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A History of Science, Volume 1, by Henry Smith Williams

Scanned by Charles Keller with OmniPage Professional OCR software







AS regards chronology, the epoch covered in the present volume is
identical with that viewed in the preceding one. But now as
regards subject matter we pass on to those diverse phases of the
physical world which are the field of the chemist, and to those
yet more intricate processes which have to do with living
organisms. So radical are the changes here that we seem to be
entering new worlds; and yet, here as before, there are
intimations of the new discoveries away back in the Greek days.
The solution of the problem of respiration will remind us that
Anaxagoras half guessed the secret; and in those diversified
studies which tell us of the Daltonian atom in its wonderful
transmutations, we shall be reminded again of the Clazomenian
philosopher and his successor Democritus.

Yet we should press the analogy much too far were we to intimate
that the Greek of the elder day or any thinker of a more recent
period had penetrated, even in the vaguest way, all of the
mysteries that the nineteenth century has revealed in the fields
of chemistry and biology. At the very most the insight of those
great Greeks and of the wonderful seventeenth-century
philosophers who so often seemed on the verge of our later
discoveries did no more than vaguely anticipate their successors
of this later century. To gain an accurate, really specific
knowledge of the properties of elementary bodies was reserved for
the chemists of a recent epoch. The vague Greek questionings as
to organic evolution were world-wide from the precise inductions
of a Darwin. If the mediaeval Arabian endeavored to dull the
knife of the surgeon with the use of drugs, his results hardly
merit to be termed even an anticipation of modern anaesthesia.
And when we speak of preventive medicine--of bacteriology in all
its phases--we have to do with a marvellous field of which no
previous generation of men had even the slightest inkling.

All in all, then, those that lie before us are perhaps the most
wonderful and the most fascinating of all the fields of science.
As the chapters of the preceding book carried us out into a
macrocosm of inconceivable magnitude, our present studies are to
reveal a microcosm of equally inconceivable smallness. As the
studies of the physicist attempted to reveal the very nature of
matter and of energy, we have now to seek the solution of the yet
more inscrutable problems of life and of mind.


The development of the science of chemistry from the "science" of
alchemy is a striking example of the complete revolution in the
attitude of observers in the field of science. As has been
pointed out in a preceding chapter, the alchemist, having a
preconceived idea of how things should be, made all his
experiments to prove his preconceived theory; while the chemist
reverses this attitude of mind and bases his conceptions on the
results of his laboratory experiments. In short, chemistry is
what alchemy never could be, an inductive science. But this
transition from one point of view to an exactly opposite one was
necessarily a very slow process. Ideas that have held undisputed
sway over the minds of succeeding generations for hundreds of
years cannot be overthrown in a moment, unless the agent of such
an overthrow be so obvious that it cannot be challenged. The
rudimentary chemistry that overthrew alchemy had nothing so
obvious and palpable.

The great first step was the substitution of the one principle,
phlogiston, for the three principles, salt, sulphur, and mercury.
We have seen how the experiment of burning or calcining such a
metal as lead "destroyed" the lead as such, leaving an entirely
different substance in its place, and how the original metal
could be restored by the addition of wheat to the calcined
product. To the alchemist this was "mortification" and
"revivification" of the metal. For, as pointed out by
Paracelsus, "anything that could be killed by man could also be
revivified by him, although this was not possible to the things
killed by God." The burning of such substances as wood, wax,
oil, etc., was also looked upon as the same "killing" process,
and the fact that the alchemist was unable to revivify them was
regarded as simply the lack of skill on his part, and in no wise
affecting the theory itself.

But the iconoclastic spirit, if not the acceptance of all the
teachings, of the great Paracelsus had been gradually taking root
among the better class of alchemists, and about the middle of the
seventeenth century Robert Boyle (1626-1691) called attention to
the possibility of making a wrong deduction from the phenomenon
of the calcination of the metals, because of a very important
factor, the action of the air, which was generally overlooked.
And he urged his colleagues of the laboratories to give greater
heed to certain other phenomena that might pass unnoticed in the
ordinary calcinating process. In his work, The Sceptical Chemist,
he showed the reasons for doubting the threefold constitution of
matter; and in his General History of the Air advanced some novel
and carefully studied theories as to the composition of the
atmosphere. This was an important step, and although Boyle is not
directly responsible for the phlogiston theory, it is probable
that his experiments on the atmosphere influenced considerably
the real founders, Becker and Stahl.

Boyle gave very definitely his idea of how he thought air might
be composed. "I conjecture that the atmospherical air consists of
three different kinds of corpuscles," he says; "the first, those
numberless particles which, in the form of vapors or dry
exhalations, ascend from the earth, water, minerals, vegetables,
animals, etc.; in a word, whatever substances are elevated by the
celestial or subterraneal heat, and thence diffused into the
atmosphere. The second may be yet more subtle, and consist of
those exceedingly minute atoms, the magnetical effluvia of the
earth, with other innumerable particles sent out from the bodies
of the celestial luminaries, and causing, by their influence, the
idea of light in us. The third sort is its characteristic and
essential property, I mean permanently elastic parts. Various
hypotheses may be framed relating to the structure of these later
particles of the air. They might be resembled to the springs of
watches, coiled up and endeavoring to restore themselves; to
wool, which, being compressed, has an elastic force; to slender
wires of different substances, consistencies, lengths, and
thickness; in greater curls or less, near to, or remote from each
other, etc., yet all continuing springy, expansible, and
compressible. Lastly, they may also be compared to the thin
shavings of different kinds of wood, various in their lengths,
breadth, and thickness. And this, perhaps, will seem the most
eligible hypothesis, because it, in some measure, illustrates the
production of the elastic particles we are considering. For no
art or curious instruments are required to make these shavings
whose curls are in no wise uniform, but seemingly casual; and
what is more remarkable, bodies that before seemed unelastic, as
beams and blocks, will afford them."[1]

Although this explanation of the composition of the air is most
crude, it had the effect of directing attention to the fact that
the atmosphere is not "mere nothingness," but a "something" with
a definite composition, and this served as a good foundation for
future investigations. To be sure, Boyle was neither the first
nor the only chemist who had suspected that the air was a mixture
of gases, and not a simple one, and that only certain of these
gases take part in the process of calcination. Jean Rey, a
French physician, and John Mayow, an Englishman, had preformed
experiments which showed conclusively that the air was not a
simple substance; but Boyle's work was better known, and in its
effect probably more important. But with all Boyle's explanations
of the composition of air, he still believed that there was an
inexplicable something, a "vital substance," which he was unable
to fathom, and which later became the basis of Stahl's phlogiston
theory. Commenting on this mysterious substance, Boyle says:
"The, difficulty we find in keeping flame and fire alive, though
but for a little time, without air, renders it suspicious that
there be dispersed through the rest of the atmosphere some odd
substance, either of a solar, astral, or other foreign nature; on
account of which the air is so necessary to the substance of
flame!" It was this idea that attracted the attention of George
Ernst Stahl (1660-1734), a professor of medicine in the
University of Halle, who later founded his new theory upon it.
Stahl's theory was a development of an earlier chemist, Johann
Joachim Becker (1635-1682), in whose footsteps he followed and
whose experiments he carried further.

In many experiments Stahl had been struck with the fact that
certain substances, while differing widely, from one another in
many respects, were alike in combustibility. From this he argued
that all combustible substances must contain a common principle,
and this principle he named phlogiston. This phlogiston he
believed to be intimately associated in combination with other
substances in nature, and in that condition not perceivable by
the senses; but it was supposed to escape as a substance burned,
and become apparent to the senses as fire or flame. In other
words, phlogiston was something imprisoned in a combustible
structure (itself forming part of the structure), and only
liberated when this structure was destroyed. Fire, or flame, was
FREE phlogiston, while the imprisoned phlogiston was called
COMBINED PHLOGISTON, or combined fire. The peculiar quality of
this strange substance was that it disliked freedom and was
always striving to conceal itself in some combustible substance.
Boyle's tentative suggestion that heat was simply motion was
apparently not accepted by Stahl, or perhaps it was unknown to

According to the phlogistic theory, the part remaining after a
substance was burned was simply the original substance deprived
of phlogiston. To restore the original combustible substance, it
was necessary to heat the residue of the combustion with
something that burned easily, so that the freed phlogiston might
again combine with the ashes. This was explained by the
supposition that the more combustible a substance was the more
phlogiston it contained, and since free phlogiston sought always
to combine with some suitable substance, it was only necessary to
mix the phlogisticating agents, such as charcoal, phosphorus,
oils, fats, etc., with the ashes of the original substance, and
heat the mixture, the phlogiston thus freed uniting at once with
the ashes. This theory fitted very nicely as applied to the
calcined lead revivified by the grains of wheat, although with
some other products of calcination it did not seem to apply at

It will be seen from this that the phlogistic theory was a step
towards chemistry and away from alchemy. It led away from the
idea of a "spirit" in metals that could not be seen, felt, or
appreciated by any of the senses, and substituted for it a
principle which, although a falsely conceived one, was still much
more tangible than the "spirit," since it could be seen and felt
as free phlogiston and weighed and measured as combined
phlogiston. The definiteness of the statement that a metal, for
example, was composed of phlogiston and an element was much less
enigmatic, even if wrong, than the statement of the alchemist
that "metals are produced by the spiritual action of the three
principles, salt, mercury, sulphur"--particularly when it is
explained that salt, mercury, and sulphur were really not what
their names implied, and that there was no universally accepted
belief as to what they really were.

The metals, which are now regarded as elementary bodies, were
considered compounds by the phlogistians, and they believed that
the calcining of a metal was a process of simplification. They
noted, however, that the remains of calcination weighed more than
the original product, and the natural inference from this would
be that the metal must have taken in some substance rather than
have given off anything. But the phlogistians had not learned
the all-important significance of weights, and their explanation
of variation in weight was either that such gain or loss was an
unimportant "accident" at best, or that phlogiston, being light,
tended to lighten any substance containing it, so that driving it
out of the metal by calcination naturally left the residue

At first the phlogiston theory seemed to explain in an
indisputable way all the known chemical phenomena. Gradually,
however, as experiments multiplied, it became evident that the
plain theory as stated by Stahl and his followers failed to
explain satisfactorily certain laboratory reactions. To meet
these new conditions, certain modifications were introduced from
time to time, giving the theory a flexibility that would allow it
to cover all cases. But as the number of inexplicable experiments
continued to increase, and new modifications to the theory became
necessary, it was found that some of these modifications were
directly contradictory to others, and thus the simple theory
became too cumbersome from the number of its modifications. Its
supporters disagreed among themselves, first as to the
explanation of certain phenomena that did not seem to accord with
the phlogistic theory, and a little later as to the theory
itself. But as yet there was no satisfactory substitute for this
theory, which, even if unsatisfactory, seemed better than
anything that had gone before or could be suggested.

