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PHYSICS AND CHEMISTRY. 453
furnished an experimental proof that both in the combustion
of metals and in the ignition of phosphorus and sulphur an
increase of weight occurred (contrary to what was postulated
by the phlogiston theory), and that such increase of weight
was due to absorption of air; he was unable to say, however,
whether this absorption was of the air as a whole or of only
a part of it. When, thanks to PRIESTLEY, he became ac­
quainted with oxygen, it occurred to him to seek in it the
cause of this phenomenon. After numerous experiments
he came to the conclusion that only one-fifth part of the
atmosphere takes part in combustion and that the air con­
sists of one part of oxygen and four parts of a gas which
subserves neither combustion nor respiration. His state­
ments upon the composition of air, water, and various acids
were confirmed and rendered more complete on some
points by CAVENDISH * The theory of phlogiston being
thus refuted, several questions asserted themselves which
had hitherto been explained by means of it or by analogous
reasoning.
LAVOISIER, finding oxygen to exist in all the acids which
he examined, pronounced it to be a necessary constituent
part of all these bodies—to be that, in fact, which bad pre­
viously been designated " primitive acid ; " he also alluded
to the part which the element plays in the oxidation or so-
called "calcination" of metals. BLACK had already cor­
rectly described the nature of caustic alkalies. LAVOISIER
disclosed more thoroughly the importance of oxygen in
respiration and its effect upon the blood, and thus led the
way to a fundamental change in the physiological explana­
tion of these processes.
The discovery of oxygen also exerted a great influence
upon pathology and therapeutics. Some doctors discerned
in it the " air of life " upon which health depends. They
thought that certain diseases depended upon the excess or
deficiency of oxygen and they employed it in therapeutics.
* KOPP: Beitrage zur Geschichte der Chemie, Braunschweig 1875, iii, 254
et seq.