he might be able to lay hold of this "igneous matter"
supposed to be taken by the limestone from the fire; but he found that
limestone loses weight when changed into quicklime. He then dissolved
limestone (or chalk) in spirits of salt (hydrochloric acid), and compared
the loss of weight undergone by the chalk in this process with the loss
suffered by an equal quantity of chalk when strongly heated. This
investigation led Black to a fuller study of the action of heat on chalk
and on "mild magnesia" (or as we now say, magnesium carbonate).
In order that his experiments might be complete and his conclusions well
established, he delayed taking the degree of Doctor of Medicine for three
years. He graduated as M. D. in 1755, and presented his thesis on "Magnesia
Alba, Quicklime and other Alkaline Substances," which contained the results
of what is probably the first accurately quantitative examination of a
chemical action which we possess.
Black prepared mild magnesia (magnesium carbonate) by boiling together
solutions of Epsom salts (magnesium sulphate) and fixed alkali (potassium
carbonate). He showed that when mild magnesia is heated--
1. It is much decreased in bulk.
2. It loses weight (twelve parts become five, according to Black).
3. It does not precipitate lime from solutions of that substance in acids
(Black had already shown that mild magnesia does precipitate lime).
He then strongly heated a weighed quantity of mild magnesia in a retort
connected with a receiver; a few drops of water were obtained in the
receiver, but the magnesia lost six or seven times as much weight as the
weight of the water produced. Black then recalls the experiments of Hales,
wherein airs other than common air had been prepared, and concludes that
the loss of weight noticed when mild magnesia is calcined is probably due
to expulsion, by the heat, of some kind of air. Dissolving some of his mild
magnesia in acid he noticed that effervescence occurred, and from this he
concluded that the same air which, according to his hypothesis, is expelled
by heat, is also driven out from the mild magnesia by the action of acid.
He then proceeded to test this hypothesis. One hundred and twenty grains of
mild magnesia were strongly calcined; the calcined matter, amounting to
seventy grains, was dissolved in dilute oil of vitriol, and this solution
was mixed with common fixed alkali (potassium carbonate). The solid which
was thus produced was collected, washed and weighed; it amounted to a
trifle less than one hundred and twenty grains, and possessed all the
properties--detailed by Black--of the original mild magnesia. But this is
exactly the result which ought to have occurred according to his
hypothesis.
The next step in the investigation was to collect the peculiar air which
Black had proved to be evolved during the calcination of mild magnesia. To
this substance he gave the name of "fixed air," because it was fixed or
held by magnesia. Black established the existence of this air in the
expired breath of animals, and also showed that it was present in the air
evolved during vinous fermentation. He demonstrated several of its
properties; among these, the fact that animals die when placed in this air.
An air with similar properties was obtained by calcining chalk. Black held
that the chemical changes which occur when chalk is calcined are exactly
analogous to those which he had proved to take place when magnesia is
strongly heated. Chalk ought therefore to lose weight when calcined; the
residue ought to neutralize an acid without evolution of any gas, and the
quantity of acid thus neutralized ought to be the same as would be
neutralized by the uncalcined chalk; lastly, it ought to be possible to
recover the uncalcined chalk by adding a fixed alkali to a solution of the
calcined chalk or quicklime.
The actual results which Black obtained were as follows:--
One hundred and twenty grains of chalk were dissolved in dilute muriatic
(hydrochloric) acid; 421 grains of the acid were needed to neutralize the
chalk, and 48 grains of fixed air were evolved. One hundred and twenty
grains of the same specimen of chalk were strongly calcined, and then
dissolved in dilute muriatic acid; 414 grains of the acid were required to
neutralize the calcined chalk. The difference between 421 and 414 is very
slight; considering the state of practical chemistry at Black's time, we
may well agree with him that he was justified in the conclusion that equal
weights of calcined and of uncalcined chalk neutralize the same amount of
acid. One hundred and twenty grains of the same specimen of chalk were
again strongly heated; the calcined chalk, amounting to 68 grains, was
digested with a solution of fixed alkali in water. The substance thus
obtained, when washed and dried, weighed 118 grains, and had all the
properties of ordinary chalk. Therefore, said Black, it is possible to
recover the whole of the chalk originally present before calcination, by
adding a fixed alkali to the calcined chalk or quicklime.
At this time it was known that water dissolves quicklime, but it was
generally held that only about one-fourth (or perhaps a little more) of any
specimen of quicklime could be dissolved by water, however much water was
employed. Black's researches had led him to regard quicklime as a
homogeneous chemical compound; he concluded that as water undoubtedly
dissolves quicklime to some extent, any specimen of this substance,
provided it be pure, must be wholly soluble in water. Carefully conducted
experiments proved that Black's conclusion was correct. Black had thus
proved that quicklime is a definite substance, with certain fixed
properties which characterize it and mark it off from all other
substances; that by absorbing, or combining with another definite substance
(fixed air), quicklime is changed into a third substance, namely chalk,
which is also characterized by properties as definite and marked as those
of quicklime or fixed air.
Black, quite as much as the alchemists, recognized the fact that change is
continually proceeding in Nature; but he clearly established the
all-important conclusion that these natural changes proceed in definite
order, and that it is possible by careful experiment and just reasoning to
acquire a knowledge of this order. He began the great work of showing that,
as in other branches of natural science, so also in chemistry, which is
pre-eminently the study of the changes of Nature, "the only distinct
meaning of that word" (natural) "is _stated_, _fixed_, or _settled_"
(Butler's "Analogy," published 1736).
This research by Black is a model of what scientific work ought to be. He
begins with a few observations of some natural phenomenon; these he
supplements by careful experiments, and thus establishes a sure basis of
fact; he then builds on this basis a general hypothesis, which he proceeds
to test by deducing from it certain necessary conclusions, and proving, or
disproving, these by an appeal to Nature. This is the scientific method; it
is common sense made accurate.
Very shortly after the publication of the thesis on magnesia and quicklime,
a vacancy occurred in the chemical chair in Glasgow University, and Black
was appointed Professor of Anatomy and Lecturer on Chemistry. As he did not
feel fully qualified to lecture on anatomy, he made an arrangement to
exchange subjects with the Professor of Medicine, and from this time he
delivered lectures on chemistry and on "The Institutes of Medicine."
Black devoted a great deal of care and time to the teaching duties of his
chair. His chemical experimental researches were not much advanced after
this time; but he delivered courses of lectures in which new light was
thrown on the whole range of chemical science.
In the years between 1759 and 1763 Black examined the phenomena of heat and
cold, and gave an explanation, founded on accurate experiments, of the
thermal changes which accompany the melting of solids and the vaporization
of liquids.
If pieces of wood, lead and ice be taken by the hand from a box in which
they have been kept cold, the wood feels cold to the touch, the lead feels
colder than the wood, and the ice feels colder than the lead; hence it was
concluded that the hand receives cold from the wood, more cold from the
lead, and most cold from the ice.
Black however showed that the wood really takes away heat from the hand,
but that as the wood soon gets warmed, the process stops before long; that
the lead, not being so quickly warmed as the wood, takes away more heat
from the hand than the wood does, and that the ice takes away more heat
than either wood or lead.
Black thought that the heat which is taken by melting ice from a warm body
remains in the water which is produced; as soon as winter came he proceeded
to test this supposition by comparing the times required to melt one pound
of ice and to raise the temperature of one pound of water through one
degree, the source of heat being the same in each case. He also compared
the time required to lower the temperature of one pound of water through
one degree with that required to freeze one pound of ice-cold water. He
found that in order to melt one pound of ice without raising its
temperature, as much heat had to be added to the ice as sufficed to raise
the temperature of one pound of water through about 140 degrees of
Fahrenheit's thermometer. But this heat which has been added to the ice to
convert it into water is not indicated by the thermometer. Black called
this "_latent heat_."
The experimental data and the complete theory of latent heat were contained
in a paper read by Black to a private society which met in the University
of Glasgow, on April 23, 1762; but it appears that Black was accustomed to
teach the theory in his ordinary lectures before this date.
The theory of latent heat ought also to explain the phenomena noticed when
liquid water is changed into steam. Black applied his theory generally to
this change, but did not fully work out the details and actually measure
the quantity of heat which is absorbed by water at the boiling point
before it is wholly converted into steam at the same temperature, until
some years later when he had the assistance of his pupil and friend James
Watt.
Taking a survey of the phenomena of Nature, Black insisted on the
importance of these experimentally established facts--that before ice melts
it must absorb a large quantity of heat, and before water is vaporized it
must absorb another large quantity of heat, which amounts of heat are
restored to surrounding substances when water vapour again becomes liquid
water and when liquid water is congealed to ice. He allows his imagination
to picture the effects of these properties of water in modifying and
ameliorating the climates of tropical and of Northern countries. In his
lectures he says, "Here we can also trace another magnificent train of
changes which are nicely accommodated to the wants of the inhabitants of
this globe. In the equatorial regions, the oppressive heat of the sun is
prevented from a destructive accumulation by copious evaporation. The
waters, stored with their vaporific heat, are then carried aloft into the
atmosphere till the rarest of the vapour reaches the very cold regions of
the air, which immediately forms a small portion of it into a fleecy cloud.
This also further tempers the scorching heat by its opacity, performing the
acceptable office of a screen. From thence the clouds are carried to the
inland countries, to form the sources in the mountains which are to supply
the numberless streams that water the fields. And by the steady operation
of causes, which are tolerably uniform, the greater part of the vapours
passes on to the circumpolar regions, there to descend in rains and dews;
and by this beneficent conversion into rain by the cold of those regions,
each particle of steam gives up the heat which was latent in it. This is
immediately diffused, and softens the rigour of those less comfortable
climates."
In the year 1766 Black was appointed Professor of Chemistry in the
University of Edinburgh, in which position he remained till his death in
1799. During these thirty-three years he devoted himself chiefly to
teaching and to encouraging the advance of chemical science. He was
especially careful in the preparation of his elementary lectures, being
persuaded that it was of the utmost importance that his pupils should be
well grounded in the principles of chemistry.
His health had never been robust, and as he grew old he was obliged to use
great care in his diet; his simple and methodical character and habits made
it easy for him to live on the plainest food, and to take meals and
exercise at stated times and in fixed quantities.
Black's life closed, as was fitting, in a quiet and honoured old age. He
had many friends, but lived pretty much alone--he was never married.