But the good effects of the era of experimental research, to
which the theory of Stahl had given such an impetus, were showing
in the attitude of the experimenters. The works of some of the
older writers, such as Boyle and Hooke, were again sought out in
their dusty corners and consulted, and their surmises as to the
possible mixture of various gases in the air were more carefully
considered. Still the phlogiston theory was firmly grounded in
the minds of the philosophers, who can hardly be censured for
adhering to it, at least until some satisfactory substitute was
offered. The foundation for such a theory was finally laid, as
we shall see presently, by the work of Black, Priestley,
Cavendish, and Lavoisier, in the eighteenth century, but the
phlogiston theory cannot be said to have finally succumbed until
the opening years of the nineteenth century.



Modern chemistry may be said to have its beginning with the work
of Stephen Hales (1677-1761), who early in the eighteenth century
began his important study of the elasticity of air. Departing
from the point of view of most of the scientists of the time, be
considered air to be "a fine elastic fluid, with particles of
very different nature floating in it" ; and he showed that these
"particles" could be separated. He pointed out, also, that
various gases, or "airs," as he called them, were contained in
many solid substances. The importance of his work, however, lies
in the fact that his general studies were along lines leading
away from the accepted doctrines of the time, and that they gave
the impetus to the investigation of the properties of gases by
such chemists as Black, Priestley, Cavendish, and Lavoisier,
whose specific discoveries are the foundation-stones of modern


The careful studies of Hales were continued by his younger
confrere, Dr. Joseph Black (1728-1799), whose experiments in the
weights of gases and other chemicals were first steps in
quantitative chemistry. But even more important than his
discoveries of chemical properties in general was his discovery
of the properties of carbonic-acid gas.

Black had been educated for the medical profession in the
University of Glasgow, being a friend and pupil of the famous Dr.
William Cullen. But his liking was for the chemical laboratory
rather than for the practice of medicine. Within three years
after completing his medical course, and when only twenty-three
years of age, he made the discovery of the properties of carbonic
acid, which he called by the name of "fixed air." After
discovering this gas, Black made a long series of experiments, by
which he was able to show how widely it was distributed
throughout nature. Thus, in 1757, be discovered that the bubbles
given off in the process of brewing, where there was vegetable
fermentation, were composed of it. To prove this, he collected
the contents of these bubbles in a bottle containing lime-water.
When this bottle was shaken violently, so that the lime-water and
the carbonic acid became thoroughly mixed, an insoluble white
powder was precipitated from the solution, the carbonic acid
having combined chemically with the lime to form the insoluble
calcium carbonate, or chalk. This experiment suggested another.
Fixing a piece of burning charcoal in the end of a bellows, he
arranged a tube so that the gas coming from the charcoal would
pass through the lime-water, and, as in the case of the bubbles
from the brewer's vat, he found that the white precipitate was
thrown down; in short, that carbonic acid was given off in
combustion. Shortly after, Black discovered that by blowing
through a glass tube inserted into lime-water, chalk was
precipitated, thus proving that carbonic acid was being
constantly thrown off in respiration.

The effect of Black's discoveries was revolutionary, and the
attitude of mind of the chemists towards gases, or "airs," was
changed from that time forward. Most of the chemists, however,
attempted to harmonize the new facts with the older theories--to
explain all the phenomena on the basis of the phlogiston theory,
which was still dominant. But while many of Black's discoveries
could not be made to harmonize with that theory, they did not
directly overthrow it. It required the additional discoveries of
some of Black's fellow-scientists to complete its downfall, as we
shall see.


This work of Black's was followed by the equally important work
of his former pupil, Henry Cavendish (1731-1810), whose discovery
of the composition of many substances, notably of nitric acid and
of water, was of great importance, adding another link to the
important chain of evidence against the phlogiston theory.
Cavendish is one of the most eccentric figures in the history of
science, being widely known in his own time for his immense
wealth and brilliant intellect, and also for his peculiarities
and his morbid sensibility, which made him dread society, and
probably did much in determining his career. Fortunately for him,
and incidentally for the cause of science, he was able to pursue
laboratory investigations without being obliged to mingle with
his dreaded fellow-mortals, his every want being provided for by
the immense fortune inherited from his father and an uncle.

When a young man, as a pupil of Dr. Black, he had become imbued
with the enthusiasm of his teacher, continuing Black's
investigations as to the properties of carbonic-acid gas when
free and in combination. One of his first investigations was
reported in 1766, when he communicated to the Royal Society his
experiments for ascertaining the properties of carbonic-acid and
hydrogen gas, in which he first showed the possibility of
weighing permanently elastic fluids, although Torricelli had
before this shown the relative weights of a column of air and a
column of mercury. Other important experiments were continued by
Cavendish, and in 1784 he announced his discovery of the
composition of water, thus robbing it of its time-honored
position as an "element." But his claim to priority in this
discovery was at once disputed by his fellow-countryman James
Watt and by the Frenchman Lavoisier. Lavoisier's claim was soon
disallowed even by his own countrymen, but for many years a
bitter controversy was carried on by the partisans of Watt and
Cavendish. The two principals, however, seem. never to have
entered into this controversy with anything like the same ardor
as some of their successors, as they remained on the best of
terms.[1] It is certain, at any rate, that Cavendish announced
his discovery officially before Watt claimed that the
announcement had been previously made by him, "and, whether right
or wrong, the honor of scientific discoveries seems to be
accorded naturally to the man who first publishes a demonstration
of his discovery." Englishmen very generally admit the justness
of Cavendish's claim, although the French scientist Arago, after
reviewing the evidence carefully in 1833, decided in favor of

It appears that something like a year before Cavendish made known
his complete demonstration of the composition of water, Watt
communicated to the Royal Society a suggestion that water was
composed of "dephlogisticated air (oxygen) and phlogiston
(hydrogen) deprived of part of its latent heat." Cavendish knew
of the suggestion, but in his experiments refuted the idea that
the hydrogen lost any of its latent heat. Furthermore, Watt
merely suggested the possible composition without proving it,
although his idea was practically correct, if we can rightly
interpret the vagaries of the nomenclature then in use. But had
Watt taken the steps to demonstrate his theory, the great "Water
Controversy" would have been avoided. Cavendish's report of his
discovery to the Royal Society covers something like forty pages
of printed matter. In this he shows how, by passing an electric
spark through a closed jar containing a mixture of hydrogen gas
and oxygen, water is invariably formed, apparently by the union
of the two gases. The experiment was first tried with hydrogen
and common air, the oxygen of the air uniting with the hydrogen
to form water, leaving the nitrogen of the air still to be
accounted for. With pure oxygen and hydrogen, however, Cavendish
found that pure water was formed, leaving slight traces of any
other, substance which might not be interpreted as being Chemical
impurities. There was only one possible explanation of this
phenomenon--that hydrogen and oxygen, when combined, form water.

"By experiments with the globe it appeared," wrote Cavendish,
"that when inflammable and common air are exploded in a proper
proportion, almost all the inflammable air, and near one-fifth
the common air, lose their elasticity and are condensed into dew.
And by this experiment it appears that this dew is plain water,
and consequently that almost all the inflammable air is turned
into pure water.

"In order to examine the nature of the matter condensed on firing
a mixture of dephlogisticated and inflammable air, I took a glass
globe, holding 8800 grain measures, furnished with a brass cock
and an apparatus for firing by electricity. This globe was well
exhausted by an air-pump, and then filled with a mixture of
inflammable and dephlogisticated air by shutting the cock,
fastening the bent glass tube into its mouth, and letting up the
end of it into a glass jar inverted into water and containing a
mixture of 19,500 grain measures of dephlogisticated air, and
37,000 of inflammable air; so that, upon opening the cock, some
of this mixed air rushed through the bent tube and filled the
globe. The cock was then shut and the included air fired by
electricity, by means of which almost all of it lost its
elasticity (was condensed into water vapors). The cock was then
again opened so as to let in more of the same air to supply the
place of that destroyed by the explosion, which was again fired,
and the operation continued till almost the whole of the mixture
was let into the globe and exploded. By this means, though the
globe held not more than a sixth part of the mixture, almost the
whole of it was exploded therein without any fresh exhaustion of
the globe."

At first this condensed matter was "acid to the taste and
contained two grains of nitre," but Cavendish, suspecting that
this was due to impurities, tried another experiment that proved
conclusively that his opinions were correct. "I therefore made
another experiment," he says, "with some more of the same air
from plants in which the proportion of inflammable air was
greater, so that the burnt air was almost completely
phlogisticated, its standard being one-tenth. The condensed
liquor was then not at all acid, but seemed pure water."

From these experiments he concludes "that when a mixture of
inflammable and dephlogisticated air is exploded, in such
proportions that the burnt air is not much phlogisticated, the
condensed liquor contains a little acid which is always of the
nitrous kind, whatever substance the dephlogisticated air is
procured from; but if the proportion be such that the burnt air
is almost entirely phlogisticated, the condensed liquor is not at
all acid, but seems pure water, without any addition

These same experiments, which were undertaken to discover the
composition of water, led him to discover also the composition of
nitric acid. He had observed that, in the combustion of hydrogen
gas with common air, the water was slightly tinged with acid, but
that this was not the case when pure oxygen gas was used. Acting
upon this observation, he devised an experiment to determine the
nature of this acid. He constructed an apparatus whereby an
electric spark was passed through a vessel containing common air.
After this process had been carried on for several weeks a small
amount of liquid was formed. This liquid combined with a solution
of potash to form common nitre, which "detonated with charcoal,
sparkled when paper impregnated with it was burned, and gave out
nitrous fumes when sulphuric acid was poured on it." In other
words, the liquid was shown to be nitric acid. Now, since nothing
but pure air had been used in the initial experiment, and since
air is composed of nitrogen and oxygen, there seemed no room to
doubt that nitric acid is a combination of nitrogen and oxygen.

This discovery of the nature of nitric acid seems to have been
about the last work of importance that Cavendish did in the field
of chemistry, although almost to the hour of his death he was
constantly occupied with scientific observations. Even in the
last moments of his life this habit asserted itself, according to
Lord Brougham. "He died on March 10, 1810, after a short
illness, probably the first, as well as the last, which he ever
suffered. His habit of curious observation continued to the end.
He was desirous of marking the progress of the disease and the
gradual extinction of the vital powers. With these ends in view,
that he might not be disturbed, he desired to be left alone. His
servant, returning sooner than he had wished, was ordered again
to leave the chamber of death, and when be came back a second
time he found his master had expired.[3]


While the opulent but diffident Cavendish was making his
important discoveries, another Englishman, a poor country
preacher named Joseph Priestley (1733-1804) was not only
rivalling him, but, if anything, outstripping him in the pursuit
of chemical discoveries. In 1761 this young minister was given a
position as tutor in a nonconformist academy at Warrington, and
here, for six years, he was able to pursue his studies in
chemistry and electricity. In 1766, while on a visit to London,
he met Benjamin Franklin, at whose suggestion he published his
History of Electricity. From this time on he made steady
progress in scientific investigations, keeping up his
ecclesiastical duties at the same time. In 1780 he removed to
Birmingham, having there for associates such scientists as James
Watt, Boulton, and Erasmus Darwin.

