PHYSIOLOGY. 
352 
accuracy. From a number of trials made upon bis own 
refpiration, he found, that ioo cubic inches of atmof¬ 
pheric air, after having once palled through the lungs, 
had loll between four and live parts of oxygen : hence 
he calculates, that 31*6 cubic inches of oxygen are con- 
fumed in a minute; this will give 45^504 inches in 24 
hours, a quantity which will weigh 15471‘36 grains. 
This eltimate coincides nearly with that of M. Lavoifier, 
though it was obtained by a different procefs, and by the 
life of a different apparatus. We may therefore conclude, 
that between 45 and 46,000 cubic inches, or about 
15,500 grains — 2 lbs. 8 oz. troy, is the average quantity 
of oxygen confumed by a man in 24 hours. (Davy’s 
Refearches, p. 431. Effay on Refpiration, p. 78.) 
In the feries of experiments performed by Meffrs. 
Allen and Pepys, an apparatus was employed, in which 
the volume of the air refpired could be meafured with 
great accuracy, and in which a large quantity (3 or 4,000 
cubic inches) could be refpired ; lo that the fource of er¬ 
ror, to which experiments on a fmalier fcale are liable, 
from the influence of the refidual air in the lungs, was 
much diminilhed. They caufed a perfon to inlpire, 
from a gafometer, 3460 cubic inches of atmofpheric air, 
which were afterwards expired into another gafometer; 
and to both gafometers graduated fcales were affixed, by 
which the quantities of air received and expelled could 
be accurately meafured. The time occupied in the ex¬ 
periment was 11 minutes: about 58 refpirations were 
made; and the deficiency in the whole volume of air, at 
the dole of the experiment, amounted only to 23 cubic 
inches. One hundred parts of the expired air afforded, 
on analyfis, 8 - 5 carbonic acid, i2'5 oxygen, and 79 nitro¬ 
gen gas. (Phil. Tranf. 1808, p. 254.) In an experiment 
on the refpiration of another fubjedl, the changes pro¬ 
duced in the air were the fame, but the quantity con- 
fumed was very different. The thermometer (Fahr.) 
being at 56°, and the barometer at 3o®.3, 3300 cubic 
inches of atmofpheric air were infpired, and 3311 ex¬ 
pired, in 5A minutes. One hundred parts of the expired 
air confided of 8’5 carbonic acid, i2‘5 oxygen, and 79 
azote. When, therefore, twice the quantity of air was 
palled through the lungs in a given time, as great a pro¬ 
portion of its oxygen was confumed, and as much carbo¬ 
nic acid formed, as in a fubjedt in whom only half the 
quantity was breathed. The experiment was repeated 
leveral timCs; and in oneinftance, 9890 cubic inches of 
air were breathed for 24J minutes, with the lofs of only 
18 cubic inches; and 100 parts of the expired air then 
afforded, on analyfis, 8 carbonic acid, 13 oxygen, and 79 
nitrogen. (Ibid. p. 257.) Now the air employed in 
thefe experiments contained, in 100 parts, 21 oxygen, 
and 79 nitrogen ; and in the numerous analyfes which 
w'ere made of this air after its refpiration, the portion of 
oxygen that difappeared was exadlly replaced by that of 
carbonic acid produced; fo that, in every inllance, thefe 
two gafes formed together '21 of the relpired air, the re¬ 
maining 79 parts being pure'nitrogen gas. It is there¬ 
fore concluded, that the quantity of carbonic acid gas 
emitted is exactly equal, bulk for bulk, to the oxygen 
confumed. (Ibid. p. 279.) 
The changes occurring in refpiration are influenced by 
various caufes, which modify the a&ions of the capillary 
veffels. Crawford eftablilhed by experiment, that lets 
oxygen is confumed at a high than at a low temperature. 