On the 26th of November 1799, "being at table with his usual fare, some
bread, a few prunes and a measured quantity of milk diluted with water,
and having the cup in his hand when the last stroke of his pulse was to be
given, he had set it down on his knees, which were joined together, and
kept it steady with his hand, in the manner of a person perfectly at ease;
and in this attitude he expired, without spilling a drop, and without a
writhe in his countenance, as if an experiment had been required to show to
his friends the facility with which he departed."
Black was characterized by "moderation and sobriety of thought;" he had a
great sense of the fitness of things--of what is called by the older
writers "propriety." But he was by no means a dull companion; he enjoyed
general society, and was able to bear a part in any kind of conversation. A
thorough student of Nature, he none the less did not wish to devote his
whole time to laboratory work or to the labours of study; indeed he seems
to have preferred the society of well-cultivated men and women to that of
specialists in his own or other branches of natural science. But with his
true scientific peers he doubtless appeared at his best. Among his more
intimate friends were the famous political economist Adam Smith, and the no
less celebrated philosopher David Hume. Dr. Hutton, one of the earliest
workers in geology, was a particular friend of Black; his friendship with
James Watt began when Watt was a student in his class, and continued during
his life.
With such men as his friends, and engaged in the study of Nature--that
boundless subject which one can never know to the full, but which one can
always know a little more year by year--Black's life could not but be
happy. His example and his teaching animated his students; he was what a
university professor ought to be, a student among students, but yet a
teacher among pupils. His work gained for him a place in the first rank of
men of science; his clearness of mind, his moderation, his gentleness, his
readiness to accept the views of others provided these views were well
established on a basis of experimentally determined facts, fitted him to be
the centre of a circle of scientific students who looked on him as at once
their teacher and their friend.
As a lecturer Black was eminently successful. He endeavoured to make all
his lectures plain and intelligible; he enlivened them by many experiments
designed simply to illustrate the special point which he had in view. He
abhorred ostentatious display and trickiness in a teacher.
Black was strongly opposed to the use of hypotheses in science. Dr. Robison
(the editor of his lectures) tells that when a student in Edinburgh he met
Black, who became interested in him from hearing him speak somewhat
enthusiastically in favour of one of the lecturers in the university. Black
impressed on him the necessity of steady experimental work in natural
science, gave him a copy of Newton's "Optics" as a model after which
scientific work ought to be conducted, and advised him "to reject, even
without examination, any hypothetical explanation, as a mere waste of time
and ingenuity." But, when we examine Black's own work, we see that by
"hypothetical explanations" he meant vague guesses. He himself made free
use of scientific (_i.e._ of exact) hypotheses; indeed the history of
science tells us that without hypotheses advance is impossible. Black
taught by his own researches that science is not an array of facts, but
that the object of the student of Nature is to explain facts. But the
method generally in vogue before the time of Black was to gather together a
few facts, or what seemed to be facts, and on these to raise a vast
superstructure of "vain imaginings." Naturalists had scarcely yet learned
that Nature is very complex, and that guessing and reasoning on guesses,
with here and there an observation added, was not the method by which
progress was to be made in learning the lessons written in this complex
book of Nature.
In place of this loose and slipshod method Black insisted that the student
must endeavour to form a clear mental image of every phenomenon which he
studied. Such an image could be obtained only by beginning with detailed
observation and experiment. From a number of definite mental images the
student must put together a picture of the whole natural phenomenon under
examination; perceiving that something was wanted here, or that the picture
was overcrowded there, he must again go to Nature and gain fresh facts, or
sometimes prove that what had been accepted as facts had no real
existence, and so at length he would arrive at a true representation of the
whole process.
So anxious was Black to define clearly what he knew and professed to teach,
that he preferred to call his lectures "On the Effects of Heat and
Mixtures," rather than to announce them as "A Systematic Course on
Chemistry."
His introductory lecture on "Heat in General" is very admirable; the
following quotation will serve to show the clearness of his style and the
methodical but yet eminently suggestive manner of his teaching:--
_"Of Heat in General._
"That this extensive subject may be treated in a profitable manner, I
propose--
"First. To ascertain what I mean by the word _heat_ in these lectures.
"Secondly. To explain the meaning of the term _cold_, and ascertain the
real difference between heat and cold.
"Thirdly. To mention some of the attempts which have been made to discover
the nature of heat, or to form an idea of what may be the immediate cause
of it.
"Fourthly and lastly. I shall begin to describe sensible effects produced
by heat on the bodies to which it is communicated.
"Any person who reflects on the ideas which we annex to the word _heat_
will perceive that this word is used for two meanings, or to express two
different things. It either means a sensation excited in our organs, or a
certain quality, affection, or condition of the bodies around us, by which
they excite in us that sensation. The word is used in the first sense when
we say, we feel heat; in the second, when we say, there is heat in the fire
or in a hot stone. There cannot be a sensation of heat in the fire, or in
the hot stone, but the matter of the fire, or of the stone, is in a state
or condition by which it excites in us the sensation of heat.
"Now, in beginning to treat of heat and its effects, I propose to use the
word in this second sense only; or as expressing that state, condition, or
quality of matter by which it excites in us the sensation of heat. This
idea of heat will be modified a little and extended as we proceed, but the
meaning of the word will continue at bottom the same, and the reason of the
modification will be easily perceived."
Black's manner of dealing with the phenomenon of combustion illustrates the
clearness of the conceptions which he formed of natural phenomena, and
shows moreover the thoroughly unbiased nature of his mind. As soon as he
had convinced himself that the balance of evidence was in favour of the new
(antiphlogistic) theory, he gave up those doctrines in which he had been
trained, and accepted the teaching of the French chemists; but he did
not--as some with less well-balanced minds might do--regard the new theory
as a final statement, but rather as one stage nearer the complete
explanation which future experiments and future reasoning would serve to
establish.
In his lectures on combustion Black first of all establishes the facts,
that when a body is burned it is changed into a kind (or kinds) of matter
which is no longer inflammable; that the presence of air is needed for
combustion to proceed; that the substance must be heated "to a certain
degree" before combustion or inflammation begins; that this degree of heat
(or we should now say this degree of temperature) differs for each
combustible substance; that the supply of air must be renewed if the
burning is to continue; and that the process of burning produces a change
in the quality of the air supplied to the burning body.
He then states the phlogistic interpretation of these phenomena: that
combustion is caused by the outrush from the burning body of a something
called the _principle of fire_, or _phlogiston_.
Black then proceeds to demonstrate certain other facts:--When the
substances produced by burning phosphorus or sulphur are heated with carbon
(charcoal) the original phosphorus or sulphur is reproduced. This
reproduction is due, according to the phlogistic chemists, to the giving
back, by carbon, of the phlogiston which had escaped during the burning.
Hence carbon contains much phlogiston. But as a similar reproduction of
phosphorus or sulphur, from the substances obtained by burning these
bodies, can be accomplished by the use of substances other than carbon, it
is evident that these other substances also contain much phlogiston, and,
moreover, that the phlogiston contained in all these substances is one and
the same _principle_. What then, he asks, is this "principle" which can so
escape, and be so restored by the action of various substances? He then
proceeds as follows:--
"But when we inquire further, and endeavour to learn what notion was formed
of the nature of this principle, and what qualities it was supposed to have
in its separate state, we find this part of the subject very obscure and
unsatisfactory, and the opinions very unsettled.
"The elder chemists, and the alchemists, considered sulphur as the
universal inflammable principle, or at least they chose to call the
inflammable part of all bodies, that are more or less inflammable, by the
name of their sulphur.... The famous German chemist Becher was, I believe,
the first who rejected the notion of sulphur being the principle of
inflammability in bodies.... His notion of the nature of the pure principle
of inflammability was afterwards more fully explained and supported by
Professor Stahl, who, agreeably to the doctrine of Becher, represented the
principle of inflammability as a dry substance, or of an earthy nature, the
particles of which were exquisitely subtile, and were much disposed to be
agitated and set in motion with inconceivable velocity.... The opinion of
Becher and Stahl concerning this _terra secunda_, or _terra inflammabilis_,
or _phlogiston_, was that the atoms of it are, more than all others,
disposed to be affected with an excessively swift whirling motion (_motus
vorticillaris_). The particles of other elementary substances are likewise
liable to be affected with the same sort of motion, but not so liable as
those of _terra secunda_; and when the particles of any body are agitated
with this sort of motion, the body exhibits the phenomena of heat, or
ignition, or inflammation according to the violence and rapidity of the
motion.... Becher and Stahl, therefore, did not suppose that heat depended
on the abundance of a peculiar matter, such as the matter of heat or fire
is now supposed to be, but on a peculiar motion of the particles of
matter....
"This very crude opinion of the earthy nature of the principle of
inflammability appears to have been deduced from a quality of many of the
inflammable substances, by which they resist the action of water as a
solvent. The greater number of the earthy substances are little, or not at
all, soluble in water.... And when Becher and Stahl found those compounds,
which they supposed contained phlogiston in the largest quantity, to be
insoluble in water, although the other matter, with which the phlogiston
was supposed to be united, was, in its separate state, exceedingly soluble
in that fluid, they concluded that _a dry nature, or an incapability to be
combined with water_, was an eminent quality of their phlogiston; and this
was what they meant by calling it an earth or earthy substance.... But
these authors supposed, at the same time, that the particles of this dry
and earthy phlogiston were much disposed to be excessively agitated with a
whirling motion; which whirling motion, exerted in all directions from the
bodies in which phlogiston is contained, produced the phenomena of
inflammation. This appears to have been the notion formed by Becher and
Stahl, concerning the nature of the principle of inflammability, or the
phlogiston; a notion which seems the least entitled to the name of
explanation of anything we can think of. I presume that few persons can
form any clear conception of this whirling motion, or, if they can, are
able to explain to themselves how it produces, or can produce, anything
like the phenomena of heat or fire."
Black then gives a clear account of the experiments of Priestley and
Lavoisier (see pp. 58, 59, and 87-89), which established the presence, in
common air, of a peculiar kind of gas which is especially concerned in the
processes of combustion; he emphasizes the fact that a substance increases
in weight when it is burned; and he gives a simple and clear statement of
that explanation of combustion which is now accepted by all, and which does
not require that the existence of any principle of fire should be assumed.
It is important to note that Black clearly connects the _physical_ fact
that heat is absorbed, or evolved, by a substance during combustion, with
the _chemical_ changes which are brought about in the properties of the
substance burned. He concludes with an admirable contrast between the
phlogistic theory and the theory of Lavoisier, which shows how wide, and at
the same time how definite, his conceptions were. Black never speaks
contemptuously of a theory which he opposes.
