572 
opinion thatfit is the oxygen only, and that 
the_azote remains the same after respiration 
as beiore it. These conclusions were the 
consequence of the experiments of Lavoisier, 
who announced the non-alteration of the 
-azote of the atmosphere at a very early period 
of his researches. This conclusion seems to 
have been the consequence ot the opinion 
which he entertained, that air is merely a 
mechanical mixture of the two gases, oxygen 
and azotic; for when he first adopted it, his 
apparatus was not delicate enough to mea- 
sure small changes ; and he does not appear 
to have afterwards examined the azotic re- 
siduum with much attention. Mr. Davy 
•has rendered it probable that a portion of the 
azote of the air as well as its oxygen disap- 
pears during respiration. 
According to Dr. Menzies, at every respi- 
ration 2.1885 cubic inches of oxygen gas are 
consumed. Now, 2.1885 cubic inches of 
that gas amount to 0.68669 grains troy. 
Supposing, with liules, that a man makes 
1200 respirations in an hour, the quantity of 
oxygen gas consumed in an hour will amount 
to ' 824.028 grains, and in 24 hours to 
18770.672 grains, or 41.2104 ounces troy. 
This quantity exceeds that found by other 
chemists considerably; but the allowance of 
oxygen for every respiration is rather too 
great. Indeed, from the nature ot Dr. 
Menzie’s apparatus, it was scarcely possible 
to measure it accurately. According to the 
last experiments of Lavoisier and Seguin, a 
man, at an average, consumes in twenty-four 
hours, by respiration, 32.48437 ounces troy 
of oxygen gas: that is to say, that a quan- 
tity of oxygen gas, equal to that weight,, dis- 
appears from the air which he respires in 24 
hours. According to Mr. Davy, the ave- 
rage quantity of "air which disappears dur- 
ing every respiration is 1.4 cubic inch; of 
which 0.2 are azote and 1.2 oxygen. This, 
allowing 26 respirations per minute, as was 
the case with Mr. Davy (the subject of the ex- 
periment), amounts in24 hours to rather more 
than 38 ounces of air ; or precisely to 4.68 
ounces of azote, and 33.54 of oxygen. ■ his 
does not differ far from the result obtained by 
Lavoisier, excepting in the azote, which the 
French chemist -'neglected altogether. We 
may consider it therefore as approximating 
to the truth as nearly as can be expected in 
the present state of the science. 
2. That the air thrown out of the lungs 
contains carbonic acid, may be easily ascer- 
tained by blowing it through a tube into 
lime - water, which immediately becomes 
milky ; and the bulk of the gas may be esti- 
mated by putting a portion of air from tire 
lungs into a graduated jar standing over mer- 
cury, introducing a little barytic, water, or 
pure soda, to absorb the carbonic acid, and 
observing the diminution of bulk in conse- 
quence of this absorption. According to 
Lavoisier, a man in 24 hours throws out from 
his lungs at an average about 15.73 ounces 
troy of carbonic acid. From the experi- 
ments of Mr. Davy, on the other hand, it 
follows, that at every expiration about 1.1 
cubic inch of carbonic acid is emitted, which 
amounts in 24 hours to no less than 37 
ounces. The difference between these two 
sets of experiments is enormous, and claims 
a more complete experimental investigation 
*o determine, whether the proportion of this 
gas emitted by different individuals, or by 
(RESPIRATION , 
the same individual at different times, does 
not differ essentially. r I his supposition is 
surely very probable, as it tallies with what 
we know to be the case in other excretions , 
and if it proves true, would throw more light 
upon the nature of respiration than any thing 
which has hitherto been ascertained. In the 
mean time, til! farther experiments decide 
the point, eve may consider Mr. Davy s con- 
clusions as nearest the medium of the two, 
because they correspond with the earlier ex- 
periments of Lavoisier, and remove a very 
striking anomaly which appears when we 
compare Lavoisier’s experimen s on the re- 
spiration of the guinea-pig with those on the 
respiration of man. He put a guinea-pig 
into 708.989 grains troy of oxygen ; and after 
the animal had breathed the gas lor an hour, 
he took it out. He found that the oxygen 
gas now amounted only to 592.C53 gr. 
Consequently there had disap- 
peared - 116.736 
Tire carbonic acid gas formed was 130.4/2 
The guinea-pig consumed in 
24 hours - 5.8368 o z. 
troy of oxygen gas, and emitted 6.5236 oz. 
of carbonic a id gas. Man, on the ^otlier 
hand, consumes in the same time 32.4843/ oz. 
of oxygen gas, and emits only 15.73 oz. ot 
carbonic acid gas. The oxygen gas con- 
sumed by the pig is to the carbonic acid gas 
emitted as 1.00 : 1.12 ; whereas in man it is 
as 1.000 : 0.484. If we could depend upon 
the accuracy of each of these experiments, 
they would prove, beyond a doubt, that the 
changes produced by the respiration ot the 
pig are different, at least in degree, from 
those produced in man ; but it is more pro- 
bable that some mistake has happened in 
one or other of the experiments. 