Eleven years later, on the anniversary of the fall of the Bastile
in Paris, a fanatical mob, knowing Priestley's sympathies with
the French revolutionists, attacked his house and chapel, burning
both and destroying a great number of valuable papers and
scientific instruments. Priestley and his family escaped violence
by flight, but his most cherished possessions were destroyed; and
three years later he quitted England forever, removing to the
United States, whose struggle for liberty he had championed. The
last ten years of his life were spent at Northumberland,
Pennsylvania, where he continued his scientific researches.

Early in his scientific career Priestley began investigations
upon the "fixed air" of Dr. Black, and, oddly enough, he was
stimulated to this by the same thing that had influenced
Black--that is, his residence in the immediate neighborhood of a
brewery. It was during the course of a series of experiments on
this and other gases that he made his greatest discovery, that of
oxygen, or "dephlogisticated air," as he called it. The story of
this important discovery is probably best told in Priestley's own

"There are, I believe, very few maxims in philosophy that have
laid firmer hold upon the mind than that air, meaning atmospheric
air, is a simple elementary substance, indestructible and
unalterable, at least as much so as water is supposed to be. In
the course of my inquiries I was, however, soon satisfied that
atmospheric air is not an unalterable thing; for that, according
to my first hypothesis, the phlogiston with which it becomes
loaded from bodies burning in it, and the animals breathing it,
and various other chemical processes, so far alters and depraves
it as to render it altogether unfit for inflammation,
respiration, and other purposes to which it is subservient; and I
had discovered that agitation in the water, the process of
vegetation, and probably other natural processes, restore it to
its original purity....

"Having procured a lens of twelve inches diameter and twenty
inches local distance, I proceeded with the greatest alacrity, by
the help of it, to discover what kind of air a great variety of
substances would yield, putting them into the vessel, which I
filled with quicksilver, and kept inverted in a basin of the same
.... With this apparatus, after a variety of experiments .... on
the 1st of August, 1774, I endeavored to extract air from
mercurius calcinatus per se; and I presently found that, by means
of this lens, air was expelled from it very readily. Having got
about three or four times as much as the bulk of my materials, I
admitted water to it, and found that it was not imbibed by it.
But what surprised me more than I can express was that a candle
burned in this air with a remarkably vigorous flame, very much
like that enlarged flame with which a candle burns in nitrous
oxide, exposed to iron or liver of sulphur; but as I had got
nothing like this remarkable appearance from any kind of air
besides this particular modification of vitrous air, and I knew
no vitrous acid was used in the preparation of mercurius
calcinatus, I was utterly at a loss to account for it."[4]

The "new air" was, of course, oxygen. Priestley at once
proceeded to examine it by a long series of careful experiments,
in which, as will be seen, he discovered most of the remarkable
qualities of this gas. Continuing his description of these
experiments, he says:

"The flame of the candle, besides being larger, burned with more
splendor and heat than in that species of nitrous air; and a
piece of red-hot wood sparkled in it, exactly like paper dipped
in a solution of nitre, and it consumed very fast; an experiment
that I had never thought of trying with dephlogisticated nitrous

". . . I had so little suspicion of the air from the mercurius
calcinatus, etc., being wholesome, that I had not even thought of
applying it to the test of nitrous air; but thinking (as my
reader must imagine I frequently must have done) on the candle
burning in it after long agitation in water, it occurred to me at
last to make the experiment; and, putting one measure of nitrous
air to two measures of this air, I found not only that it was
diminished, but that it was diminished quite as much as common
air, and that the redness of the mixture was likewise equal to a
similar mixture of nitrous and common air.... The next day I was
more surprised than ever I had been before with finding that,
after the above-mentioned mixture of nitrous air and the air from
mercurius calcinatus had stood all night, . . . a candle burned
in it, even better than in common air."

A little later Priestley discovered that "dephlogisticated air .
. . is a principal element in the composition of acids, and may
be extracted by means of heat from many substances which contain
them.... It is likewise produced by the action of light upon
green vegetables; and this seems to be the chief means employed
to preserve the purity of the atmosphere."

This recognition of the important part played by oxygen in the
atmosphere led Priestley to make some experiments upon mice and
insects, and finally upon himself, by inhalations of the pure
gas. "The feeling in my lungs," he said, "was not sensibly
different from that of common air, but I fancied that my
breathing felt peculiarly light and easy for some time
afterwards. Who can tell but that in time this pure air may
become a fashionable article in luxury? . . . Perhaps we may from
these experiments see that though pure dephlogisticated air might
be useful as a medicine, it might not be so proper for us in the
usual healthy state of the body."

This suggestion as to the possible usefulness of oxygen as a
medicine was prophetic. A century later the use of oxygen had
become a matter of routine practice with many physicians. Even in
Priestley's own time such men as Dr. John Hunter expressed their
belief in its efficacy in certain conditions, as we shall see,
but its value in medicine was not fully appreciated until several
generations later.

Several years after discovering oxygen Priestley thus summarized
its properties: "It is this ingredient in the atmospheric air
that enables it to support combustion and animal life. By means
of it most intense heat may be produced, and in the purest of it
animals will live nearly five times as long as in an equal
quantity of atmospheric air. In respiration, part of this air,
passing the membranes of the lungs, unites with the blood and
imparts to it its florid color, while the remainder, uniting with
phlogiston exhaled from venous blood, forms mixed air. It is
dephlogisticated air combined with water that enables fishes to
live in it."[5]


The discovery of oxygen was the last but most important blow to
the tottering phlogiston theory, though Priestley himself would
not admit it. But before considering the final steps in the
overthrow of Stahl's famous theory and the establishment of
modern chemistry, we must review the work of another great
chemist, Karl Wilhelm Scheele (1742-1786), of Sweden, who
discovered oxygen quite independently, although later than
Priestley. In the matter of brilliant discoveries in a brief
space of time Scheele probably eclipsed all his great
contemporaries. He had a veritable genius for interpreting
chemical reactions and discovering new substances, in this
respect rivalling Priestley himself. Unlike Priestley, however,
he planned all his experiments along the lines of definite
theories from the beginning, the results obtained being the
logical outcome of a predetermined plan.

Scheele was the son of a merchant of Stralsund, Pomerania, which
then belonged to Sweden. As a boy in school he showed so little
aptitude for the study of languages that he was apprenticed to an
apothecary at the age of fourteen. In this work he became at
once greatly interested, and, when not attending to his duties in
the dispensary, he was busy day and night making experiments or
studying books on chemistry. In 1775, still employed as an
apothecary, he moved to Stockholm, and soon after he sent to
Bergman, the leading chemist of Sweden, his first discovery--that
of tartaric acid, which he had isolated from cream of tartar.
This was the beginning of his career of discovery, and from that
time on until his death he sent forth accounts of new discoveries
almost uninterruptedly. Meanwhile he was performing the duties of
an ordinary apothecary, and struggling against poverty. His
treatise upon Air and Fire appeared in 1777. In this remarkable
book he tells of his discovery of oxygen--"empyreal" or
"fire-air," as he calls it--which he seems to have made
independently and without ever having heard of the previous
discovery by Priestley. In this book, also, he shows that air is
composed chiefly of oxygen and nitrogen gas.

Early in his experimental career Scheele undertook the solution
of the composition of black oxide of manganese, a substance that
had long puzzled the chemists. He not only succeeded in this,
but incidentally in the course of this series of experiments he
discovered oxygen, baryta, and chlorine, the last of far greater
importance, at least commercially, than the real object of his
search. In speaking of the experiment in which the discovery was
made he says:

"When marine (hydrochloric) acid stood over manganese in the cold
it acquired a dark reddish-brown color. As manganese does not
give any colorless solution without uniting with phlogiston
[probably meaning hydrogen], it follows that marine acid can
dissolve it without this principle. But such a solution has a
blue or red color. The color is here more brown than red, the
reason being that the very finest portions of the manganese,
which do not sink so easily, swim in the red solution; for
without these fine particles the solution is red, and red mixed
with black is brown. The manganese has here attached itself so
loosely to acidum salis that the water can precipitate it, and
this precipitate behaves like ordinary manganese. When, now, the
mixture of manganese and spiritus salis was set to digest, there
arose an effervescence and smell of aqua regis."[6]

The "effervescence" he refers to was chlorine, which he proceeded
to confine in a suitable vessel and examine more fully. He
described it as having a "quite characteristically suffocating
smell," which was very offensive. He very soon noted the
decolorizing or bleaching effects of this now product, finding
that it decolorized flowers, vegetables, and many other

Commercially this discovery of chlorine was of enormous
importance, and the practical application of this new chemical in
bleaching cloth soon supplanted the, old process of
crofting--that is, bleaching by spreading the cloth upon the
grass. But although Scheele first pointed out the bleaching
quality of his newly discovered gas, it was the French savant,
Berthollet, who, acting upon Scheele's discovery that the new gas
would decolorize vegetables and flowers, was led to suspect that
this property might be turned to account in destroying the color
of cloth. In 1785 he read a paper before the Academy of Sciences
of Paris, in which he showed that bleaching by chlorine was
entirely satisfactory, the color but not the substance of the
cloth being affected. He had experimented previously and found
that the chlorine gas was soluble in water and could thus be made
practically available for bleaching purposes. In 1786 James Watt
examined specimens of the bleached cloth made by Berthollet, and
upon his return to England first instituted the process of
practical bleaching. His process, however, was not entirely
satisfactory, and, after undergoing various modifications and
improvements, it was finally made thoroughly practicable by Mr.
Tennant, who hit upon a compound of chlorine and lime--the
chloride of lime--which was a comparatively cheap chemical
product, and answered the purpose better even than chlorine

To appreciate how momentous this discovery was to cloth
manufacturers, it should be remembered that the old process of
bleaching consumed an entire summer for the whitening of a single
piece of linen; the new process reduced the period to a few
hours. To be sure, lime had been used with fair success previous
to Tennant's discovery, but successful and practical bleaching by
a solution of chloride of lime was first made possible by him and
through Scheele's discovery of chlorine.

Until the time of Scheele the great subject of organic chemistry
had remained practically unexplored, but under the touch of his
marvellous inventive genius new methods of isolating and studying
animal and vegetable products were introduced, and a large number
of acids and other organic compounds prepared that had been
hitherto unknown. His explanations of chemical phenomena were
based on the phlogiston theory, in which, like Priestley, he
always, believed. Although in error in this respect, he was,
nevertheless, able to make his discoveries with extremely
accurate interpretations. A brief epitome of the list of some of
his more important discoveries conveys some idea, of his
fertility of mind as well as his industry. In 1780 he discovered
lactic acid,[7] and showed that it was the substance that caused
the acidity of sour milk; and in the same year he discovered
mucic acid. Next followed the discovery of tungstic acid, and in
1783 he added to his list of useful discoveries that of
glycerine. Then in rapid succession came his announcements of the
new vegetable products citric, malic, oxalic, and gallic acids.
Scheele not only made the discoveries, but told the world how he
had made them--how any chemist might have made them if he
chose--for he never considered that he had really discovered any
substance until he had made it, decomposed it, and made it again.