"According to this theory" (_i.e._ the theory of Lavoisier), "the
inflammable bodies, sulphur for example, or phosphorus, are simple
substances. The acid into which they are changed by inflammation is a
compound. The chemists, on the contrary" (_i.e._ the followers of Stahl),
"consider the inflammable bodies as compounds, and the uninflammable matter
as more simple. In the common theory the heat and light are supposed to
emanate from, or to be furnished by, the burning body. But, in Mr.
Lavoisier's theory, both are held to be furnished by the air, of which they
are held to be constituent parts, or ingredients, while in its state of
fire-supporting air."
Black was not a brilliant discoverer, but an eminently sound and at the
same time imaginative worker; whatever he did he did well, but he did not
exhaust any field of inquiry. Many of the facts established by him have
served as the basis of important work done by those who came after him. The
number of new facts added by Black to the data of chemistry was not large;
but by his lectures--which are original dissertations of the highest
value--he did splendid service in advancing the science of chemistry. Black
possessed that which has generally distinguished great men of science, a
marked honesty of character; and to this he added comprehensiveness of
mental vision: he saw beyond the limits of the facts which formed the
foundations of chemical science in his day. He was not a fact-collector,
but a philosopher.
* * * * *
JOSEPH PRIESTLEY, the son of Jonas Priestley, "a maker and dresser of
woollen cloth," was born at Fieldhead, near Leeds, in the year 1733. His
mother, who was the daughter of a farmer near Wakefield, died when he was
seven years old. From that time he was brought up by a sister of his
father, who was possessed of considerable private means.
Priestley's surroundings in his young days were decidedly religious, and
evidently gave a tone to his whole after life. We shall find that
Priestley's work as a man of science can scarcely be separated from his
theological and metaphysical work. His cast of mind was decidedly
metaphysical; he was altogether different from Black, who, as we have seen,
was a typical student of natural phenomena.
The house of Priestley's aunt was a resort for all the Dissenting ministers
of that part of the county. She herself was strictly Calvinistic in her
theological views, but not wholly illiberal.
Priestley's early schooling was chiefly devoted to learning languages; he
acquired a fair knowledge of Latin, a little Greek, and somewhat later he
learned the elements of Hebrew. At one time he thought of going into trade,
and therefore, as he tells us in his "Memoirs," he acquired some knowledge
of French, Italian and High Dutch. With the help of a friend, a Dissenting
minister, he learned something of geometry, mathematics and natural
philosophy, and also got some smattering of the Chaldee and Syriac tongues.
At the age of nineteen Priestley went to an "academy" at Daventry. The
intellectual atmosphere here seems to have been suitable to the rapid
development of Priestley's mind. Great freedom of discussion was allowed;
even during the teachers' lectures the students were permitted "to ask
whatever questions and to make whatever remarks" they pleased; and they did
it, Priestley says, "with the greatest, but without any offensive,
freedom."
The students were required to read and to give an account of the more
important arguments for and against the questions discussed in the
teachers' lectures. Theological disputations appear to have been the
favourite topics on which the students exercised their ingenuity among
themselves. Priestley tells us that he "saw reason to embrace what is
generally called the heterodox side of almost every question."
Leaving this academy, Priestley went, in 1755, as assistant to the
Dissenting minister at Needham, in Suffolk. Here he remained for three
years, living on a salary of about L30 a year, and getting more and more
into bad odour because of his peculiar theological views.
From Needham he moved to Nantwich, in Cheshire, where he was more
comfortable, and, having plenty of work to do, he had little time for
abstruse speculations. School work engaged most of his time at Nantwich; he
also began to collect a few scientific instruments, such as an electrical
machine and an air-pump. These he taught his scholars to use and to keep in
good order. He gave lectures on natural phenomena, and encouraged his
scholars to make experiments and sometimes to exhibit their experiments
before their parents and friends. He thus extended the reputation of his
school and implanted in his scholars a love of natural knowledge.
In the year 1761 Priestley removed to Warrington, to act as tutor in a
newly established academy, where he taught languages--a somewhat wide
subject, as it included lectures on "The Theory of Languages," on "Oratory
and Criticism," and on "The History, Laws, and Constitution of England." He
says, "It was my province to teach elocution, and also logic and Hebrew.
The first of these I retained, but after a year or two I exchanged the two
last articles with Dr. Aikin for the civil law, and one year I gave a
course of lectures on anatomy."
During his stay at Warrington, which lasted until 1767, Priestley married a
daughter of Mr. Isaac Wilkinson, an ironmaster of Wrexham, in Wales. He
describes his wife as "a woman of an excellent understanding much improved
by reading, of great fortitude and strength of mind, and of a temper in
the highest degree affectionate and generous, feeling strongly for others
and little for herself, also greatly excelling in everything relating to
household affairs."
About this time Priestley met Dr. Franklin more than once in London. His
conversation seems to have incited Priestley to a further study of natural
philosophy. He began to examine electrical phenomena, and this led to his
writing and publishing a "History of Electricity," in the course of which
he found it necessary to make new experiments. The publication of the
results of these experiments brought him more into notice among scientific
men, and led to his election as a Fellow of the Royal Society, and to his
obtaining the degree of LL.D. from the University of Edinburgh. In the year
1767 Priestley removed to Leeds, where he spent six years as minister of
Millhill Chapel.
He was able to give freer expression to his theological views in Leeds than
could be done in smaller places, such as Needham and Nantwich. During this
time he wrote and published many theological and metaphysical treatises.
But, what is of more importance to us, he happened to live near a brewery.
Now, the accidental circumstances, as we call them, of Priestley's life
were frequently of the greatest importance in their effects on his
scientific work. Black had established the existence and leading properties
of fixed air about twelve or thirteen years before the time when Priestley
came to live near the brewery in Leeds. He had shown that this fixed air
is produced during alcoholic fermentation. Priestley knowing this used to
collect the fixed air which came off from the vats in the neighbouring
brewery, and amuse himself with observing its properties. But removing from
this part of the town his supplies of fixed air were stopped. As however he
had become interested in working with airs, he began to make fixed air for
himself from chalk, and in order to collect this air he devised a very
simple piece of apparatus which has played a most important part in the
later development of the chemistry of gases, or pneumatic chemistry.
Priestley's _pneumatic trough_ is at this day to be found in every
laboratory; it is extremely simple and extremely perfect. A dish of glass,
or earthenware, or wood is partly filled with water; a shelf runs across
the dish at a little distance beneath the surface of the water; a
wide-mouthed bottle is filled with water and placed, mouth downwards, over
a hole in this shelf. The gas which is to be collected in this bottle is
generated in a suitable vessel, from which a piece of glass or metal tubing
passes under the shelf and stops just where the hole is made. The gas which
comes from the apparatus bubbles up into the bottle, drives out the water,
and fills the bottle. When the bottle is full of gas, it is moved to one
side along the shelf, and another bottle filled with water is put in its
place. As the mouth of each bottle is under water there is no connection
between the gas inside and the air outside the bottle; the gas may
therefore be kept in the bottle until the experimenter wants it. (See Fig.
1. which is reduced from the cut in Priestley's "Air.")
[Illustration: Fig. 1.]
Priestley tells us that at this time he knew very little chemistry, but he
thinks that this was a good thing, else he might not have been led to make
so many new discoveries as he did afterwards make.
Experimenting with fixed air, he found that water could be caused to
dissolve some of the gas. In 1772 he published a pamphlet on the method of
impregnating water with fixed air; this solution of fixed air in water was
employed medicinally, and from this time we date the manufacture of
artificial mineral waters.
The next six years of Priestley's life (1773-1779) are very important in
the history of chemistry; it was during these years that much of his best
work on various airs was performed. During this time he lived as a kind of
literary companion (nominally as librarian) with the Earl of Shelburne
(afterwards Marquis of Lansdowne.) His wife and family--he had now three
children--lived at Calne, in Wiltshire, near Lord Shelburne's seat of
Bowood. Priestley spent most of the summer months with his family, and the
greater part of each winter with Lord Shelburne at his London residence;
during this time he also travelled in Holland and Germany, and visited
Paris in 1774.
In a paper published in November 1772, Priestley says that he examined a
specimen of air which he had extracted from saltpetre above a year before
this date. This air "had by some means or other become noxious, but," he
supposed, "had been restored to its former wholesome state, so as to
effervesce with nitrous air" (in modern language, to combine with nitric
oxide) "and to admit a candle to burn in it, in consequence of agitation
with water." He tells us, in his "Observations on Air" (1779), that at this
time he was altogether in the dark as to the nature of this air obtained
from saltpetre. In August 1774, he was amusing himself by observing the
action of heat on various substances--"without any particular view," he
says, "except that of extracting air from a variety of substances by means
of a burning lens in quicksilver, which was then a new process with me, and
which I was very proud of"--when he obtained from _red precipitate_ (oxide
of mercury) an air in which a candle burned with a "remarkably vigorous
flame." The production of this peculiar air "surprised me more than I can
well express;" "I was utterly at a loss how to account for it." At first he
thought that the specimen of _red precipitate_ from which the air had been
obtained was not a proper preparation, but getting fresh specimens of this
salt, he found that they all yielded the same kind of air. Having satisfied
himself by experiment that this peculiar air had "all the properties of
common air, only in much greater perfection," he gave to it the name of
_dephlogisticated air_. Later experiments taught him that the same air
might be obtained from red lead, from manganese oxide, etc., by the action
of heat, and from various other salts by the action of acids.
Priestley evidently regards the new "dephlogisticated air" simply as very
pure ordinary air; indeed, he seems to look on all airs, or gases, as
easily changeable one into the other. He always interprets his experimental
results by the help of the theory of phlogiston. One would indeed think
from Priestley's papers that the existence of this substance phlogiston was
an unquestioned and unquestionable fact. Thus, he says in the preface to
his "Experiments on Air:" "If any opinion in all the modern doctrine
concerning air be well founded, it is certainly this, that nitrous air is
highly charged with phlogiston, and that from this quality only it renders
pure air noxious.... If I have completely ascertained anything at all
relating to air it is this." Priestley thought that "very pure air" would
take away phlogiston from some metals without the help of heat or any acid,
and thus cause these metals to rust. He therefore placed some clean iron
nails in _dephlogisticated air_ standing over mercury; after three months
he noticed that about one-tenth of the air in the vessel had disappeared,
and he concluded, although no rust appeared, that the dephlogisticated air
had as a fact withdrawn phlogiston from the iron nails. This is the kind of
reasoning which Black described to his pupils as "mere waste of time and
ingenuity." The experiment with the nails was made in 1779; at this time,
therefore, Priestley had no conception as to what his _dephlogisticated
air_ really was.