He obferved alfo, that in an animal placed in a warm me¬ 
dium, the venous blood approached to the arterial in co¬ 
lour. Hence it appears, that the high temperature coun- 
teradfs thofe chemical changes whicli the blood under¬ 
goes in the extreme veffels ; and that the diminution in 
the confnmption of oxygen by refpiration is owing to- 
this caufe, and not, as has been fuppofed, to the rarity 
■of the air at the high temperature. If the confumption 
of oxygen were diminilhed from the latter caufe, the 
blood ought to be even more completely venous than 
tifual. The fadl, formerly known, that the confumption 
of oxygen is influenced by the food and the Hate of d'gef- 
tion, was confirmed and more accurately demonftrated 
by Lavoifier and Seguin. They found, that during di- 
geftion the confumption of oxygen was increafed to 1800 
or 1900 cubic inches in an hour. Exercife, too, increafed 
the proportion confumed. It is Hated that Seguin, in 
continuing the exercife of raifing a weight of 15 pounds 
to a height of 613 feet during a quarter of an hour, con¬ 
fumed 800 cubic inches, which is at the rate of 3200 in 
an hour; and the fame exercife, made during digeftion, 
occafioned a confumption equal to 4600N 
Notwithftanding, however, fays Mr. Ellis, the neceffity 
of oxygen gas to the continuance of refpiration, and the 
great quantity of it that is thus daily confumed, many 
faffs tend to prove, that, by the very conllitution of 
that funffion, a neceffary limit is placed to its confump¬ 
tion; and that this limit is determined, not by the pu¬ 
rity of the air employed, but by fome circutnllances in¬ 
herent in the animal fyltem. It has been found, that the 
growth of vegetables is retarded by a great fuperabun- 
dance of oxygen ; (Ellis’s Inquiry, § 14—40.) and that, 
although infefls will live a confiderable time in this gas, 
yet their breathing becomes oppreflive, and they die (ibid. 
§ 53.) long before the whole of it is confumed. More¬ 
over, the inferior animals, under the fame circumftances, 
fuffer in the fame manner, as has been proved by experi¬ 
ment. Another curious fafl is, that an animal confumes 
much lefs oxygen gas when he refpires it pure, than when, 
for the fame length of time, he breathes atmofpheric air; 
and the quantity of carbonic acid formed in the firft cafe, 
is but little more than half that obtained by the refpira¬ 
tion, for the fame time, of atmofpheric air. Confe- 
quently, we may conclude, that the atmofphere, as it is 
naturally compofed, is bell adapted to the economy of 
the animal fyftem ; but that this fyftem is, at the lame 
time, fo conllituted, as to be able to bear great variations 
in the compofition of the air, without immediate injury 
to the powers of animal life. 
Having afcertained the proportion of oxygen which is 
confumed in refpiration, it next remains for us to deter¬ 
mine the quantity of carbonic acid gas which is produced. 
It appears that Dr. Black firll demonftrated its exiftence 
in air emitted from the lungs, and that Lavoifier after¬ 
wards examined it with more accuracy, and found that the 
air, in which an animal had expired, contained about 
one fixth of its bulk of carbonic acid gas. M. Jurine of 
Geneva, firft attempted to calculate the abfolute quantity 
of carbonic acid formed by the refpiration of man; he 
imagined that it conllituted about one-tenth part of the 
air emitted from the lungs. (Encycl. Method. Medecine, 
i. 494.) Dr. Menzies inftituted a fet of experiments to 
difcover the abfolute quantity generated in a given time ; 
he infers from them, that -Jgth part of air which has 
been once refpired is carbonic acid, and ellimates, that 
a man in 24 hours fends out from the lungs 51,840 cu¬ 
bic inches, or nearly 4lbs. troy ; but this eftimate is pro¬ 
bably over-rated. See his Effay, p. 50. 
The circumftances which have been already pointed 
out, as influencing the confumption of oxygen, have at 
leaft as powerful an effeCt upon the production of carbo¬ 
nic acid gas. Accordingly we fliall find the calculations 
of the molt accurate experimenters upon this fubjeCl fo 
widely different from each other, that it feems fcarcely 
poflible to arrive at any tolerable degree of certainty. 
If it fliould be allowed, however, as the greateft number 
of inveftigations on the fubjeCt tends to prove, that the 
carbonic acid formed in refpiration is lei's in bulk than 
the oxygen gas which difappears, yet the weight of the 
compound is increafed by the additio,n of carbon, de¬ 
rived from the animal fyftem. Lavoifier and Seguin efti¬ 
mate d the weight of oxygen gas confumed by a man in 
24 hours, at i566r66 grains; and that of carbonic acid 
produced in the fame lpace of time, at 17720^9 grains; 
the carbon conftituting -g^th of the weight of the acid 
formed. In the experiments of Mr. Davy, the volume 
