Science and Education | Page 6

tubs, he discovered
more new gases than all his predecessors put together had done. He laid
the foundations of gas analysis; he discovered the complementary
actions of animal and vegetable life upon the constituents of the
atmosphere; and, finally, he crowned his work, this day one hundred
years ago, by the discovery of that "pure dephlogisticated air" to which
the French chemists subsequently gave the name of oxygen. Its
importance, as the constituent of the atmosphere which disappears in
the processes of respiration and combustion, and is restored by green
plants growing in sunshine, was proved somewhat later. For these
brilliant discoveries, the Royal Society elected Priestley a fellow and
gave him their medal, while the Academies of Paris and St. Petersburg
conferred their membership upon him. Edinburgh had made him an
honorary doctor of laws at an early period of his career; but, I need
hardly add, that a man of Priestley's opinions received no recognition
from the universities of his own country.
That Priestley's contributions to the knowledge of chemical fact were of
the greatest importance, and that they richly deserve all the praise that
has been awarded to them, is unquestionable; but it must, at the same
time, be admitted that he had no comprehension of the deeper
significance of his work; and, so far from contributing anything to the
theory of the facts which he discovered, or assisting in their rational
explanation, his influence to the end of his life was warmly exerted in
favour of error. From first to last, he was a stiff adherent of the
phlogiston doctrine which was prevalent when his studies commenced;
and, by a curious irony of fate, the man who by the discovery of what
he called "dephlogisticated air" furnished the essential datum for the

true theory of combustion, of respiration, and of the composition of
water, to the end of his days fought against the inevitable corollaries
from his own labours. His last scientific work, published in 1800, bears
the title, "The Doctrine of Phlogiston established, and that of the
Composition of Water refuted."
When Priestley commenced his studies, the current belief was, that
atmospheric air, freed from accidental impurities, is a simple
elementary substance, indestructible and unalterable, as water was
supposed to be. When a combustible burned, or when an animal
breathed in air, it was supposed that a substance, "phlogiston," the
matter of heat and light, passed from the burning or breathing body into
it, and destroyed its powers of supporting life and combustion. Thus,
air contained in a vessel in which a lighted candle had gone out, or a
living animal had breathed until it could breathe no longer, was called
"phlogisticated." The same result was supposed to be brought about by
the addition of what Priestley called "nitrous gas" to common air.
In the course of his researches, Priestley found that the quantity of
common air which can thus become "phlogisticated," amounts to about
one-fifth the volume of the whole quantity submitted to experiment.
Hence it appeared that common air consists, to the extent of four-fifths
of its volume, of air which is already "phlogisticated"; while the other
fifth is free from phlogiston, or "dephlogisticated." On the other hand,
Priestley found that air "phlogisticated" by combustion or respiration
could be "dephlogisticated," or have the properties of pure common air
restored to it, by the action of green plants in sunshine. The question,
therefore, would naturally arise--as common air can be wholly
phlogisticated by combustion, and converted into a substance which
will no longer support combustion, is it possible to get air that shall be
less phlogisticated than common air, and consequently support
combustion better than common air does?
Now, Priestley says that, in 1774, the possibility of obtaining air less
phlogisticated than common air had not occurred to him. [6] But in
pursuing his experiments on the evolution of air from various bodies by
means of heat, it happened that, on the 1st of August 1774, he threw the
heat of the sun, by means of a large burning glass which he had
recently obtained, upon a substance which was then called mercurius
calcinatus per se, and which is commonly known as red precipitate.

"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 well 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 air,
exposed to iron or lime of sulphur; but as I had got nothing like this
remarkable appearance from any kind