His experiments on Prussian blue are most interesting, not only
because of the enormous amount of work involved and the skill he
displayed in his experiments, but because all the time the
chemist was handling, smelling, and even tasting a compound of
one of the most deadly poisons, ignorant of the fact that the
substance was a dangerous one to handle. His escape from injury
seems almost miraculous; for his experiments, which were most
elaborate, extended over a considerable period of time, during
which he seems to have handled this chemical with impunity.

While only forty years of age and just at the zenith of his fame,
Scheele was stricken by a fatal illness, probably induced by his
ceaseless labor and exposure. It is gratifying to know, however,
that during the last eight or nine years of his life he had been
less bound down by pecuniary difficulties than before, as Bergman
had obtained for him an annual grant from the Academy. But it
was characteristic of the man that, while devoting one-sixth of
the amount of this grant to his personal wants, the remaining
five-sixths was devoted to the expense of his experiments.


The time was ripe for formulating the correct theory of chemical
composition: it needed but the master hand to mould the materials
into the proper shape. The discoveries in chemistry during the
eighteenth century had been far-reaching and revolutionary in
character. A brief review of these discoveries shows how
completely they had subverted the old ideas of chemical elements
and chemical compounds. Of the four substances earth, air, fire,
and water, for many centuries believed to be elementary bodies,
not one has stood the test of the eighteenth-century chemists.
Earth had long since ceased to be regarded as an element, and
water and air had suffered the same fate in this century. And
now at last fire itself, the last of the four "elements" and the
keystone to the phlogiston arch, was shown to be nothing more
than one of the manifestations of the new element, oxygen, and
not "phlogiston" or any other intangible substance.

In this epoch of chemical discoveries England had produced such
mental giants and pioneers in science as Black, Priestley, and
Cavendish; Sweden had given the world Scheele and Bergman, whose
work, added to that of their English confreres, had laid the
broad base of chemistry as a science; but it was for France to
produce a man who gave the final touches to the broad but rough
workmanship of its foundation, and establish it as the science of
modern chemistry. It was for Antoine Laurent Lavoisier
(1743-1794) to gather together, interpret correctly, rename, and
classify the wealth of facts that his immediate predecessors and
contemporaries had given to the world.

The attitude of the mother-countries towards these illustrious
sons is an interesting piece of history. Sweden honored and
rewarded Scheele and Bergman for their efforts; England received
the intellectuality of Cavendish with less appreciation than the
Continent, and a fanatical mob drove Priestley out of the
country; while France, by sending Lavoisier to the guillotine,
demonstrated how dangerous it was, at that time at least, for an
intelligent Frenchman to serve his fellowman and his country

"The revolution brought about by Lavoisier in science," says
Hoefer, "coincides by a singular act of destiny with another
revolution, much greater indeed, going on then in the political
and social world. Both happened on the same soil, at the same
epoch, among the same people; and both marked the commencement of
a new era in their respective spheres."[8]

Lavoisier was born in Paris, and being the son of an opulent
family, was educated under the instruction of the best teachers
of the day. With Lacaille he studied mathematics and astronomy;
with Jussieu, botany; and, finally, chemistry under Rouelle. His
first work of importance was a paper on the practical
illumination of the streets of Paris, for which a prize had been
offered by M. de Sartine, the chief of police. This prize was not
awarded to Lavoisier, but his suggestions were of such importance
that the king directed that a gold medal be bestowed upon the
young author at the public sitting of the Academy in April, 1776.
Two years later, at the age of thirty-five, Lavoisier was
admitted a member of the Academy.

In this same year he began to devote himself almost exclusively
to chemical inquiries, and established a laboratory in his home,
fitted with all manner of costly apparatus and chemicals. Here he
was in constant communication with the great men of science of
Paris, to all of whom his doors were thrown open. One of his
first undertakings in this laboratory was to demonstrate that
water could not be converted into earth by repeated
distillations, as was generally advocated; and to show also that
there was no foundation to the existing belief that it was
possible to convert water into a gas so "elastic" as to pass
through the pores of a vessel. He demonstrated the fallaciousness
of both these theories in 1768-1769 by elaborate experiments, a
single investigation of this series occupying one hundred and one

In 1771 he gave the first blow to the phlogiston theory by his
experiments on the calcination of metals. It will be recalled
that one basis for the belief in phlogiston was the fact that
when a metal was calcined it was converted into an ash, giving up
its "phlogiston" in the process. To restore the metal, it was
necessary to add some substance such as wheat or charcoal to the
ash. Lavoisier, in examining this process of restoration, found
that there was always evolved a great quantity of "air," which he
supposed to be "fixed air" or carbonic acid--the same that
escapes in effervescence of alkalies and calcareous earths, and
in the fermentation of liquors. He then examined the process of
calcination, whereby the phlogiston of the metal was supposed to
have been drawn off. But far from finding that phlogiston or any
other substance had been driven off, he found that something had
been taken on: that the metal "absorbed air," and that the
increased weight of the metal corresponded to the amount of air
"absorbed." Meanwhile he was within grasp of two great
discoveries, that of oxygen and of the composition of the air,
which Priestley made some two years later.

The next important inquiry of this great Frenchman was as to the
composition of diamonds. With the great lens of Tschirnhausen
belonging to the Academy he succeeded in burning up several
diamonds, regardless of expense, which, thanks to his
inheritance, he could ignore. In this process he found that a gas
was given off which precipitated lime from water, and proved to
be carbonic acid. Observing this, and experimenting with other
substances known to give off carbonic acid in the same manner, he
was evidently impressed with the now well-known fact that diamond
and charcoal are chemically the same. But if he did really
believe it, he was cautious in expressing his belief fully. "We
should never have expected," he says, "to find any relation
between charcoal and diamond, and it would be unreasonable to
push this analogy too far; it only exists because both substances
seem to be properly ranged in the class of combustible bodies,
and because they are of all these bodies the most fixed when kept
from contact with air."

As we have seen, Priestley, in 1774, had discovered oxygen, or
"dephlogisticated air." Four years later Lavoisier first
advanced his theory that this element discovered by Priestley was
the universal acidifying or oxygenating principle, which, when
combined with charcoal or carbon, formed carbonic acid; when
combined with sulphur, formed sulphuric (or vitriolic) acid; with
nitrogen, formed nitric acid, etc., and when combined with the
metals formed oxides, or calcides. Furthermore, he postulated the
theory that combustion was not due to any such illusive thing as
"phlogiston," since this did not exist, and it seemed to him that
the phenomena of combustion heretofore attributed to phlogiston
could be explained by the action of the new element oxygen and
heat. This was the final blow to the phlogiston theory, which,
although it had been tottering for some time, had not been
completely overthrown.

In 1787 Lavoisier, in conjunction with Guyon de Morveau,
Berthollet, and Fourcroy, introduced the reform in chemical
nomenclature which until then had remained practically unchanged
since alchemical days. Such expressions as "dephlogisticated" and
"phlogisticated" would obviously have little meaning to a
generation who were no longer to believe in the existence of
phlogiston. It was appropriate that a revolution in chemical
thought should be accompanied by a corresponding revolution in
chemical names, and to Lavoisier belongs chiefly the credit of
bringing about this revolution. In his Elements of Chemistry he
made use of this new nomenclature, and it seemed so clearly an
improvement over the old that the scientific world hastened to
adopt it. In this connection Lavoisier says: "We have,
therefore, laid aside the expression metallic calx altogether,
and have substituted in its place the word oxide. By this it may
be seen that the language we have adopted is both copious and
expressive. The first or lowest degree of oxygenation in bodies
converts them into oxides; a second degree of additional
oxygenation constitutes the class of acids of which the specific
names drawn from their particular bases terminate in ous, as in
the nitrous and the sulphurous acids. The third degree of
oxygenation changes these into the species of acids distinguished
by the termination in ic, as the nitric and sulphuric acids; and,
lastly, we can express a fourth or higher degree of oxygenation
by adding the word oxygenated to the name of the acid, as has
already been done with oxygenated muriatic acid."[9]

This new work when given to the world was not merely an
epoch-making book; it was revolutionary. It not only discarded
phlogiston altogether, but set forth that metals are simple
elements, not compounds of "earth" and "phlogiston." It upheld
Cavendish's demonstration that water itself, like air, is a
compound of oxygen with another element. In short, it was
scientific chemistry, in the modern acceptance of the term.

Lavoisier's observations on combustion are at once important and
interesting: "Combustion," he says, ". . . is the decomposition
of oxygen produced by a combustible body. The oxygen which forms
the base of this gas is absorbed by and enters into combination
with the burning body, while the caloric and light are set free.
Every combustion necessarily supposes oxygenation; whereas, on
the contrary, every oxygenation does not necessarily imply
concomitant combustion; because combustion properly so called
cannot take place without disengagement of caloric and light.
Before combustion can take place, it is necessary that the base
of oxygen gas should have greater affinity to the combustible
body than it has to caloric; and this elective attraction, to use
Bergman's expression, can only take place at a certain degree of
temperature which is different for each combustible substance;
hence the necessity of giving the first motion or beginning to
every combustion by the approach of a heated body. This necessity
of heating any body we mean to burn depends upon certain
considerations which have not hitherto been attended to by any
natural philosopher, for which reason I shall enlarge a little
upon the subject in this place:

"Nature is at present in a state of equilibrium, which cannot
have been attained until all the spontaneous combustions or
oxygenations possible in an ordinary degree of temperature had
taken place.... To illustrate this abstract view of the matter by
example: Let us suppose the usual temperature of the earth a
little changed, and it is raised only to the degree of boiling
water; it is evident that in this case phosphorus, which is
combustible in a considerably lower degree of temperature, would
no longer exist in nature in its pure and simple state, but would
always be procured in its acid or oxygenated state, and its
radical would become one of the substances unknown to chemistry.
By gradually increasing the temperature of the earth, the same
circumstance would successively happen to all the bodies capable
of combustion; and, at the last, every possible combustion having
taken place, there would no longer exist any combustible body
whatever, and every substance susceptible of the operation would
be oxygenated and consequently incombustible.