Trying a great many experiments, and finding that the new air was obtained
by the action of acids on earthy substances, Priestley was inclined to
regard this air, and if this then all other airs, as made up of an acid (or
acids) and an earthy substance. We now know how completely erroneous this
conclusion was, but we must remember that in Priestley's time chemical
substances were generally regarded as of no very definite or fixed
composition; that almost any substance, it was supposed, might be changed
into almost any other; that no clear meaning was attached to the word
"element;" and that few, if any, careful measurements of the quantities of
different kinds of matter taking part in chemical actions had yet been
made.
But at the same time we cannot forget that the books of Hooke and Mayow had
been published years before this time, and that twenty years before
Priestley began his work on airs, Black had published his exact, scientific
investigation on fixed air.
Although we may agree with Priestley that, had he made himself acquainted
with what others had done before he began his own experiments, he might not
have made so many new discoveries as he did, yet one cannot but think that
his discoveries, although fewer, would have been more accurate.
We are told by Priestley that, when he was in Paris in 1774, he exhibited
the method of obtaining dephlogisticated air from _red precipitate_ to
Lavoisier and other French chemists. We shall see hereafter what important
results to science followed from this visit to Lavoisier.
Let us shortly review Priestley's answer to the question, "What happens
when a substance burns in air?"
Beginning to make chemical experiments when he had no knowledge of
chemistry, and being an extremely rapid worker and thinker, he naturally
adopted the prevalent theory, and as naturally interpreted the facts which
he discovered in accordance with this theory.
When a substance burns, phlogiston, it was said, rushes out of it. But why
does rapid burning only take place in air? Because, said Priestley, air has
a great affinity for phlogiston, and draws it out of the burning substance.
What then becomes of this phlogiston? we next inquire. The answer is,
obviously it remains in the air around the burning body, and this is proved
by the fact that this air soon becomes incapable of supporting the process
of burning, it becomes phlogisticated. Now, if phlogisticated air cannot
support combustion, the greater the quantity of phlogiston in air, the less
will it support burning; but we know that if a substance is burnt in a
closed tube containing air, the air which remains when the burning is quite
finished at once extinguishes a lighted candle. Priestley also proved that
an air can be obtained by heating _red precipitate_, characterized by its
power of supporting combustion with great vigour. What is this but common
air completely deprived of phlogiston? It is dephlogisticated air. Now, if
common air draws phlogiston out of substances, surely this dephlogisticated
air will even more readily do the same. That it really does this Priestley
thought he had proved by his experiment with clean iron nails (see p. 60).
Water was regarded as a substance which, like air, readily combined with
phlogiston; but Priestley thought that a candle burned less vigorously in
dephlogisticated air which had been shaken with water than in the same air
before this treatment; hence he concluded that phlogiston had been taken
from the water.
After Cavendish had discovered (or rather rediscovered) hydrogen, and had
established the fact that this air is extremely inflammable, most chemists
began to regard this gas as pure or nearly pure phlogiston, or, at least,
as a substance very highly charged with phlogiston. "Now," said Priestley,
"when a metal burns phlogiston rushes out of it; if I restore this
phlogiston to the metallic calx, I shall convert it back into the metal."
He then showed by experiment that when calx of iron is heated with
hydrogen, the hydrogen disappears and the metal iron is produced.
He seemed, therefore, to have a large experimental basis for his answer to
the question, "What happens when a substance burns?" But at a later time it
was proved that iron was also produced by heating the calx of iron with
carbon. The antiphlogistic chemists regarded fixed air as composed of
carbon and dephlogisticated air; the phlogisteans said it was a substance
highly charged with phlogiston. The antiphlogistic school said that calx of
iron is composed of iron and dephlogisticated air; the phlogisteans said it
was iron deprived of its phlogiston. Here was surely an opportunity for a
crucial experiment: when calx of iron is heated with carbon, and iron is
produced, there must either be a production of fixed air (which is a
non-inflammable gas, and forms a white solid substance when brought into
contact with limewater), or there must be an outrush of phlogiston from the
carbon. The experiment was tried: a gas was produced which had no action on
limewater and which was very inflammable; what could this be but
phlogiston, already recognized by this very property of extreme
inflammability? Thus the phlogisteans appeared to triumph. But if we
examine these experiments made by Priestley with the light thrown on them
by subsequent research, we find that they bear the interpretation which he
put on them only because they were not accurate; thus, two gases are
inflammable, but it by no means follows that these gases are one and the
same. We must have more accurate knowledge of the properties of these
gases.
The air around a burning body, such as iron, after a time loses the power
of supporting combustion; but this is merely a qualitative fact. Accurately
to trace the change in the properties of this air, it is absolutely
necessary that exact measurements should be made; when this is done, we
find that the volume of air diminishes during the combustion, that the
burning body gains weight, and that this gain in weight is just equal to
the loss in weight undergone by the air. When the inflammable gas produced
by heating calx of iron with carbon was carefully and _quantitatively_
analyzed, it was found to consist of carbon and oxygen (dephlogisticated
air), but to contain these substances in a proportion different from that
in which they existed in fixed air. It was a new kind of air or gas; it
was _not_ hydrogen.
This account of Priestley's experiments and conclusions regarding
combustion shows how easy it is in natural science to interpret
experimental results, especially when these results are not very accurate,
in accordance with a favourite theory; and it also illustrates one of the
lessons so emphatically taught by all scientific study, viz. the necessity
of suspending one's judgment until accurate measurements have been made,
and the great wisdom of then judging cautiously.
About 1779 Priestley left Lord Shelburne, and went as minister of a chapel
to Birmingham, where he remained until 1791.
During his stay in Birmingham, Priestley had a considerable amount of
pecuniary help from his friends. He had from Lord Shelburne, according to
an agreement made when he entered his service, an annuity of L150 a year
for life; some of his friends raised a sum of money annually for him, in
order that he might be able to prosecute his researches without the
necessity of taking pupils. During the ten years or so after he settled in
Birmingham, Priestley did a great deal of chemical work, and made many
discoveries, almost entirely in the field of pneumatic chemistry.
Besides the discovery of dephlogisticated air (or oxygen) which has been
already described, Priestley discovered and gave some account of the
properties of _nitrous air_ (nitric acid), _vitriolic acid air_ (sulphur
dioxide), _muriatic acid air_ (hydrochloric acid), and _alkaline air_
(ammonia), etc.
In the course of his researches on the last-named air he showed, that when
a succession of electric sparks is passed through this gas a great increase
in the volume of the gas occurs. This fact was further examined at a later
time by Berthollet, who, by measuring the increase in volume undergone by a
measured quantity of ammonia gas, and determining the nature of the gases
produced by the passage of the electric sparks, proved that ammonia is a
compound of hydrogen and nitrogen, and that three volumes of the former gas
combine with one volume of the latter to produce two volumes of ammonia
gas.
Priestley's experiments on "inflammable air"--or hydrogen--are important
and interesting. The existence of this substance as a definite kind of air
had been proved by the accurate researches of Cavendish in 1766. Priestley
drew attention to many actions in which this inflammable air is produced,
chiefly to those which take place between acids and metals. He showed that
inflammable air is not decomposed by electric sparks; but he thought that
it was decomposed by long-continued heating in closed tubes made of
lead-glass. Priestley regarded inflammable air as an air containing much
phlogiston. He found that tubes of lead-glass, filled with this air, were
blackened when strongly heated for a long time, and he explained this by
saying that the lead in the glass had a great affinity for phlogiston, and
drew it out of the inflammable air.
When inflammable air burns in a closed vessel containing common air, the
latter after a time loses its property of supporting combustion. Priestley
gave what appeared to be a fairly good explanation of this fact, when he
said that the inflammable air parted with phlogiston, which, becoming mixed
with the ordinary air in the vessel, rendered it unable to support the
burning of a candle. He gave a few measurements in support of this
explanation; but we now know that the method of analysis which he employed
was quite untrustworthy.
Thinking that by measuring the extent to which the _phlogistication_ (we
would now say the _deoxidation_) of common air was carried by mixing
measured quantities of common and inflammable airs and exploding this
mixture, he might be able to determine the amount of phlogiston in a given
volume of inflammable air, he mixed the two airs in glass tubes, through
the sides of which he had cemented two pieces of wire, sealed the tubes,
and exploded the mixture by passing electric sparks from wire to wire. The
residual air now contained, according to Priestley, more phlogiston, and
therefore relatively less dephlogisticated air than before the explosion.
He made various measurements of the quantities of dephlogisticated air in
the tubes, but without getting any constant results. He noticed that after
the explosions the insides of the tubes were covered with moisture. At a
later time he exploded a mixture of dephlogisticated and inflammable airs
(oxygen and hydrogen) in a copper globe, and recorded the fact that after
the explosion the globe contained a little water. Priestley was here
apparently on the eve of a great discovery. "In looking for one thing,"
says Priestley, "I have generally found another, and sometimes a thing of
much more value than that which I was in quest of." Had he performed the
experiment of exploding dephlogisticated and inflammable airs with more
care, and had he made sure that the airs used were quite dry before the
explosion, he would probably have found a thing of indeed much more value
than that of which he was in quest; he would probably have discovered the
compound nature of water--a discovery which was made by Cavendish three or
four years after these experiments described by Priestley.
Some very curious observations were made by Priestley regarding the colour
of the gas obtained by heating "spirit of nitre" (_i.e._ nitric acid). He
showed that a yellow gas or air is obtained by heating colourless liquid
spirit of nitre in a sealed glass tube, and that as the heating is
continued the colour of the gas gets darker, until it is finally very dark
orange red. These experiments have found an explanation only in quite
recent times.
Another discovery made by Priestley while in Birmingham, viz. that an acid
is formed when electric sparks are passed through ordinary air for some
time, led, in the hands of Cavendish--an experimenter who was as careful
and deliberate as Priestley was rapid and careless--to the demonstration of
the composition of nitric acid.
