351 
PHYSIOLOGY. 
which that explanation was partly founded no longer 
exift, yet it thould never be forgotten that his experi¬ 
ments and difcoveries firft pointed out the true path of 
iuveftigation ; and have contributed, in a pre-eminent 
degree, to advance our knowledge of this raoft important 
function. The caufe of the unfitnefs of air, beyond a 
certain extent, to fupport life and flame, he proved to 
arife from the deftrudlion of its pure part, or what has 
fince been called its oxygen gas; and he concluded, that, 
in refpiration, combuftion, and calcination, which, in 
confequence of his peculiar theory, he flyles phlogiftic 
procefl’es, it underwent precifely the fame changes. 
(Phil. Tranf. 1776. Obf. on Air, vol. iii.) 
About a year after the publication of Dr. Prieftley’s 
experiments, the celebrated and unfortunate Lavoifier 
prefented a memoir on refpiration to the French Aca¬ 
demy of Sciences. He agrees with Prieftley in fuppoiing, 
that the proportion of oxygen is diminifhed in air which 
has been refpired ; but, upon a careful analyfis of the 
refidue, he finds it to differ from the air left after the 
calcination of metals, which is merely azotic gas, in 
containing a quantity of carbonic acid. He alfo obferved, 
that the bulk of the air W’as fomewhat diminifhed ; and we 
learn in general from thefe experiments, that the changes 
produced in air by refpiration, confift in the removal of 
part of the oxygen, in the addition of a quantity of 
carbonic acid gas, and in the diminution of its volume. 
He fuppofes that the azote is not affefled by the procefs, 
and that it ferves merely to dilute the oxygenous part of 
the atmofphere. In this paper, M. Lavoilier does not 
mention the aqueous vapour which is fo evidently dif- 
charged from the lungs by refpiration; it is impoflible 
that it could have been overlooked by fo accurate an 
obferver ; we may therefore conjecture, that he omitted 
to mention it, becaufe at this period he confidered it as 
only diffufed through the air expired from the lungs, by 
the procefs of evaporation, and not formed in confe¬ 
quence of the operation of any chemical affinities. 
A difficult and interefting queftion refpedling the con- 
fumption of oxygen, is the abfolute quantity of this gas 
confumed by refpiration in a given time. The firfl calcu¬ 
lations which were made upon this fubjedl, in confe¬ 
quence of the imperfeCf nature of the apparatus employed, 
and of the want of a fufficient dexterity in the manage¬ 
ment of pneumatic experiments, were unavoidably vague 
and inaccurate. The difficulty was much increafed by a 
circumftance firft noticed by Dr. Crawford, and after¬ 
wards more fully inveftigated by M, Jurine of Geneva, 
and M. Lavoilier, that the refpiration of the fame animal 
in different ftates of the fyftem, and under the operation 
of different external circumftances, affefts the air in very 
different degrees. This curious fa ft, which affords an 
infight into lome of the moll important operations of 
the animal economy, mull unavoidably produce great 
differences in the refults of the beft-conduCted experi¬ 
ments, and will render it impoflible for us to arrive at 
more than an approximation to the truth. The circum¬ 
ftances which have been difcovered to influence the che¬ 
mical efteCts of the refpiration are, the temperature of the 
air refpired, the degree of mufcular exertion, the Hate of 
the digeftive organs, and the condition of the fyllem as 
afteCted by fever ; it is highly probable that other circum¬ 
ftances will be difcovered, by multiplying and varying our 
experiments upon the living body. 
^ An experiment performed by Lavoilier, upon a guinea- 
pig, feems to have been the firft in which a perfect appa¬ 
ratus, and the neceffary degree ofaccuracy,were employed. 