"There cannot, therefore, exist, as far as relates to us, any
combustible body but such as are non-combustible at the ordinary
temperature of the earth, or, what is the same thing in other
words, that it is essential to the nature of every combustible
body not to possess the property of combustion unless heated, or
raised to a degree of temperature at which its combustion
naturally takes place. When this degree is once produced,
combustion commences, and the caloric which is disengaged by the
decomposition of the oxygen gas keeps up the temperature which is
necessary for continuing combustion. When this is not the
case--that is, when the disengaged caloric is not sufficient for
keeping up the necessary temperature--the combustion ceases. This
circumstance is expressed in the common language by saying that a
body burns ill or with difficulty."[10]

It needed the genius of such a man as Lavoisier to complete the
refutation of the false but firmly grounded phlogiston theory,
and against such a book as his Elements of Chemistry the feeble
weapons of the supporters of the phlogiston theory were hurled in

But while chemists, as a class, had become converts to the new
chemistry before the end of the century, one man, Dr. Priestley,
whose work had done so much to found it, remained unconverted.
In this, as in all his life-work, he showed himself to be a most
remarkable man. Davy said of him, a generation later, that no
other person ever discovered so many new and curious substances
as he; yet to the last he was only an amateur in science, his
profession, as we know, being the ministry. There is hardly
another case in history of a man not a specialist in science
accomplishing so much in original research as did this chemist,
physiologist, electrician; the mathematician, logician, and
moralist; the theologian, mental philosopher, and political
economist. He took all knowledge for his field; but how he found
time for his numberless researches and multifarious writings,
along with his every-day duties, must ever remain a mystery to
ordinary mortals.

That this marvellously receptive, flexible mind should have
refused acceptance to the clearly logical doctrines of the new
chemistry seems equally inexplicable. But so it was. To the
very last, after all his friends had capitulated, Priestley kept
up the fight. From America he sent out his last defy to the
enemy, in 1800, in a brochure entitled "The Doctrine of
Phlogiston Upheld," etc. In the mind of its author it was little
less than a paean of victory; but all the world beside knew that
it was the swan-song of the doctrine of phlogiston. Despite the
defiance of this single warrior the battle was really lost and
won, and as the century closed "antiphlogistic" chemistry had
practical possession of the field.



Small beginnings as have great endings--sometimes. As a case in
point, note what came of the small, original effort of a
self-trained back-country Quaker youth named John Dalton, who
along towards the close of the eighteenth century became
interested in the weather, and was led to construct and use a
crude water-gauge to test the amount of the rainfall. The simple
experiments thus inaugurated led to no fewer than two hundred
thousand recorded observations regarding the weather, which
formed the basis for some of the most epochal discoveries in
meteorology, as we have seen. But this was only a beginning. The
simple rain-gauge pointed the way to the most important
generalization of the nineteenth century in a field of science
with which, to the casual observer, it might seem to have no
alliance whatever. The wonderful theory of atoms, on which the
whole gigantic structure of modern chemistry is founded, was the
logical outgrowth, in the mind of John Dalton, of those early
studies in meteorology.

The way it happened was this: From studying the rainfall, Dalton
turned naturally to the complementary process of evaporation. He
was soon led to believe that vapor exists, in the atmosphere as
an independent gas. But since two bodies cannot occupy the same
space at the same time, this implies that the various atmospheric
gases are really composed of discrete particles. These ultimate
particles are so small that we cannot see them--cannot, indeed,
more than vaguely imagine them--yet each particle of vapor, for
example, is just as much a portion of water as if it were a drop
out of the ocean, or, for that matter, the ocean itself. But,
again, water is a compound substance, for it may be separated, as
Cavendish has shown, into the two elementary substances hydrogen
and oxygen. Hence the atom of water must be composed of two
lesser atoms joined together. Imagine an atom of hydrogen and one
of oxygen. Unite them, and we have an atom of water; sever them,
and the water no longer exists; but whether united or separate
the atoms of hydrogen and of oxygen remain hydrogen and oxygen
and nothing else. Differently mixed together or united, atoms
produce different gross substances; but the elementary atoms
never change their chemical nature--their distinct personality.

It was about the year 1803 that Dalton first gained a full grasp
of the conception of the chemical atom. At once he saw that the
hypothesis, if true, furnished a marvellous key to secrets of
matter hitherto insoluble--questions relating to the relative
proportions of the atoms themselves. It is known, for example,
that a certain bulk of hydrogen gas unites with a certain bulk of
oxygen gas to form water. If it be true that this combination
consists essentially of the union of atoms one with another (each
single atom of hydrogen united to a single atom of oxygen), then
the relative weights of the original masses of hydrogen and of
oxygen must be also the relative weights of each of their
respective atoms. If one pound of hydrogen unites with five and
one-half pounds of oxygen (as, according to Dalton's experiments,
it did), then the weight of the oxygen atom must be five and
one-half times that of the hydrogen atom. Other compounds may
plainly be tested in the same way. Dalton made numerous tests
before he published his theory. He found that hydrogen enters
into compounds in smaller proportions than any other element
known to him, and so, for convenience, determined to take the
weight of the hydrogen atom as unity. The atomic weight of
oxygen then becomes (as given in Dalton's first table of 1803)
5.5; that of water (hydrogen plus oxygen) being of course 6.5.
The atomic weights of about a score of substances are given in
Dalton's first paper, which was read before the Literary and
Philosophical Society of Manchester, October 21, 1803. I wonder
if Dalton himself, great and acute intellect though he had,
suspected, when he read that paper, that he was inaugurating one
of the most fertile movements ever entered on in the whole
history of science?

Be that as it may, it is certain enough that Dalton's
contemporaries were at first little impressed with the novel
atomic theory. Just at this time, as it chanced, a dispute was
waging in the field of chemistry regarding a matter of empirical
fact which must necessarily be settled before such a theory as
that of Dalton could even hope for a bearing. This was the
question whether or not chemical elements unite with one another
always in definite proportions. Berthollet, the great co-worker
with Lavoisier, and now the most authoritative of living
chemists, contended that substances combine in almost
indefinitely graded proportions between fixed extremes. He held
that solution is really a form of chemical combination--a
position which, if accepted, left no room for argument.

But this contention of the master was most actively disputed, in
particular by Louis Joseph Proust, and all chemists of repute
were obliged to take sides with one or the other. For a time the
authority of Berthollet held out against the facts, but at last
accumulated evidence told for Proust and his followers, and
towards the close of the first decade of our century it came to
be generally conceded that chemical elements combine with one
another in fixed and definite proportions.

More than that. As the analysts were led to weigh carefully the
quantities of combining elements, it was observed that the
proportions are not only definite, but that they bear a very
curious relation to one another. If element A combines with two
different proportions of element B to form two compounds, it
appears that the weight of the larger quantity of B is an exact
multiple of that of the smaller quantity. This curious relation
was noticed by Dr. Wollaston, one of the most accurate of
observers, and a little later it was confirmed by Johan Jakob
Berzelius, the great Swedish chemist, who was to be a dominating
influence in the chemical world for a generation to come. But
this combination of elements in numerical proportions was exactly
what Dalton had noticed as early as 1802, and what bad led him
directly to the atomic weights. So the confirmation of this
essential point by chemists of such authority gave the strongest
confirmation to the atomic theory.

During these same years the rising authority of the French
chemical world, Joseph Louis Gay-Lussac, was conducting
experiments with gases, which he had undertaken at first in
conjunction with Humboldt, but which later on were conducted
independently. In 1809, the next year after the publication of
the first volume of Dalton's New System of Chemical Philosophy,
Gay-Lussac published the results of his observations, and among
other things brought out the remarkable fact that gases, under
the same conditions as to temperature and pressure, combine
always in definite numerical proportions as to volume. Exactly
two volumes of hydrogen, for example, combine with one volume of
oxygen to form water. Moreover, the resulting compound gas
always bears a simple relation to the combining volumes. In the
case just cited, the union of two volumes of hydrogen and one of
oxygen results in precisely two volumes of water vapor.

Naturally enough, the champions of the atomic theory seized upon
these observations of Gay-Lussac as lending strong support to
their hypothesis--all of them, that is, but the curiously
self-reliant and self-sufficient author of the atomic theory
himself, who declined to accept the observations of the French
chemist as valid. Yet the observations of Gay-Lussac were
correct, as countless chemists since then have demonstrated anew,
and his theory of combination by volumes became one of the
foundation-stones of the atomic theory, despite the opposition of
the author of that theory.

The true explanation of Gay-Lussac's law of combination by
volumes was thought out almost immediately by an Italian savant,
Amadeo, Avogadro, and expressed in terms of the atomic theory.
The fact must be, said Avogadro, that under similar physical
conditions every form of gas contains exactly the same number of
ultimate particles in a given volume. Each of these ultimate
physical particles may be composed of two or more atoms (as in
the case of water vapor), but such a compound atom conducts
itself as if it were a simple and indivisible atom, as regards
the amount of space that separates it from its fellows under
given conditions of pressure and temperature. The compound atom,
composed of two or more elementary atoms, Avogadro proposed to
distinguish, for purposes of convenience, by the name molecule.
It is to the molecule, considered as the unit of physical
structure, that Avogadro's law applies.

This vastly important distinction between atoms and molecules,
implied in the law just expressed, was published in 1811. Four
years later, the famous French physicist Ampere outlined a
similar theory, and utilized the law in his mathematical
calculations. And with that the law of Avogadro dropped out of
sight for a full generation. Little suspecting that it was the
very key to the inner mysteries of the atoms for which they were
seeking, the chemists of the time cast it aside, and let it fade
from the memory of their science.

This, however, was not strange, for of course the law of Avogadro
is based on the atomic theory, and in 1811 the atomic theory was
itself still being weighed in the balance. The law of multiple
proportions found general acceptance as an empirical fact; but
many of the leading lights of chemistry still looked askance at
Dalton's explanation of this law. Thus Wollaston, though from the
first he inclined to acceptance of the Daltonian view, cautiously
suggested that it would be well to use the non-committal word
"equivalent" instead of "atom"; and Davy, for a similar reason,
in his book of 1812, speaks only of "proportions," binding
himself to no theory as to what might be the nature of these

At least two great chemists of the time, however, adopted the
atomic view with less reservation. One of these was Thomas
Thomson, professor at Edinburgh, who, in 1807, had given an
outline of Dalton's theory in a widely circulated book, which
first brought the theory to the general attention of the chemical
world. The other and even more noted advocate of the atomic
theory was Johan Jakob Berzelius. This great Swedish chemist at
once set to work to put the atomic theory to such tests as might
be applied in the laboratory. He was an analyst of the utmost
skill, and for years be devoted himself to the determination of
the combining weights, "equivalents" or "proportions," of the
different elements. These determinations, in so far as they were
accurately made, were simple expressions of empirical facts,
independent of any theory; but gradually it became more and more
plain that these facts all harmonize with the atomic theory of
Dalton. So by common consent the proportionate combining weights
of the elements came to be known as atomic weights--the name
Dalton had given them from the first--and the tangible conception
of the chemical atom as a body of definite constitution and
weight gained steadily in favor.