Many observations were made by Priestley on the effects of various airs on
growing plants and living animals; indeed, one of his customary methods of
testing different airs was to put a mouse into each and watch the effects
of the air on its breathing. He grew sprigs of mint in common air, in
dephlogisticated air (oxygen), and in phlogisticated air (nitrogen, but
probably not pure); the sprig in the last-named air grew best, while that
in the dephlogisticated air soon appeared sickly. He also showed that air
which has been rendered "noxious" by the burning of a candle in it, or by
respiration or putrefaction, could be restored to its original state by the
action of growing plants. He thought that the air was in the first instance
rendered noxious by being impregnated with phlogiston, and that the plant
restored the air by removing this phlogiston. Thus Priestley distinctly
showed that (to use his own words) "it is very probable that the injury
which is continually done to the atmosphere by the respiration of such a
number of animals as breathe it, and the putrefaction of such vast masses,
both of vegetable and animal substances, exposed to it, is, in part at
least, repaired by the vegetable creation." But from want of quantitative
experiments he failed to give any just explanation of the process whereby
this "reparation" is accomplished.
During his stay in Birmingham, Priestley was busily engaged, as was his
wont during life, in writing metaphysical and theological treatises and
pamphlets.
At this time the minds of men in England were much excited by the events of
the French Revolution, then being enacted before them. Priestley and some
of his friends were known to sympathize with the French people in this
great struggle, as they had been on the side of the Americans in the War of
Independence. Priestley's political opinions had, in fact, always been more
advanced than the average opinion of his age; by some he was regarded as a
dangerous character. But if we read what he lays down as a fundamental
proposition in the "Essay on the First Principles of Civil Government"
(1768), we cannot surely find anything very startling.
"It must be understood, whether it be expressed or not, that all people
live in society for their mutual advantage; so that the good and happiness
of the members, that is the majority of the members of any state, is the
great standard by which everything relating to that state must be finally
determined. And though it may be supposed that a body of people may be
bound by a voluntary resignation of all their rights to a single person, or
to a few, it can never be supposed that the resignation is obligatory on
their posterity, because it is manifestly contrary to the good of the whole
that it should be so."
Priestley proposed many political reforms, but he was decidedly of opinion
that these ought to be brought about gradually. He was in favour of
abolishing all religious State establishments, and was a declared enemy to
the Church of England. His controversies with the clergy of Birmingham
helped to stir up a section of public opinion against him, and to bring
about the condemnation of his writings in many parts of the country; he was
also unfortunate in making an enemy of Mr. Burke, who spoke against him and
his writings in the House of Commons.
In the year 1791, the day of the anniversary of the taking of the Bastille
was celebrated by some of Priestley's friends in Birmingham. On that day a
senseless mob, raising the cry of "Church and King," caused a riot in the
town. Finding that they were not checked by those in authority, they after
a time attacked and burned Dr. Priestley's meeting-house, and then
destroyed his dwelling-house, and the houses of several other Dissenters in
the town. One of his sons barely escaped with his life. He himself found it
necessary to leave Birmingham for London, as he considered his life to be
in danger. Many of his manuscripts, his library, and much of his apparatus
were destroyed, and his house was burned.
A congregation at Hackney had the courage at this time to invite Priestley
to become their minister. Here he remained for about three years,
ministering to the congregation, and pursuing his chemical and other
experiments with the help of apparatus and books which had been supplied by
his friends, and by the expenditure of part of the sum, too small to cover
his losses, given him by Government in consideration of the damage done to
his property in the riots at Birmingham.
But finding himself more and more isolated and lonely, especially after the
departure of his three sons to America, which occurred during these years,
he at last resolved to follow them, and spend the remainder of his days in
the New World. Although Priestley had been very badly treated by a
considerable section of the English people, yet he left his native country
"without any resentment or ill will." "When the time for reflection," he
says, "shall come, my countrymen will, I am confident, do me more justice."
He left England in 1795, and settled at Northumberland, in Pennsylvania,
about a hundred and thirty miles north-west of Philadelphia. By the help of
his friends in England he was enabled to build a house and establish a
laboratory and a library; an income was also secured sufficient to maintain
him in moderate comfort.
The chair of chemistry in the University of Philadelphia was offered to
him, and he was also invited to the charge of a Unitarian chapel in New
York; but he preferred to remain quietly at work in his laboratory and
library, rather than again to enter into the noisy battle of life. In
America he published several writings. Of his chemical discoveries made
after leaving England, the most important was that an inflammable gas is
obtained by heating metallic calces with carbon. The production of this gas
was regarded by Priestley as an indisputable proof of the justness of the
theory of phlogiston (see pp. 63, 64).
His health began to give way about 1801; gradually his strength declined,
and in February 1804, the end came quietly and peacefully.
A list of the books and pamphlets published by Priestley on theological,
metaphysical, philological, historical, educational and scientific subjects
would fill several pages of this book. His industry was immense. To
accomplish the vast amount of work which he did required the most careful
outlay of time. In his "Memoirs," partly written by himself, he tells us
that he inherited from his parents "a happy temperament of body and mind;"
his father especially was always in good spirits, and "could have been
happy in a workhouse." His paternal ancestors had, as a race, been healthy
and long-lived. He was not himself robust as a youth, yet he was always
able to study: "I have never found myself," he says, "less disposed or less
qualified for mental exertion of any kind at one time of the day more than
another; but all seasons have been equal to me, early or late, before
dinner or after."
His peculiar evenness of disposition enabled him quickly to recover from
the effects of any unpleasant occurrence; indeed, he assures us that "the
most perfect satisfaction" often came a day or two after "an event that
afflicted me the most, and without any change having taken place in the
state of things."
Another circumstance which tended to make life easy to him was his fixed
resolution, that in any controversy in which he might be engaged, he would
frankly acknowledge every mistake he perceived himself to have fallen into.
Priestley's scientific work is marked by rapidity of execution. The
different parts do not hang together well; we are presented with a
brilliant series of discoveries, but we do not see the connecting strings
of thought. We are not then astonished when he tells us that sometimes he
forgot that he had made this or that experiment, and repeated what he had
done weeks before. He says that he could not work in a hurry, and that he
was therefore always methodical; but he adds that he sometimes blamed
himself for "doing to-day what had better have been put off until
to-morrow."
Many of his most startling discoveries were the results of chance
operations, "not of themes worked out and applied." He was led to the
discovery of oxygen, he says, by a succession of extraordinary accidents.
But that he was able to take advantage of the chance observations, and from
these to advance to definite facts, constitutes the essential difference
between him and ordinary plodding investigators. Although he rarely, if
ever, saw all the bearings of his own discoveries, although none of his
experiments was accurately worked out to its conclusion, yet he did see,
rapidly and as it appeared almost at one glance, something of their
meanings, and this something was enough to urge him on to fresh
experimental work.
Although we now condemn Priestley's theories as quite erroneous, yet we
must admire his undaunted devotion to experiment. He was a true student of
science in one essential point, viz. Nature was for him the first and the
last court of appeal. He theorized and speculated much, he experimented
rapidly and not accurately, but he was ever appealing to natural facts; and
in doing this he could not but lay some foundation which should remain. The
facts discovered by him are amongst the very corner-stones on which the
building of chemical science was afterwards raised.
So enthusiastic was Priestley in the prosecution of his experiments, that
when he began, he tells us, "I spent all the money I could possibly raise,
carried on by my ardour in philosophical investigation, and entirely
regardless of consequences, except so far as never to contract any debts."
He seems all through his life to have been perfectly free from anxiety
about money affairs.
Priestley's manner of work shows how kindly and genial he was. He trained
himself to talk and think and write with his family by the fireside;
"nothing but reading aloud, or speaking without interruption," was an
obstruction to his work.
Priestley was just the man who was wanted in the early days of chemical
science. By the vast number, variety and novelty of his experimental
results, he astonished scientific men--he forcibly drew attention to the
science in which he laboured so hard; by the brilliancy of some of his
experiments he obliged chemists to admit that a new field of research was
opened before them, and the instruments for the prosecution of this
research were placed in their hands; and even by the unsatisfactoriness of
his reasoning he drew attention to the difficulties and contradictions of
the theories which then prevailed in chemistry.
That the work of Priestley should bear full fruit it was necessary that a
greater than he should interpret it, and should render definite that which
Priestley had but vaguely shown to exist.
The man who did this, and who in doing it really established chemistry as a
science, was Lavoisier.
But before considering the work of Lavoisier, I should like to point out
that many of the physical characters of common air had been clearly
established in the later years of the seventeenth century by the Honourable
Robert Boyle. In the "Sceptical Chymist," published in 1661, Mr. Boyle had
established the fact that air is a material substance possessed of weight,
that this air presses on the surface of all things, and that by removing
part of the air in an enclosed space the pressure within that space is
diminished. He had demonstrated that the boiling point of water is
dependent on the pressure of the air on the surface of the water. Having
boiled some water "a pretty while, that by the heat it might be freed from
the latitant air," he placed the vessel containing the hot water within the
receiver of an arrangement which he had invented for sucking air out of an
enclosed space; as soon as he began to suck out air from this receiver, the
water boiled "as if it had stood over a very quick fire.... Once, when the
air had been drawn out, the liquor did, upon a single exsuction, boil so
long with prodigiously vast bubbles, that the effervescence lasted almost
as long as was requisite for the rehearsing of a _Pater noster_." Boyle had
gone further than the qualitative fact that the volume of an enclosed
quantity of air alters with changes in the pressure to which that air is
subjected; he had shown by simple and accurate experiments that "the volume
varies inversely as the pressure." He had established the generalization of
so much importance in physical science now known as _Boyle's law_.
The work of the Honourable Henry Cavendish will be considered in some
detail in the book on "The Physicists" belonging to this series, but I must
here briefly allude to the results of his experiments on air published in
the _Philosophical Transactions_ for 1784 and 1785.
Cavendish held the ordinary view that when a metal burns in air, the air is
thereby phlogisticated; but why is it, he asked, that the volume of air is
decreased by this process? It was very generally said that fixed air was
produced during the calcination of metals, and was absorbed by the calx.
But Cavendish instituted a series of experiments which proved that no fixed
air could be obtained from metallic calces. In 1766 inflammable air
(hydrogen) was discovered by Cavendish; he now proved that when this air is
exploded with dephlogisticated air (oxygen), water is produced. He showed
that when these two airs are mixed in about the proportion of two volumes
of hydrogen to one volume of oxygen, the greater part, if not the whole of
the airs is condensed into water by the action of the electric spark. He
then proceeded to prove by experiments that when common air is exploded
with inflammable air water is likewise produced, and phlogisticated air
(_i.e._ nitrogen) remains.