(Mem. de l’Acad. des Sciences, 1780, p. 401.) The 
animal was confined over mercury, in a jarcontaining248 
cubic inches of gas, confiding principally of oxygen. In 
an hour and a quarter, the animal breathed with much 
difficulty; and, being removed from the apparatus, the 
ftate of the air was examined. Its bulk was found to 
be diminifhed by eight cubic inches; and, of the remain¬ 
ing 240 inches, 40 was abforbed by cauftic potalh, and 
confequently confided of carbonic acid gas. Taking 100 
parts of this air, thefe numbers will be as follows : the 
air was diminilhed to 96-5, or by 3-5 cubic inches ; and, 
of the remainder, i 6 - 5 w'ere converted into carbonic acid 
gas, and abforbed by potalh, which reduces the quantity 
of air to 80 cubic inches. Towards the conclufion of the 
experiment, the air would be neceffarily much lefs fit for 
performing the funflions of the lungs than the air of 
the atmofphere, in confequence of the carbonic acid gas 
which it contained ; but, as the air employed was origi¬ 
nally much purer than the atmofphere, the author 
fuppofes, that the quantity of oxygen deflroyed was 
probably about the fame which would have been con- 
fumed under the ordinary circumftances of refpiration. 
The fame philofopher performed a fecond experiment 
upon the fame fpecies of animal, with Hill more accuracy, 
in which pure oxygen were employed. (Ann. de Chimie, 
t. v. p. a6i.l This experiment continued during an hour 
and a half; and, the animal being then removed from the 
jar, the air was analyfed as in the former cafe ; 1728 
cubic inches of air were found to be reduced to 1673 ; 
i. e. had luffered a diminution of 55 inches; cauftic 
potalh abforbed about 229‘5 inches, leaving a refidue of 
pure oxygen. Thefe numbers, eftimated as in the former 
cafe, will be nearly as follows: 100 inches were reduced 
to 96-82, or by 3*18 inches, the potafh abforbed about 19 
inches, reducing the whole quantity of air to 77-82 parts. 
The quantity of carbonic acid was here fomewhat 
greater than in any former experiment, which may be 
attributed to the air employed being pure oxygen, and to 
the procefs having been continued fora fomewhat longer 
fpace of time than in the former inftance. Upon the 
whole, the refults correfpond as nearly as can be expelled, 
from the very delicate nature of the experiments. 
Dr. Menzies firft attempted to afcertain the quantity of 
oxygen conlumed by a man in the courfe of a day. He 
found by experiment, that one-twentieth part of air, 
which had been once refpired, is converted into carbonic 
acid gas; this he concludes mull have been oxygen, as 
that part of the air alone is affected by refpiration. He 
conceives that 720 cubic inches of air are refpired in a 
minute, of which confequently 36 will be confumed. 
From thefe data he ellimates, that in the fpace of 24 
hours, 51,840 cubic inches, or 17625-6 grains, of oxygen, 
are confumed and converted into carbonic acid gas. In 
this calculation feveral important particulars appear to 
have been overlooked; and accordingly it will be found 
to differ from the refults of the more accurate experiments 
which have been fince performed by Lavoifier and Mr. 
Davy. 
The experiments made by M. Seguin, which were 
conduced with every poffible attention to accuracy, and 
with an apparatus more complete than any which had 
ever been employed in phyfiological relearches, induce us 
to conclude, that the mean confumption of oxygen by a 
man, during 24 hours, is fomewhat more than 22 French 
cubic feet, or 46037-38 Englilh cubic inches; a quantity 
of gas which will weigh 15661-66 grains troy. 
Lavoifier was Hill continuing to purfue his experiments 
on this fubjedt, and had conftrucled a very expenfive ap¬ 
paratus, for afcertaining with Hill more precillon the 
amount of the feveral changes produced by refpiration, 
when he fell a facrifice to the fury of Robefpierre, and 
received fentence of death. He had already performed 
a number of experiments with his new apparatus, and 
earneltly requefted a refpite of a few days, in order to 
prepare them for publication; but his requeft was not 
granted. M. de la Place, who pronounced his eulogy, 
has fortunately given us the moll important refults : they 
will be found to differ in fome particulars from the for¬ 
mer experiments, though, with refpedl to the quantity 
of oxygen confumed, they nearly coincide. It is ftated 
that a man in 24 hours confumes 15592-5 grains. 
There are fome experiments on this fubjeft by Mr. 
Davy, which appear to have been executed with great 
accuracy. 