From the outset the idea had had the utmost tangibility in the
mind of Dalton. He had all along represented the different atoms
by geometrical symbols--as a circle for oxygen, a circle
enclosing a dot for hydrogen, and the like--and had represented
compounds by placing these symbols of the elements in
juxtaposition. Berzelius proposed to improve upon this method by
substituting for the geometrical symbol the initial of the Latin
name of the element represented--O for oxygen, H for hydrogen,
and so on--a numerical coefficient to follow the letter as an
indication of the number of atoms present in any given compound.
This simple system soon gained general acceptance, and with
slight modifications it is still universally employed. Every
school-boy now is aware that H2O is the chemical way of
expressing the union of two atoms of hydrogen with one of oxygen
to form a molecule of water. But such a formula would have had
no meaning for the wisest chemist before the day of Berzelius.

The universal fame of the great Swedish authority served to give
general currency to his symbols and atomic weights, and the new
point of view thus developed led presently to two important
discoveries which removed the last lingering doubts as to the
validity of the atomic theory. In 1819 two French physicists,
Dulong and Petit, while experimenting with heat, discovered that
the specific heats of solids (that is to say, the amount of heat
required to raise the temperature of a given mass to a given
degree) vary inversely as their atomic weights. In the same year
Eilhard Mitscherlich, a German investigator, observed that
compounds having the same number of atoms to the molecule are
disposed to form the same angles of crystallization--a property
which he called isomorphism.

Here, then, were two utterly novel and independent sets of
empirical facts which harmonize strangely with the supposition
that substances are composed of chemical atoms of a determinate
weight. This surely could not be coincidence--it tells of law.
And so as soon as the claims of Dulong and Petit and of
Mitscherlich had been substantiated by other observers, the laws
of the specific heat of atoms, and of isomorphism, took their
place as new levers of chemical science. With the aid of these
new tools an impregnable breastwork of facts was soon piled about
the atomic theory. And John Dalton, the author of that theory,
plain, provincial Quaker, working on to the end in
semi-retirement, became known to all the world and for all time
as a master of masters.


During those early years of the nineteenth century, when Dalton
was grinding away at chemical fact and theory in his obscure
Manchester laboratory, another Englishman held the attention of
the chemical world with a series of the most brilliant and widely
heralded researches. This was Humphry Davy, a young man who had
conic to London in 1801, at the instance of Count Rumford, to
assume the chair of chemical philosophy in the Royal Institution,
which the famous American had just founded.

Here, under Davy's direction, the largest voltaic battery yet
constructed had been put in operation, and with its aid the
brilliant young experimenter was expected almost to perform
miracles. And indeed he scarcely disappointed the expectation,
for with the aid of his battery he transformed so familiar a
substance as common potash into a metal which was not only so
light that it floated on water, but possessed the seemingly
miraculous property of bursting into flames as soon as it came in
contact with that fire-quenching liquid. If this were not a
miracle, it had for the popular eye all the appearance of the

What Davy really had done was to decompose the potash, which
hitherto had been supposed to be elementary, liberating its
oxygen, and thus isolating its metallic base, which he named
potassium. The same thing was done with soda, and the closely
similar metal sodium was discovered--metals of a unique type,
possessed of a strange avidity for oxygen, and capable of seizing
on it even when it is bound up in the molecules of water.
Considered as mere curiosities, these discoveries were
interesting, but aside from that they were of great theoretical
importance, because they showed the compound nature of some
familiar chemicals that had been regarded as elements. Several
other elementary earths met the same fate when subjected to the
electrical influence; the metals barium, calcium, and strontium
being thus discovered. Thereafter Davy always referred to the
supposed elementary substances (including oxygen, hydrogen, and
the rest) as "unde-compounded" bodies. These resist all present
efforts to decompose them, but how can one know what might not
happen were they subjected to an influence, perhaps some day to
be discovered, which exceeds the battery in power as the battery
exceeds the blowpipe?

Another and even more important theoretical result that flowed
from Davy's experiments during this first decade of the century
was the proof that no elementary substances other than hydrogen
and oxygen are produced when pure water is decomposed by the
electric current. It was early noticed by Davy and others that
when a strong current is passed through water, alkalies appear at
one pole of the battery and acids at the other, and this though
the water used were absolutely pure. This seemingly told of the
creation of elements--a transmutation but one step removed from
the creation of matter itself--under the influence of the new
"force." It was one of Davy's greatest triumphs to prove, in the
series of experiments recorded in his famous Bakerian lecture of
1806, that the alleged creation of elements did not take place,
the substances found at the poles of the battery having been
dissolved from the walls of the vessels in which the water
experimented upon had been placed. Thus the same implement which
had served to give a certain philosophical warrant to the fading
dreams of alchemy banished those dreams peremptorily from the
domain of present science.

"As early as 1800," writes Davy, "I had found that when separate
portions of distilled water, filling two glass tubes, connected
by moist bladders, or any moist animal or vegetable substances,
were submitted to the electrical action of the pile of Volta by
means of gold wires, a nitro-muriatic solution of gold appeared
in the tube containing the positive wire, or the wire
transmitting the electricity, and a solution of soda in the
opposite tube; but I soon ascertained that the muriatic acid owed
its existence to the animal or vegetable matters employed; for
when the same fibres of cotton were made use of in successive
experiments, and washed after every process in a weak solution of
nitric acid, the water in the apparatus containing them, though
acted on for a great length of time with a very strong power, at
last produced no effects upon nitrate of silver.

"In cases when I had procured much soda, the glass at its point
of contact with the wire seemed considerably corroded; and I was
confirmed in my idea of referring the production of the alkali
principally to this source, by finding that no fixed saline
matter could be obtained by electrifying distilled water in a
single agate cup from two points of platina with the Voltaic

"Mr. Sylvester, however, in a paper published in Mr. Nicholson's
journal for last August, states that though no fixed alkali or
muriatic acid appears when a single vessel is employed, yet that
they are both formed when two vessels are used. And to do away
with all objections with regard to vegetable substances or glass,
he conducted his process in a vessel made of baked tobacco-pipe
clay inserted in a crucible of platina. I have no doubt of the
correctness of his results; but the conclusion appears
objectionable. He conceives, that he obtained fixed alkali,
because the fluid after being heated and evaporated left a matter
that tinged turmeric brown, which would have happened had it been
lime, a substance that exists in considerable quantities in all
pipe-clay; and even allowing the presence of fixed alkali, the
materials employed for the manufacture of tobacco-pipes are not
at all such as to exclude the combinations of this substance.

"I resumed the inquiry; I procured small cylindrical cups of
agate of the capacity of about one-quarter of a cubic inch each.
They were boiled for some hours in distilled water, and a piece
of very white and transparent amianthus that had been treated in
the same way was made then to connect together; they were filled
with distilled water and exposed by means of two platina wires to
a current of electricity, from one hundred and fifty pairs of
plates of copper and zinc four inches square, made active by
means of solution of alum. After forty-eight hours the process
was examined: Paper tinged with litmus plunged into the tube
containing the transmitting or positive wire was immediately
strongly reddened. Paper colored by turmeric introduced into the
other tube had its color much deepened; the acid matter gave a
very slight degree of turgidness to solution of nitrate of soda.
The fluid that affected turmeric retained this property after
being strongly boiled; and it appeared more vivid as the quantity
became reduced by evaporation; carbonate of ammonia was mixed
with it, and the whole dried and exposed to a strong heat; a
minute quantity of white matter remained, which, as far as my
examinations could go, had the properties of carbonate of soda. I
compared it with similar minute portions of the pure carbonates
of potash, and similar minute portions of the pure carbonates of
potash and soda. It was not so deliquescent as the former of
these bodies, and it formed a salt with nitric acid, which, like
nitrate of soda, soon attracted moisture from a damp atmosphere
and became fluid.

"This result was unexpected, but it was far from convincing me
that the substances which were obtained were generated. In a
similar process with glass tubes, carried on under exactly the
same circumstances and for the same time, I obtained a quantity
of alkali which must have been more than twenty times greater,
but no traces of muriatic acid. There was much probability that
the agate contained some minute portion of saline matter, not
easily detected by chemical analysis, either in combination or
intimate cohesion in its pores. To determine this, I repeated
this a second, a third, and a fourth time. In the second
experiment turbidness was still produced by a solution of nitrate
of silver in the tube containing the acid, but it was less
distinct; in the third process it was barely perceptible; and in
the fourth process the two fluids remained perfectly clear after
the mixture. The quantity of alkaline matter diminished in every
operation; and in the last process, though the battery had been
kept in great activity for three days, the fluid possessed, in a
very slight degree, only the power of acting on paper tinged with
turmeric; but its alkaline property was very sensible to litmus
paper slightly reddened, which is a much more delicate test; and
after evaporation and the process by carbonate of ammonia, a
barely perceptible quantity of fixed alkali was still left. The
acid matter in the other tube was abundant; its taste was sour;
it smelled like water over which large quantities of nitrous gas
have been long kept; it did not effect solution of muriate of
barytes; and a drop of it placed upon a polished plate of silver
left, after evaporation, a black stain, precisely similar to that
produced by extremely diluted nitrous acid.

"After these results I could no longer doubt that some saline
matter existing in the agate tubes had been the source of the
acid matter capable of precipitating nitrate of silver and much
of the alkali. Four additional repetitions of the process,
however, convinced me that there was likewise some other cause
for the presence of this last substance; for it continued to
appear to the last in quantities sufficiently distinguishable,
and apparently equal in every case. I had used every precaution,
I had included the tube in glass vessels out of the reach of the
circulating air; all the acting materials had been repeatedly
washed with distilled water; and no part of them in contact with
the fluid had been touched by the fingers.

"The only substance that I could now conceive as furnishing the
fixed alkali was the water itself. This water appeared pure by
the tests of nitrate of silver and muriate of barytes; but potash
of soda, as is well known, rises in small quantities in rapid
distillation; and the New River water which I made use of
contains animal and vegetable impurities, which it was easy to
conceive might furnish neutral salts capable of being carried
over in vivid ebullition."[1] Further experiment proved the
correctness of this inference, and the last doubt as to the
origin of the puzzling chemical was dispelled.

Though the presence of the alkalies and acids in the water was
explained, however, their respective migrations to the negative
and positive poles of the battery remained to be accounted for.
Davy's classical explanation assumed that different elements
differ among themselves as to their electrical properties, some
being positively, others negatively, electrified. Electricity
and "chemical affinity," he said, apparently are manifestations
of the same force, acting in the one case on masses, in the other
on particles. Electro-positive particles unite with
electro-negative particles to form chemical compounds, in virtue
of the familiar principle that opposite electricities attract one
another. When compounds are decomposed by the battery, this
mutual attraction is overcome by the stronger attraction of the
poles of the battery itself.