Priestley and Cavendish had thus distinctly established the existence of
three kinds of air, viz. dephlogisticated air, phlogisticated air, and
inflammable air. Cavendish had shown that when the last named is exploded
with common air water is produced (which is composed of dephlogisticated
and inflammable airs), and phlogisticated air remains. Common air had thus
been proved to consist of these two--phlogisticated and dephlogisticated
airs (nitrogen and oxygen). Applying these results to the phenomenon of the
calcination of metals, Cavendish gave reasons for thinking that the metals
act towards common air in a manner analogous to that in which inflammable
air acts--that they withdraw dephlogisticated and leave phlogisticated air;
but, as he was a supporter of the phlogistic theory, he rather preferred to
say that the burning metals withdraw dephlogisticated air and phlogisticate
that which remains; in other words, while admitting that a metal in the
process of burning gains dephlogisticated air, he still thought that the
metal also loses _something_; viz. phlogiston.
That Cavendish in 1783-84 had proved air to consist of two distinct gases,
and water to be produced by the union of two gases, must be remembered as
we proceed with the story of the discoveries of Lavoisier.
* * * * *
ANTOINE LAURENT LAVOISIER, born in Paris in 1743, was the son of a wealthy
merchant, who, judging from his friendship with many of the men of science
of that day, was probably of a scientific bent of mind, and who certainly
showed that he was a man of sense by giving his son the best education
which he could obtain. After studying in the Mazarin College, Lavoisier
entered on a course of training in physical, astronomical, botanical and
chemical science. The effects of this training in the accurate methods of
physics are apparent in the chemical researches of Lavoisier.
At the age of twenty-one Lavoisier wrote a memoir which gained the prize
offered by the French Government for the best and most economical method of
lighting the streets of a large city. While making experiments, the
results of which were detailed in this paper, Lavoisier lived for six weeks
in rooms lighted only by artificial light, in order that his eyesight might
become accustomed to small differences in the intensities of light from
various sources. When he was twenty-five years old Lavoisier was elected a
member of the Academy of Sciences. During the next six years (1768-1774) he
published various papers, some on chemical, some on geological, and some on
mathematical subjects. Indeed at this time, although an ardent cultivator
of natural science, he appears to have been undecided as to which branch of
science he should devote his strength.
The accuracy and thoroughness of Lavoisier's work, and the acuteness of his
reasoning powers, are admirably illustrated in two papers, published in the
Memoirs of the Academy for 1770, on the alleged conversion of water into
earth.
When water is boiled for a long time in a glass vessel a considerable
quantity of white siliceous earth is found in the vessel. This apparent
conversion or transmutation of water into earthy matter was quite in
keeping with the doctrines which had been handed down from the times of the
alchemists; the experiment was generally regarded as conclusively proving
the possibility of changing water into earth. Lavoisier found that after
heating water for a hundred and one days in a closed _and weighed_ glass
vessel, there was no change in the total weight of the vessel and its
contents; when he poured out the water and evaporated it to dryness, he
obtained 20.4 grains of solid earthy matter; but he also found, what had
been before overlooked, that the glass vessel had lost weight. The actual
loss amounted to 17.4 grains. The difference between this and the weight of
the earthy matter in the water, viz. three grains, was set down (and as we
now know justly set down) by Lavoisier to errors of experiment. Lavoisier
therefore concluded that water, when boiled, is not changed into earth, but
that a portion of the earthy matter of which glass is composed is dissolved
by the water. This conclusion was afterwards confirmed by the Swedish
chemist _Scheele_, who proved that the composition of the earthy matter
found in the water is identical with that of some of the constituents of
glass.
By this experiment Lavoisier proved the old alchemical notion of
transmutation to be erroneous; he showed that water is not transmuted into
earth, but that each of these substances is possessed of definite
properties which belong to it and to it only. He established the
all-important generalization--which subsequent research has more amply
confirmed, until it is to-day accepted as the very foundation of every
branch of physical science--that in no process of change is there any
alteration in the total mass of matter taking part in that change. The
glass vessel in which Lavoisier boiled water for so many days lost weight;
but the matter lost by the glass was found dissolved in the water.
We know that this generalization holds good in all chemical changes. Solid
sulphur may be converted into liquid oil of vitriol, but it is only by the
sulphur combining with other kinds of matter; the weight of oil of vitriol
produced is always exactly equal to the sum of the weights of the sulphur,
hydrogen and oxygen which have combined to form it. The colourless gases,
hydrogen and oxygen, combine, and the limpid liquid water is the result;
but the weight of the water produced is equal to the sum of the weights of
hydrogen and oxygen which combined together. It is impossible to overrate
the importance of the principle of the _conservation of mass_, first
definitely established by Lavoisier.
Some time about the year 1770 Lavoisier turned his attention seriously to
chemical phenomena. In 1774 he published a volume entitled "Essays Physical
and Chemical," wherein he gave an historical account of all that had been
done on the subject of airs from the time of Paracelsus to the year 1774,
and added an account of his own experiments, in which he had established
the facts that a metal in burning absorbs air, and that when the metallic
calx is reduced to metal by heating with charcoal, an air is produced of
the same nature as the fixed air of Dr. Black.
In November 1772 Lavoisier deposited a sealed note in the hands of the
Secretary to the Academy of Sciences. This note was opened on the 1st of
May 1773, and found to run as follows[4]:--
"About eight days ago I discovered that sulphur in
burning, far from losing, augments in weight; that is
to say, that from one pound of sulphur much more than
one pound of vitriolic acid is obtained, without
reckoning the humidity of the air. Phosphorus presents
the same phenomenon. This augmentation of weight arises
from a great quantity of air which becomes fixed during
the combustion, and which combines with the vapours.
"This discovery, confirmed by experiments which I
regard as decisive, led me to think that what is
observed in the combustion of sulphur and phosphorus
might likewise take place with respect to all the
bodies which augment in weight by combustion and
calcination; and I was persuaded that the augmentation
of weight in the calces of metals proceeded from the
same cause. The experiment fully confirmed my
conjectures.
"I operated the reduction of litharge in closed vessels
with Hale's apparatus, and I observed that at the
moment of the passage of the calx into the metallic
state, there was a disengagement of air in considerable
quantity, and that this air formed a volume at least
one thousand times greater than that of the litharge
employed.
"As this discovery appears to me one of the most
interesting which has been made since Stahl, I thought
it expedient to secure to myself the property, by
depositing the present note in the hands of the
Secretary of the Academy, to remain secret till the
period when I shall publish my experiments.
"LAVOISIER.
"Paris, 11th November 1772."
In his paper "On the Calcination of Tin in Closed Vessels, and on the Cause
of Increase of Weight acquired by the Metal during this Process" (published
in 1774), we see and admire Lavoisier's manner of working. A weighed
quantity (about half a pound) of tin was heated to melting in a glass
retort, the beak of which was drawn out to a very small opening; the air
within the retort having expanded, the opening was closed by melting the
glass before the blowpipe. The weight of retort and tin was now noted; the
tin was again heated to its melting point, and kept at this temperature as
long as the process of calcination appeared to proceed; the retort and its
contents were then allowed to cool and again weighed. No change was caused
by the heating process in the total weight of the whole apparatus. The end
of the retort beak was now broken off; air rushed in with a hissing sound.
The retort and contents were again weighed, and the increase over the
weight at the moment of sealing the retort was noted. The calcined tin in
the retort was now collected and weighed. It was found that the increase in
the weight of the tin was equal to the weight of the air which rushed into
the retort. Hence Lavoisier concluded that the calcination of tin was
accompanied by an absorption of air, and that the difference between the
weights of the tin and the calx of tin was equal to the weight of air
absorbed; but he states that probably only a part of the air had combined
with the tin, and that hence air is not a simple substance, but is composed
of two or more constituents.
Between the date of this publication and that of Lavoisier's next paper on
combustion we know that Priestley visited Paris. In his last work, "The
Doctrine of Phlogiston established" (published in 1800), Priestley says,
"Having made the discovery of dephlogisticated air some time before I was
in Paris in 1774, I mentioned it at the table of Mr. Lavoisier, when most
of the philosophical people in the city were present; saying that it was a
kind of air in which a candle burned much better than in common air, but I
had not then given it any name. At this all the company, and Mr. and Mrs.
Lavoisier as much as any, expressed great surprise. I told them that I had
got it from _precipitatum per se_, and also from _red lead_."
In 1775 Lavoisier's paper, "On the Nature of the Principle which combines
with the Metals during their Calcination, and which augments their Weight,"
was read before the Academy. The preparation and properties of an air
obtained, in November 1774, from _red precipitate_ are described, but
Priestley's name is not mentioned. It seems probable, however, that
Lavoisier learned the existence and the mode of preparation of this air
from Priestley;[5] but we have seen that even in 1779 Priestley was quite
in the dark as to the true nature of the air discovered by him (p. 60).
In papers published in the next three or four years Lavoisier gradually
defined and more thoroughly explained the phenomenon of combustion. He
burned phosphorus in a confined volume of air, and found that about
one-fourth of the air disappeared, that the residual portion of air was
unable to support combustion or to sustain animal life, that the phosphorus
was converted into a white substance deposited on the sides of the vessel
in which the experiment was performed, and that for each grain of
phosphorus used about two and a half grains of this white solid were
obtained. He further described the properties of the substance produced by
burning phosphorus, gave it the name of _phosphoric acid_, and described
some of the substances formed by combining it with various bases.
The burning of candles in air was about this time studied by Lavoisier. He
regarded his experiments as proving that the air which remained after
burning a candle, and in which animal life could not be sustained, was
really present before the burning; that common air consisted of about
one-fourth part of dephlogisticated air and three-fourths of _azotic air_
(_i.e._ air incapable of sustaining life); and that the burning candle
simply combined with, and so removed the former of these, and at the same
time produced more or less fixed air.
