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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
whatever.”[2]
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]
JOSEPH PRIESTLEYWhile 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
words:
“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
air.
“… 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]
KARL WILHELM SCHEELEThe 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
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