3. It is not so easy to determine the pro- 
portion of water emitted from the lungs 
mixed with the air expired, as it is that of 
the carbonic acid. According to the experi- 
ments of Dr. Hales, it amounts in a day to 
20.4 oz.; but his method was not susceptible 
of great accuracy. Mr. Lavoisier, on the 
other hand, estimates it at 28.55 ounces; 
but this proportion seems rather to have 
been the result of calculation than of any 
direct measurement. It can only be con- 
sidered therefore as an approximation to the 
truth, and most probably a very imperfect 
one. 
III. Let us now endeavour to ascertain the 
changes produced on the blood by respira- 
tion. The whole of the blood is propelled 
from the heart to the lungs, circulates 
through the vessels of that organ, and during 
that circulation it is exposed to the influ- 
ence of the air which the animal is constantly 
drawing into the lungs. Now certain 
changes are produced upon it by this action, 
w'hich have been partly traced by the experi- 
ments of Priestley, Cigna, Fourcroy, Has- 
senfratz, Beddoes, Watt, and above all by 
those of Mr. Davy. These changes, as far 
as we are acquainted with them, are the fol- 
lowing: 1. The blood absorbs air. 2. It 
acquires a florid red colour, and the chyle 
disappears. 3. It emits carbonic acid, and 
perhaps carbon. 4. It emits water, and per- 
haps hydrogen. 
As the azote which lias separated from the 
air during respiration is not to be found in 
the products of respiration, we must conclude 
that it has been absorbed by the bloodl Ths* 
experiments of Mr. Davy have rendered it. 
exceedingly probable that the air is absorb- 
ed unaltered by the blood ; that it is after- 
wards decomposed by that liquid; and that 
the portion of azote which is useless is given 
out again, and mixed with the air in the 
lungs.' The following facts render this opi- 
nion probable : When hydrogen gas is re- 
spired, no part of it is absorbed or disap- 
pears, nor are any positive changes produced 
on the blood. But when tire gaseous oxide of 
azote is respired, it diminishes in quantity, 
while at the same time carbonic acid is 
evolved as usual, and a quantity of azotic 
gas makes its appearance. Now, as this 
azotic gas did not exist separately in the air 
before respiration, it must have been pro- 
duced by the decomposition of the oxide of 
azote; but its quantity being much less than 
the azote contained in the oxide of a/ole- 
which had disappeared, it follows that at 
least a part of this last gas had been absorbed 
by the blood unaltered ; and it a part is 
thus absorbed, why not the whole ? In that 
ca->e the azotic gas must have been separated 
from the blood, in consequence ot the sub- 
sequent decomposition of the oxide of azote 
absorbed. Now, as air is composed of pre- 
cisely the same ingredients with the oxide 
of azote ; and as a portion of the azote, as 
well as of the oxygen,, ot the air respired, 
disappears ; it is reasonable to suppose that 
the air is absorbed by the blood, and that the 
azotic gas which is developed is thrown out 
of the blood in consequence of the decom- 
position of the air absorbed. But farther, if 
the oxygen of the air was alone absorbed by 
the blood during respiration while the azote- 
remains unaltered, oxygen gas ought to an- 
swer the same purposes as air. This gas, 
however, cannot be respired without occa- 
sioning death at last; and when it is respir- 
ed, the proportion of oxygen which disap- 
pears in a given time is much smaller than, 
when the air is respired. Thus when 182: 
cubfc indues of oxygen gas were breathed 
by Mr. Davy for half a minute, 11.. 4 cubic 
inches of the gas disappeared, whereas 15.6 
cubic inches disappear in the same time when 
common air is respired. This is a demon- 
stration that the whole of air is useful in re- 
spiration, and not merely its oxygen and. 
if so, the air must be absorbed. 
2. It has been long known that the blood 
which flows in the veins is of a dark-reddish, 
purple colour, whereas the arterial blood is 
of a florid scarlet colour. Lower observed, 
that the colour of venous blood was converted 
into that of arterial during its passage through: 
the lungs. No chyle can be distinguished by 
its white colour in the blood alter it has pass- 
ed through the lungs. The changes, then,, 
which take place upon the appearance of the- 
blood, are two : 1. It acquires a florid red, 
colour; 2. the chyle totally disappears. 
Lower himself knew that the change was 
produced by the air, and Mayow attempted, 
to prove that it was by absorbing a part of 
the air. But it was not till Dr. Priestley 
discovered that venous blood acquires a 
scarlet colour when put in contact with oxy- 
gen gas, and arterial blood a dark red co- 
lour when put in contact with hydrogen 
gas ; or, which is the same thing, th At oxy- 
gen gas instantly gives venous blood the 
colour of arterial, and hydrogen, on tire 
6 