In his treatise on chemistry Lavoisier describes more fully his proof that
the calcination of a metal consists in the removal, by the metal, of
dephlogisticated air (or oxygen) from the atmosphere, and that the metallic
calx is simply a compound of metal and oxygen. The experiments are strictly
quantitative and are thoroughly conclusive. He placed four ounces of pure
mercury in a glass balloon, the neck of which dipped beneath the surface of
mercury in a glass dish, and then passed a little way up into a jar
containing fifty cubic inches of air, and standing in the mercury in the
dish. There was thus free communication between the air in the balloon and
that in the glass jar, but no communication between the air inside and that
outside the whole apparatus. The mercury in the balloon was heated nearly
to its boiling point for twelve days, during which time red-coloured specks
gradually formed on the surface of the metal; at the end of this time it
was found that the air in the glass jar measured between forty-two and
forty-three cubic inches. The red specks when collected amounted to
forty-five grains; they were heated in a very small retort connected with a
graduated glass cylinder containing mercury. Between seven and eight cubic
inches of pure dephlogisticated air (oxygen) were obtained in this
cylinder, and forty-one and a half grains of metallic mercury remained when
the decomposition of the red substance was completed.
The conclusion drawn by Lavoisier from these experiments was that mercury,
when heated nearly to boiling in contact with air, withdraws oxygen from
the air and combines with this gas to form _red precipitate_, and that when
the red precipitate which has been thus formed is strongly heated, it parts
with the whole of its oxygen, and is changed back again into metallic
mercury.
Lavoisier had now (1777-78) proved that the calces of mercury, tin and lead
are compounds of these metals with oxygen; and that the oxygen is obtained
from the atmosphere when the metal burns. But the phlogistic chemistry was
not yet overthrown. We have seen that the upholders of phlogiston believed
that in the inflammable air of Cavendish they had at last succeeded in
obtaining the long-sought-for phlogiston. Now they triumphantly asked, Why,
when metals dissolve in diluted vitriolic or muriatic acid with evolution
of inflammable air, are calces of these metals produced? And they answered
as triumphantly, Because these metals lose phlogiston by this process, and
we know that a calx is a metal deprived of its phlogiston.
Lavoisier contented himself with observing that a metallic calx always
weighed more than the metal from which it was produced; and that as
inflammable air, although much lighter than common air, was distinctly
possessed of weight, it was not possible that a metallic calx could be
metal deprived of inflammable air. He had given a simple explanation of the
process of calcination, and had proved, by accurate experiments, that this
explanation was certainly true in some cases. Although all the known facts
about solution of metals in acids could not as yet be brought within his
explanation, yet none of these facts was absolutely contradictory of that
explanation. He was content to wait for further knowledge. And to gain this
further knowledge he set about devising and performing new experiments. The
upholders of the theory of phlogiston laid considerable stress on the fact
that metals are produced by heating metallic calces in inflammable air; the
air is absorbed, they said, and so the metal is reproduced. It was
obviously of the utmost importance that Lavoisier should learn more about
this inflammable air, and especially that he should know exactly what
happened when this air was burned. He therefore prepared to burn a large
quantity of inflammable air, arranging the experiment so that he should be
able to collect and examine the product of this burning, whatever should be
the nature of that product. But at this time the news was brought to Paris
that Cavendish had obtained water by burning mixtures of inflammable and
dephlogisticated airs. This must have been a most exciting announcement to
Lavoisier; he saw how much depended on the accuracy of this statement, and
as a true student of Nature, he at once set about to prove or disprove it.
On the 24th of June 1783, in the presence of the King and several
notabilities (including Sir Charles Blagden, Secretary of the Royal
Society, who had told Lavoisier of the experiments of Cavendish), Lavoisier
and Laplace burned inflammable and dephlogisticated airs, and obtained
water. As the result of these experiments they determined that one volume
of dephlogisticated air combines with 1.91 volumes of inflammable air to
form water.
A little later Lavoisier completed the proof of the composition of water by
showing that when steam is passed through a tube containing iron filings
kept red hot, inflammable air is evolved and calx of iron remains in the
tube.
Lavoisier could now explain the conversion of a metallic calx into metal by
the action of inflammable air; this air decomposes the calx--that is, the
metallic oxide--combines with its oxygen to form water, and so the metal is
produced.
When a metal is dissolved in diluted vitriolic or muriatic acid a calx is
formed, because, according to Lavoisier, the water present is decomposed by
the metal, inflammable air is evolved, and the dephlogisticated air of the
water combines with the metal forming a calx, which then dissolves in the
acid.
Lavoisier now studied the properties of the compounds produced by burning
phosphorus, sulphur and carbon in dephlogisticated air. He found that
solutions of these compounds in water had a more or less sour taste and
turned certain blue colouring matters red; but these were the properties
regarded as especially belonging to acids. These products of combustion in
dephlogisticated air were therefore acids; but as phosphorus, carbon and
sulphur were not themselves acids, the acid character of the substances
obtained by burning these bodies in dephlogisticated air must be due to the
presence in them of this air. Hence Lavoisier concluded that this air is
the substance the presence of which in a compound confers acid properties
on that compound. This view of the action of dephlogisticated air he
perpetuated in the name "oxygen" (from Greek, = _acid-producer_), which he
gave to dephlogisticated air, and by which name this gas has ever since
been known.
Priestley was of opinion that the atmosphere is rendered noxious by the
breathing of animals, because it is thereby much phlogisticated, and he
thought that his experiments rendered it very probable that plants are able
to purify this noxious air by taking away phlogiston from it (see p. 69).
But Lavoisier was now able to give a much more definite account of the
effects on the atmosphere of animal and vegetable life. He had already
shown that ordinary air contains oxygen and azote (nitrogen), and that the
former is alone concerned in the process of combustion. He was now able to
show that animals during respiration draw in air into their lungs: that a
portion of the oxygen is there combined with carbon to form carbonic acid
gas (as the fixed air of Black was now generally called), which is again
expired along with unaltered azote. Respiration was thus proved to be a
process chemically analogous to that of calcination.
Thus, about the year 1784-85, the theory of phlogiston appeared to be quite
overthrown. The arguments of its upholders, after this time, were not
founded on facts; they consisted of fanciful interpretations of crudely
performed experiments. Cavendish was the only opponent to be dreaded by the
supporters of the new chemistry. But we have seen that although Cavendish
retained the language of the phlogistic theory (see pp. 78, 79) as in his
opinion equally applicable to the facts of combustion with that of the new
or Lavoisierian theory, he nevertheless practically admitted the essential
point of the latter, viz. that calces are compounds of metal and oxygen (or
dephlogisticated air). Although Cavendish was the first to show that water
is produced when the two gases hydrogen and oxygen are exploded together,
it would yet appear that he did not fully grasp the fact that water is a
compound of these two gases; it was left to Lavoisier to give a clear
statement of this all-important fact, and thus to remove the last prop from
under the now tottering, but once stately edifice built by Stahl and his
successors.
The explanation given by Lavoisier of combustion was to a great extent
based on a conception of element and compound very different from that of
the older chemists. In the "Sceptical Chymist" (1661) Boyle had argued
strongly against the doctrine of the four "elementary principles," earth,
air, fire and water, as held by the "vulgar chymists." The existence of
these principles, or some of them, in every compound substance was firmly
held by most chemists in Boyle's time. They argued thus: when a piece of
green wood bums, the existence in the wood of the principle of fire is made
evident by the flame, of the principle of air by the smoke which ascends,
of that of water by the hissing and boiling sound, and of the principle of
earth by the ashes which remain when the burning is finished.[6]
Boyle combated the inference that because a flame is visible round the
burning wood, and a light air or smoke ascends from it, _therefore_ these
principles were contained in the wood before combustion began. He tried to
prove by experiments that one substance may be obtained from another in
which the first substance did not already exist; thus, he heated water for
a year in a closed glass vessel, and obtained solid particles heavier than,
and as he supposed formed from, the water. We have already learned the
true interpretation of this experiment from the work of Lavoisier. Boyle
grew various vegetables in water only, and thought that he had thus changed
water into solid vegetable matter. He tells travellers' tales of the growth
of pieces of iron and other metals in the earth or while kept in
underground cellars.
We now know how erroneous in most points this reasoning was, but we must
admit that Boyle established one point most satisfactorily, viz. that
because earth, or air, or fire, or water is obtained by heating or
otherwise decomposing a substance, it does not necessarily follow that the
earth, or air, or fire, or water existed as such in the original substance.
He overthrew the doctrine of elementary principles held by the "vulgar
chymists." Defining elements as "certain primitive and simple 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," Boyle admitted the _possible_
existence, but thought that the facts known at his time did not warrant the
assertion of the _certain_ existence, of such "elements." The work of Hooke
and Mayow on combustion tended to strengthen this definition of "element"
given by Boyle.
Black, as we have seen, clearly proved that certain chemical substances
were possessed of definite and unvarying composition and properties; and
Lavoisier, indirectly by his explanation of combustion, and directly in
his "Treatise on Chemistry", laid down the definition of "element" which is
now universally adopted.
An element is a substance from which no simpler forms of matter--that is,
no forms of matter each weighing less than the original substance--have _as
yet_ been obtained.
In the decade 1774-1784 chemical science was thus established on a sure
foundation by Lavoisier. Like most great builders, whether of physical or
mental structures, he used the materials gathered by those who came before
him, but the merit of arranging these materials into a well-laid
foundation, on which the future building might firmly rest, is due to him
alone.
The value of Lavoisier's work now began to be recognized by his
fellow-chemists in France. In 1785 Berthollet, one of the most rising of
the younger French chemists, declared himself a convert to the views of
Lavoisier on combustion. Fourcroy, another member of the Academy, soon
followed the example of Berthollet. Fourcroy, knowing the weakness of his
countrymen, saw that if the new views could be made to appear as especially
the views of Frenchmen, the victory would be won; he therefore gave to the
theory of Lavoisier the name "_La chimie Francaise_". Although this name
was obviously unfair to Lavoisier, it nevertheless caused the
antiphlogistic theory to be identified with the French chemists, and
succeeded in impressing the French public generally with the idea that to
hold to the old theory was to be a traitor to the glory of one's country.
M. de Morveau, who held a prominent place both in politics and science, was
invited to Paris, and before long was persuaded to embrace the new theory.
This conversion--for "the whole matter was managed as if it had been a
political intrigue rather than a philosophical inquiry"--was of great
importance to Lavoisier and his friends. M. de Morveau was editor of the
chemical part of the "Encyclopedie Methodique;" in that part of this work
which had appeared before 1784 De Morveau had skilfully opposed the
opinions of Lavoisier, but in the second part of the work he introduced an
advertisement announcing the change in his opinions on the subject of
combustion, and giving his reasons for this change.