This theory of binary composition of all chemical compounds,
through the union of electro-positive and electro-negative atoms
or molecules, was extended by Berzelius, and made the basis of
his famous system of theoretical chemistry. This theory held
that all inorganic compounds, however complex their composition,
are essentially composed of such binary combinations. For many
years this view enjoyed almost undisputed sway. It received what
seemed strong confirmation when Faraday showed the definite
connection between the amount of electricity employed and the
amount of decomposition produced in the so-called electrolyte.
But its claims were really much too comprehensive, as subsequent
discoveries proved.


When Berzelius first promulgated his binary theory he was careful
to restrict its unmodified application to the compounds of the
inorganic world. At that time, and for a long time thereafter,
it was supposed that substances of organic nature had some
properties that kept them aloof from the domain of inorganic
chemistry. It was little doubted that a so-called "vital force"
operated here, replacing or modifying the action of ordinary
"chemical affinity." It was, indeed, admitted that organic
compounds are composed of familiar elements--chiefly carbon,
oxygen, hydrogen, and nitrogen; but these elements were supposed
to be united in ways that could not be imitated in the domain of
the non-living. It was regarded almost as an axiom of chemistry
that no organic compound whatever could be put together from its
elements--synthesized--in the laboratory. To effect the synthesis
of even the simplest organic compound, it was thought that the
"vital force" must be in operation.

Therefore a veritable sensation was created in the chemical world
when, in the year 1828, it was announced that the young German
chemist, Friedrich Wohler, formerly pupil of Berzelius, and
already known as a coming master, had actually synthesized the
well-known organic product urea in his laboratory at Sacrow. The
"exception which proves the rule" is something never heard of in
the domain of logical science. Natural law knows no exceptions.
So the synthesis of a single organic compound sufficed at a blow
to break down the chemical barrier which the imagination of the
fathers of the science had erected between animate and inanimate
nature. Thenceforth the philosophical chemist would regard the
plant and animal organisms as chemical laboratories in which
conditions are peculiarly favorable for building up complex
compounds of a few familiar elements, under the operation of
universal chemical laws. The chimera "vital force" could no
longer gain recognition in the domain of chemistry.

Now a wave of interest in organic chemistry swept over the
chemical world, and soon the study of carbon compounds became as
much the fashion as electrochemistry had been in the, preceding

Foremost among the workers who rendered this epoch of organic
chemistry memorable were Justus Liebig in Germany and Jean
Baptiste Andre Dumas in France, and their respective pupils,
Charles Frederic Gerhardt and Augustus Laurent. Wohler, too,
must be named in the same breath, as also must Louis Pasteur,
who, though somewhat younger than the others, came upon the scene
in time to take chief part in the most important of the
controversies that grew out of their labors.

Several years earlier than this the way had been paved for the
study of organic substances by Gay-Lussac's discovery, made in
1815, that a certain compound of carbon and nitrogen, which he
named cyanogen, has a peculiar degree of stability which enables
it to retain its identity and enter into chemical relations after
the manner of a simple body. A year later Ampere discovered that
nitrogen and hydrogen, when combined in certain proportions to
form what he called ammonium, have the same property. Berzelius
had seized upon this discovery of the compound radical, as it was
called, because it seemed to lend aid to his dualistic theory. He
conceived the idea that all organic compounds are binary unions
of various compound radicals with an atom of oxygen, announcing
this theory in 1818. Ten years later, Liebig and Wohler undertook
a joint investigation which resulted in proving that compound
radicals are indeed very abundant among organic substances. Thus
the theory of Berzelius seemed to be substantiated, and organic
chemistry came to be defined as the chemistry of compound

But even in the day of its seeming triumph the dualistic theory
was destined to receive a rude shock. This came about through
the investigations of Dumas, who proved that in a certain organic
substance an atom of hydrogen may be removed and an atom of
chlorine substituted in its place without destroying the
integrity of the original compound--much as a child might
substitute one block for another in its play-house. Such a
substitution would be quite consistent with the dualistic theory,
were it not for the very essential fact that hydrogen is a
powerfully electro-positive element, while chlorine is as
strongly electro-negative. Hence the compound radical which
united successively with these two elements must itself be at one
time electro-positive, at another electro-negative--a seeming
inconsistency which threw the entire Berzelian theory into

In its place there was elaborated, chiefly through the efforts of
Laurent and Gerhardt, a conception of the molecule as a unitary
structure, built up through the aggregation of various atoms, in
accordance with "elective affinities" whose nature is not yet
understood A doctrine of "nuclei" and a doctrine of "types" of
molecular structure were much exploited, and, like the doctrine
of compound radicals, became useful as aids to memory and guides
for the analyst, indicating some of the plans of molecular
construction, though by no means penetrating the mysteries of
chemical affinity. They are classifications rather than
explanations of chemical unions. But at least they served an
important purpose in giving definiteness to the idea of a
molecular structure built of atoms as the basis of all
substances. Now at last the word molecule came to have a distinct
meaning, as distinct from "atom," in the minds of the generality
of chemists, as it had had for Avogadro a third of a century
before. Avogadro's hypothesis that there are equal numbers of
these molecules in equal volumes of gases, under fixed
conditions, was revived by Gerhardt, and a little later, under
the championship of Cannizzaro, was exalted to the plane of a
fixed law. Thenceforth the conception of the molecule was to be
as dominant a thought in chemistry as the idea of the atom had
become in a previous epoch.


Of course the atom itself was in no sense displaced, but
Avogadro's law soon made it plain that the atom had often usurped
territory that did not really belong to it. In many cases the
chemists had supposed themselves dealing with atoms as units
where the true unit was the molecule. In the case of elementary
gases, such as hydrogen and oxygen, for example, the law of equal
numbers of molecules in equal spaces made it clear that the atoms
do not exist isolated, as had been supposed. Since two volumes
of hydrogen unite with one volume of oxygen to form two volumes
of water vapor, the simplest mathematics show, in the light of
Avogadro's law, not only that each molecule of water must contain
two hydrogen atoms (a point previously in dispute), but that the
original molecules of hydrogen and oxygen must have been composed
in each case of two atoms---else how could one volume of oxygen
supply an atom for every molecule of two volumes of water?

What, then, does this imply? Why, that the elementary atom has
an avidity for other atoms, a longing for companionship, an
"affinity"--call it what you will--which is bound to be satisfied
if other atoms are in the neighborhood. Placed solely among
atoms of its own kind, the oxygen atom seizes on a fellow oxygen
atom, and in all their mad dancings these two mates cling
together--possibly revolving about each other in miniature
planetary orbits. Precisely the same thing occurs among the
hydrogen atoms. But now suppose the various pairs of oxygen atoms
come near other pairs of hydrogen atoms (under proper conditions
which need not detain us here), then each oxygen atom loses its
attachment for its fellow, and flings itself madly into the
circuit of one of the hydrogen couplets, and--presto!--there are
only two molecules for every three there were before, and free
oxygen and hydrogen have become water. The whole process, stated
in chemical phraseology, is summed up in the statement that under
the given conditions the oxygen atoms had a greater affinity for
the hydrogen atoms than for one another.

As chemists studied the actions of various kinds of atoms, in
regard to their unions with one another to form molecules, it
gradually dawned upon them that not all elements are satisfied
with the same number of companions. Some elements ask only one,
and refuse to take more; while others link themselves, when
occasion offers, with two, three, four, or more. Thus we saw that
oxygen forsook a single atom of its own kind and linked itself
with two atoms of hydrogen. Clearly, then, the oxygen atom, like
a creature with two hands, is able to clutch two other atoms.
But we have no proof that under any circumstances it could hold
more than two. Its affinities seem satisfied when it has two
bonds. But, on the other hand, the atom of nitrogen is able to
hold three atoms of hydrogen, and does so in the molecule of
ammonium (NH3); while the carbon atom can hold four atoms of
hydrogen or two atoms of oxygen.

Evidently, then, one atom is not always equivalent to another
atom of a different kind in combining powers. A recognition of
this fact by Frankland about 1852, and its further investigation
by others (notably A. Kekule and A. S. Couper), led to the
introduction of the word equivalent into chemical terminology in
a new sense, and in particular to an understanding of the
affinities or "valency" of different elements, which proved of
the most fundamental importance. Thus it was shown that, of the
four elements that enter most prominently into organic compounds,
hydrogen can link itself with only a single bond to any other
element--it has, so to speak, but a single hand with which to
grasp--while oxygen has capacity for two bonds, nitrogen for
three (possibly for five), and carbon for four. The words
monovalent, divalent, trivalent, tretrava-lent, etc., were coined
to express this most important fact, and the various elements
came to be known as monads, diads, triads, etc. Just why
different elements should differ thus in valency no one as yet
knows; it is an empirical fact that they do. And once the nature
of any element has been determined as regards its valency, a most
important insight into the possible behavior of that element has
been secured. Thus a consideration of the fact that hydrogen is
monovalent, while oxygen is divalent, makes it plain that we must
expect to find no more than three compounds of these two
elements--namely, H--O--(written HO by the chemist, and called
hydroxyl); H--O--H (H2O, or water), and H--O--O--H (H2O2, or
hydrogen peroxide). It will be observed that in the first of
these compounds the atom of oxygen stands, so to speak, with one
of its hands free, eagerly reaching out, therefore, for another
companion, and hence, in the language of chemistry, forming an
unstable compound. Again, in the third compound, though all hands
are clasped, yet one pair links oxygen with oxygen; and this also
must be an unstable union, since the avidity of an atom for its
own kind is relatively weak. Thus the well-known properties of
hydrogen peroxide are explained, its easy decomposition, and the
eagerness with which it seizes upon the elements of other

But the molecule of water, on the other hand, has its atoms
arranged in a state of stable equilibrium, all their affinities
being satisfied. Each hydrogen atom has satisfied its own
affinity by clutching the oxygen atom; and the oxygen atom has
both its bonds satisfied by clutching back at the two hydrogen
atoms. Therefore the trio, linked in this close bond, have no
tendency to reach out for any other companion, nor, indeed, any
power to hold another should it thrust itself upon them. They
form a "stable" compound, which under all ordinary circumstances
will retain its identity as a molecule of water, even though the
physical mass of which it is a part changes its condition from a
solid to a gas from ice to vapor.

But a consideration of this condition of stable equilibrium in
the molecule at once suggests a new question: How can an
aggregation of atoms, having all their affinities satisfied, take
any further part in chemical reactions? Seemingly such a
molecule, whatever its physical properties, must be chemically
inert, incapable of any atomic readjustments. And so in point of
fact it is, so long as its component atoms cling to one another
unremittingly. But this, it appears, is precisely what the atoms
are little prone to do. It seems that they are fickle to the last
degree in their individual attachments, and are as prone to break
away from bondage as they are to enter into it. Thus the oxygen
atom which has just flung itself into the circuit of two hydrogen
atoms, the next moment flings itself free again and seeks new
companions. It is for all the world like the incessant change of
partners in a rollicking dance. This incessant dissolution and
reformation of molecules in a substance which as a whole remains
apparently unchanged was first fully appreciated by Ste.-Claire
Deville, and by him named dissociation. It is a process which
goes on much more actively in some compounds than in others, and
very much more actively under some physical conditions (such as
increase of temperature) than under others. But apparently no
substances at ordinary temperatures, and no temperature above the
absolute zero, are absolutely free from its disturbing influence.
Hence it is that molecules having all the valency of their atoms
fully satisfied do not lose their chemical activity--since each
atom is momentarily free in the exchange of partners, and may
seize upon different atoms from its former partners, if those it
prefers are at hand.