The importance of having a definite language in every science is apparent
at each step of advance. Lavoisier found great difficulty in making his
opinions clear because he was obliged to use a language which had been
introduced by the phlogistic chemists, and which bore the impress of that
theory on most of its terms. About the years 1785-1787, Lavoisier,
Berthollet, Fourcroy and De Morveau drew up a new system of chemical
nomenclature. The fundamental principles of that system have remained as
those of every nomenclature since proposed. They are briefly these:--
An element is a substance from which no form of matter simpler than itself
has as yet been obtained.
Every substance is to be regarded as an element until it is proved to be
otherwise.
The name of every compound is to tell of what elements the substance is
composed, and it is to express as far as possible the relative amounts of
the elements which go to form the compound.
Thus the compounds of oxygen with any other element were called oxides,
_e.g._ iron oxide, mercury oxide, tin oxide, etc. When two oxides of iron
came to be known, one containing more oxygen relatively to the amount of
iron present than the other, that with the greater quantity of oxygen was
called iron peroxide, and that with the smaller quantity iron protoxide.
We now generally prefer to use the name of the element other than oxygen in
adjectival form, and to indicate the relatively smaller or greater quantity
of oxygen present by modifications in the termination of this adjective.
Thus iron protoxide is now generally known as ferr_ous_ oxide, and iron
peroxide as ferr_ic_ oxide. But the principles laid down by the four French
chemists in 1785-1787 remain as the groundwork of our present system of
nomenclature.
The antiphlogistic theory was soon adopted by all French chemists of note.
We have already seen that Black, with his usual candour and openness to
conviction, adopted and taught this theory, and we are assured by Dr.
Thomas Thomson that when he attended Black's classes, nine years after the
publication of the French system of nomenclature, that system was in
general use among the chemical students of the university. The older theory
was naturally upheld by the countrymen of the distinguished Stahl after it
had been given up in France. In the year 1792 Klaproth, who was then
Professor of Chemistry in Berlin, proposed to the Berlin Academy of
Sciences to repeat the more important experiments on which the Lavoisierian
theory rested, before the Academy. His offer was accepted, and from that
time most of the Berlin chemists declared themselves in favour of the new
theory.
By the close of last century the teaching of Lavoisier regarding combustion
found almost universal assent among chemists. But this teaching carried
with it, as necessary parts, the fundamental distinction between element
and compound; the denial of the existence of "principles" or "essences;"
the recognition of the study of actually occurring reactions between
substances as the basis on which all true chemical knowledge was to be
built; and the full acknowledgment of the fact that matter is neither
created nor destroyed, but only changed as to its form, in any chemical
reaction.
Of Lavoisier's other work I can only mention the paper on "Specific Heats"
contributed by Laplace and Lavoisier to the Memoirs of the Academy for
1780. In this paper is described the ice calorimeter, whereby the amount of
heat given out by a substance in cooling from one definite temperature to
another is determined, by measuring the amount of ice converted into water
by the heated substance in cooling through the stated interval of
temperature. The specific heats of various substances, _e.g._ iron, glass,
mercury, quicklime, etc., were determined by the help of this instrument.
As we read the record of work done by Lavoisier during the years between
1774 and 1794--work which must have involved a great amount of concentrated
thought as well as the expenditure of much time--we find it hard to realize
that the most tremendous political and social revolution which the modern
world has seen was raging around him during this time.
In the earlier days of the French Revolution, and in the time immediately
preceding that movement, many minds had been stirred to see the importance
of the study of Nature; but it was impossible that natural science should
continue to flourish when the tyrant Robespierre had begun the Reign of
Terror.
The roll of those who perished during this time contains no more
illustrious name than that of Antoine Laurent Lavoisier. In the year 1794
Lavoisier, who had for some time acted as a _fermier-general_ under the
Government, was accused of mixing with the tobacco "water and other
ingredients hurtful to the health of the citizens." On this pretext he and
some of his colleagues were condemned to death. For some days Lavoisier
found a hiding-place among his friends, but hearing that his colleagues had
been arrested, he delivered himself up to the authorities, only asking
that the death sentence should not be executed until he had completed the
research in which he was engaged; "not" that he was "unwilling to part with
life," but because he thought the results would be "for the good of
humanity."
"The Republic has no need of chemists; the course of justice cannot be
suspended," was the reply.
On the 8th of May 1794, the guillotine did its work; and in his fifty-first
year Lavoisier "joined the majority." To the honour of the Academy of which
he was so illustrious a member it is recorded that a deputation of his
fellow-workers in science, braving the wrath of Robespierre, penetrated to
the dungeons of the prison and placed a wreath on the grave of their
comrade.
* * * * *
The period of the infancy of chemical science which I have now briefly
described is broadly contemporaneous with the second half of the eighteenth
century.
At this time the minds of men were greatly stirred. Opinions and beliefs
consecrated by the assent of generations of men were questioned or denied;
the pretensions of civil and ecclesiastical authorities were withstood;
assertions however strongly made, and by whatever authority supported, were
met by demands for reasons. In France this revolt against mere authority
was especially marked. Led by the great thinker Voltaire, the French
philosophers attacked the generally accepted views in moral, theological
and historical matters. A little later they began to turn with eager
attention and hope to the facts of external Nature. Physical science was
cultivated with wonderful vigour and with surprising success.
In the sciences of heat and light we have at this time the all-important
works of Fourier, Prevost and Fresnel; in geology and natural history we
have Buffon and Cuvier; the name of Bichat marks the beginning of
biological science, and chemistry takes rank as a science only from the
time of Lavoisier.
From the philosophers an interest in natural science spread through the
mass of the people. About the year 1870 the lecture-rooms of the great
teachers of chemistry, astronomy, electricity, and even anatomy were
crowded with ladies and gentlemen of fashion in the French capital. A
similar state of matters was noticeable in this country. Dr. Black's
lecture theatre was filled by an audience which comprised many young men of
good position. To know something of chemistry became an essential part of
the training of all who desired to be liberally educated.
The secrets of Nature were now rapidly explored; astonishing advances were
made, and as a matter of course much opposition was raised.
In this active, inquiring atmosphere the young science of chemistry grew
towards maturity.
Priestley, ever seeking for new facts, announcing discovery after
discovery, attacking popular belief in most matters, yet satisfied to
interpret his scientific discoveries in terms of the hypothesis with which
he was most familiar, was the pioneer of the advancing science. He may be
compared to the advance-guard sent forward by the explorers of a new
country with orders to clear a way for the main body: his work was not to
level the rough parts of the way, or to fill in the miry places with
well-laid metal, but rather rapidly to make a road as far into the heart of
the country as possible.
And we have seen how well he did the work. In his discovery of various
kinds of airs, notably of oxygen, he laid the basis of the great
generalizations of Lavoisier, and, what was perhaps of even more
importance, he introduced a new method into chemistry. He showed the
existence of a new and unexplored region. Before his time, Hooke and Mayow
had proved the existence of more than one kind of air, but the chemistry of
gases arose with the discoveries of Priestley.
Although Black's chief research, on fixed air and on latent heat, was
completed fifteen or twenty years before Priestley's discovery of oxygen,
yet the kind of work done by Black, and its influence on chemical science,
mark him as coming after Priestley in order of development. We have seen
that the work of Black was characterized by thoroughness and
suggestiveness. The largeness of scope, the breadth of view, of this great
philosopher are best illustrated in his discourses on heat; he there leads
us with him in his survey of the domain of Nature, and although he tells us
that hypotheses are a "mere waste of time," we find that it is by the
strength of his imagination that he commands assent. But he never allows
the imagination to degenerate into fanciful guesses; he vigorously tests
the fundamental facts of his theory, and then he uses the imagination in
developing the necessary consequences of these facts.
To Black we owe not only the first rigorously accurate chemical
investigation, but also the establishment of just ideas concerning the
nature of heat.
But Lavoisier came before us as a greater than either Priestley or Black.
To great accuracy and great breadth of view he added wonderful power of
generalizing; with these, aided by marked mental activity and, on the
whole, favourable external circumstances, he was able finally to overthrow
the loose opinions regarding combustion and elementary principles which
prevailed before his time, and so to establish chemistry as one of the
natural sciences.
At the close of the first period of advance we find that the sphere of
chemistry has been defined; that the object of the science has been laid
down, as being to find an explanation of the remarkable changes noticed in
the properties of bodies; that as a first step towards the wished-for
explanation, all material substances have been divided by the chemist into
elements and compounds; that an element has been defined as any kind of
matter from a given weight of which no simpler forms of matter--that is,
no kinds of matter each weighing less than the original matter--have as yet
been obtained; that the great principle of the indestructibility of matter
has been established, viz. that however the properties of matter may be
altered, yet the total mass (or quantity) remains unchanged; and lastly, we
find that an explanation of one important class of chemical changes--those
changes which occur when substances burn--has been found.
And we have also learned that the method by which these results were
obtained was this--to go to Nature, to observe and experiment accurately,
to consider carefully the results of these experiments, and so to form a
general hypothesis; by the use of the mental powers, and notably by the use
of the imagination, to develop the necessary deductions from this
hypothesis; and finally, to try these deductions by again inquiring from
Nature "whether these things were so."
Before the time which we have been considering the paths of chemical
science had scarcely yet been trodden. Each discovery was full of promise,
each advance displayed the possibility of further progress; the atmosphere
was filled as with "a mighty rushing wind" ready to sweep away the old
order of things. The age was an age of doubt and of freedom from the
trammels of authority; it was a time eminently suited for making advances
in natural knowledge.
In the unceasing activity of Priestley and Lavoisier we may trace the
influence of the restlessness of the age; but in the quietness and strength
of the best work of these men, and notably in the work of Black; in the
calmness with which Priestley bore his misfortunes at Birmingham; in the
noble words of Lavoisier, "I am not unwilling to part with life, but I ask
time to finish my experiments, because the results will, I believe, be for
the good of humanity"--we see the truth of the assertion made by one who
was himself a faithful student of Nature--
"Nature never did betray
The heart that loved her."
FOOTNOTES:
[4] The translation is taken from Thomson's "History of Chemistry."
[5] Nevertheless, in other places Lavoisier most readily acknowledges the
merits of Priestley.
[6] A similar method of reasoning was employed so far back as the tenth
century: thus, in an Anglo-Saxon "Manual of Astronomy" we read, "There is
no corporeal thing which has not in it the four elements, that is, air and
fire, earth and water.... Take a stick and rub it on something, it becomes
hot directly with the fire which lurks in it; burn one end, then goeth the
moisture out at the other end with the smoke."