While, however, an appreciation of this ceaseless activity of the
atom is essential to a proper understanding of its chemical
efficiency, yet from another point of view the "saturated"
molecule--that is, the molecule whose atoms have their valency
all satisfied--may be thought of as a relatively fixed or stable
organism. Even though it may presently be torn down, it is for
the time being a completed structure; and a consideration of the
valency of its atoms gives the best clew that has hitherto been
obtainable as to the character of its architecture. How
important this matter of architecture of the molecule--of space
relations of the atoms--may be was demonstrated as long ago as
1823, when Liebig and Wohler proved, to the utter bewilderment of
the chemical world, that two substances may have precisely the
same chemical constitution--the same number and kind of
atoms--and yet differ utterly in physical properties. The word
isomerism was coined by Berzelius to express this anomalous
condition of things, which seemed to negative the most
fundamental truths of chemistry. Naming the condition by no
means explained it, but the fact was made clear that something
besides the mere number and kind of atoms is important in the
architecture of a molecule. It became certain that atoms are not
thrown together haphazard to build a molecule, any more than
bricks are thrown together at random to form a house.

How delicate may be the gradations of architectural design in
building a molecule was well illustrated about 1850, when Pasteur
discovered that some carbon compounds--as certain sugars--can
only be distinguished from one another, when in solution, by the
fact of their twisting or polarizing a ray of light to the left
or to the right, respectively. But no inkling of an explanation
of these strange variations of molecular structure came until the
discovery of the law of valency. Then much of the mystery was
cleared away; for it was plain that since each atom in a molecule
can hold to itself only a fixed number of other atoms, complex
molecules must have their atoms linked in definite chains or
groups. And it is equally plain that where the atoms are
numerous, the exact plan of grouping may sometimes be susceptible
of change without doing violence to the law of valency. It is in
such cases that isomerism is observed to occur.

By paying constant heed to this matter of the affinities,
chemists are able to make diagrammatic pictures of the plan of
architecture of any molecule whose composition is known. In the
simple molecule of water (H2O), for example, the two hydrogen
atoms must have released each other before they could join the
oxygen, and the manner of linking must apparently be that
represented in the graphic formula H--O--H. With molecules
composed of a large number of atoms, such graphic representation
of the scheme of linking is of course increasingly difficult,
yet, with the affinities for a guide, it is always possible. Of
course no one supposes that such a formula, written in a single
plane, can possibly represent the true architecture of the
molecule: it is at best suggestive or diagrammatic rather than
pictorial. Nevertheless, it affords hints as to the structure of
the molecule such as the fathers of chemistry would not have
thought it possible ever to attain.


These utterly novel studies of molecular architecture may seem at
first sight to take from the atom much of its former prestige as
the all-important personage of the chemical world. Since so much
depends upon the mere position of the atoms, it may appear that
comparatively little depends upon the nature of the atoms
themselves. But such a view is incorrect, for on closer
consideration it will appear that at no time has the atom been
seen to renounce its peculiar personality. Within certain limits
the character of a molecule may be altered by changing the
positions of its atoms (just as different buildings may be
constructed of the same bricks), but these limits are sharply
defined, and it would be as impossible to exceed them as it would
be to build a stone building with bricks. From first to last the
brick remains a brick, whatever the style of architecture it
helps to construct; it never becomes a stone. And just as closely
does each atom retain its own peculiar properties, regardless of
its surroundings.

Thus, for example, the carbon atom may take part in the formation
at one time of a diamond, again of a piece of coal, and yet again
of a particle of sugar, of wood fibre, of animal tissue, or of a
gas in the atmosphere; but from first to last--from glass-cutting
gem to intangible gas--there is no demonstrable change whatever
in any single property of the atom itself. So far as we know, its
size, its weight, its capacity for vibration or rotation, and its
inherent affinities, remain absolutely unchanged throughout all
these varying fortunes of position and association. And the same
thing is true of every atom of all of the seventy-odd elementary
substances with which the modern chemist is acquainted. Every one
appears always to maintain its unique integrity, gaining nothing
and losing nothing.

All this being true, it would seem as if the position of the
Daltonian atom as a primordial bit of matter, indestructible and
non-transmutable, had been put to the test by the chemistry of
our century, and not found wanting. Since those early days of the
century when the electric battery performed its miracles and
seemingly reached its limitations in the hands of Davy, many new
elementary substances have been discovered, but no single element
has been displaced from its position as an undecomposable body.
Rather have the analyses of the chemist seemed to make it more
and more certain that all elementary atoms are in truth what John
Herschel called them, "manufactured articles"--primordial,
changeless, indestructible.

And yet, oddly enough, it has chanced that hand in hand with the
experiments leading to such a goal have gone other experiments
arid speculations of exactly the opposite tenor. In each
generation there have been chemists among the leaders of their
science who have refused to admit that the so-called elements are
really elements at all in any final sense, and who have sought
eagerly for proof which might warrant their scepticism. The first
bit of evidence tending to support this view was furnished by an
English physician, Dr. William Prout, who in 1815 called
attention to a curious relation to be observed between the atomic
weight of the various elements. Accepting the figures given by
the authorities of the time (notably Thomson and Berzelius), it
appeared that a strikingly large proportion of the atomic weights
were exact multiples of the weight of hydrogen, and that others
differed so slightly that errors of observation might explain the
discrepancy. Prout felt that it could not be accidental, and he
could think of no tenable explanation, unless it be that the
atoms of the various alleged elements are made up of different
fixed numbers of hydrogen atoms. Could it be that the one true
element--the one primal matter--is hydrogen, and that all other
forms of matter are but compounds of this original substance?

Prout advanced this startling idea at first tentatively, in an
anonymous publication; but afterwards he espoused it openly and
urged its tenability. Coming just after Davy's dissociation of
some supposed elements, the idea proved alluring, and for a time
gained such popularity that chemists were disposed to round out
the observed atomic weights of all elements into whole numbers.
But presently renewed determinations of the atomic weights seemed
to discountenance this practice, and Prout's alleged law fell
into disrepute. It was revived, however, about 1840, by Dumas,
whose great authority secured it a respectful hearing, and whose
careful redetermination of the weight of carbon, making it
exactly twelve times that of hydrogen, aided the cause.

Subsequently Stas, the pupil of Dumas, undertook a long series of
determinations of atomic weights, with the expectation of
confirming the Proutian hypothesis. But his results seemed to
disprove the hypothesis, for the atomic weights of many elements
differed from whole numbers by more, it was thought, than the
limits of error of the experiments. It was noteworthy, however,
that the confidence of Dumas was not shaken, though he was led to
modify the hypothesis, and, in accordance with previous
suggestions of Clark and of Marignac, to recognize as the
primordial element, not hydrogen itself, but an atom half the
weight, or even one-fourth the weight, of that of hydrogen, of
which primordial atom the hydrogen atom itself is compounded. But
even in this modified form the hypothesis found great opposition
from experimental observers.

In 1864, however, a novel relation between the weights of the
elements and their other characteristics was called to the
attention of chemists by Professor John A. R. Newlands, of
London, who had noticed that if the elements are arranged
serially in the numerical order of their atomic weights, there is
a curious recurrence of similar properties at intervals of eight
elements This so-called "law of octaves" attracted little
immediate attention, but the facts it connotes soon came under
the observation of other chemists, notably of Professors Gustav
Hinrichs in America, Dmitri Mendeleeff in Russia, and Lothar
Meyer in Germany. Mendeleeff gave the discovery fullest
expression, explicating it in 1869, under the title of "the
periodic law."

Though this early exposition of what has since been admitted to
be a most important discovery was very fully outlined, the
generality of chemists gave it little heed till a decade or so
later, when three new elements, gallium, scandium, and germanium,
were discovered, which, on being analyzed, were quite
unexpectedly found to fit into three gaps which Mendeleeff had
left in his periodic scale. In effect the periodic law had
enabled Mendeleeff to predicate the existence of the new elements
years before they were discovered. Surely a system that leads to
such results is no mere vagary. So very soon the periodic law
took its place as one of the most important generalizations of
chemical science.

This law of periodicity was put forward as an expression of
observed relations independent of hypothesis; but of course the
theoretical bearings of these facts could not be overlooked. As
Professor J. H. Gladstone has said, it forces upon us "the
conviction that the elements are not separate bodies created
without reference to one another, but that they have been
originally fashioned, or have been built up, from one another,
according to some general plan." It is but a short step from
that proposition to the Proutian hypothesis.


But the atomic weights are not alone in suggesting the compound
nature of the alleged elements. Evidence of a totally different
kind has contributed to the same end, from a source that could
hardly have been imagined when the Proutian hypothesis, was
formulated, through the tradition of a novel weapon to the
armamentarium of the chemist--the spectroscope. The perfection
of this instrument, in the hands of two German scientists, Gustav
Robert Kirchhoff and Robert Wilhelm Bunsen, came about through
the investigation, towards the middle of the century, of the
meaning of the dark lines which had been observed in the solar
spectrum by Fraunhofer as early as 1815, and by Wollaston a
decade earlier. It was suspected by Stokes and by Fox Talbot in
England, but first brought to demonstration by Kirchhoff and
Bunsen, that these lines, which were known to occupy definite
positions in the spectrum, are really indicative of particular
elementary substances. By means of the spectroscope, which is
essentially a magnifying lens attached to a prism of glass, it is
possible to locate the lines with great accuracy, and it was soon
shown that here was a new means of chemical analysis of the most
exquisite delicacy. It was found, for example, that the
spectroscope could detect the presence of a quantity of sodium so
infinitesimal as the one two-hundred-thousandth of a grain. But
what was even more important, the spectroscope put no limit upon
the distance of location of the substance it tested, provided
only that sufficient light came from it. The experiments it
recorded might be performed in the sun, or in the most distant
stars or nebulae; indeed, one of the earliest feats of the
instrument was to wrench from the sun the secret of his chemical

To render the utility of the spectroscope complete, however, it
was necessary to link with it another new chemical
agency--namely, photography. This now familiar process is based
on the property of light to decompose certain unstable compounds
of silver, and thus alter their chemical composition. Davy and
Wedgwood barely escaped the discovery of the value of the
photographic method early in the nineteenth century. Their
successors quite overlooked it until about 1826, when Louis J. M.
Daguerre, the French chemist, took the matter in hand, and after

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