contrary, gives arterial bloorl the colour of 
venous bloorl ; it was not till then that phi- 
losophers began to attempt any thing like 
an explanation of the pneno.nena of respi- 
ration. 
The blood is a fluid of so complex a nature 
that it is not easy to ascertain the changes 
produced in it by exposure to different gases 
out of the body; and even if that could be 
done, we have no method of proving that 
the effects of these gaseous bodies upon the 
coagulated blood are the same as they would 
be on the blood in its natural state, circu- 
lating in the vessels of a living animal. The 
facts which have been ascertained are tire 
following : 
1st. It appears from the experiments of 
Priestley, Girtanner, and Hassenfratz, that 
when venous blood is exposed to oxvgen 
gas confined over it, the blood instantly as- 
sumes a scarlet colour, and the gas is dimi- 
nished in bulk; therefore part of the gas 
has been absorbed. Mr. Davy indeed could 
not perceive any sensible diminution of the 
bulk of the gas. 
2d. Ibe same change of colour takes 
place when blood is exposed to common 
air; and in that case the diminution of the 
bulk of the air is rather more sensible.. 
3d. Venous blood exposed to the action 
of azotic gas continues unaltered in colour; 
neither does any perceptible diminution of 
the gas en-;ue. 
4th. Venous blood exposed to the action 
of nitrous gas becomes of a deep purple, and 
about one-eighth of the gas is absorbed. 
5th. Venous blood exposed to nitrous 
oxide becomes of a brighter purple, especi- ; 
ally on the surface, and a considerable por- 
tion of the gas is absorbed. 
6th. Venous blood exposed to carbonic 
acid gas becomes of a brownish-red colour, 
much darker than usual, and the gas is slight- 
ly diminished in bulk. 
• 7th. Carbureted hydrogen gas gives venous 
blood a fine red colour, a shade darker than 
oxygen gas does, as was first observed by 
Dr. Beddoes, and at the same time a small 
portion of the gas is absorbed. This gas 
has the property of preventing, or at least 
greatly retarding, the putrefaction of blood, 
as was first observed by Mr. Watt. 
8th. . When arterial blood is put in contact 
with azotic gas, or carbonic acid gas, it gra- 
dually assumes the dark colour of venous 
blood, as Dr. Priestley found. The same 
philosopher also observed, that arterial blood 
acquired the colour of venous blood 
when placed in vacuo. Consequently this 
alteration of colour is owing to some change 
which takes place in the blood itself, inde- 
pendant of any external agent. 
The arterial blood becomes much more 
rapidly and deeply dark-coloured when it is 
left in contact with hydrogen gas placed 
above it. We must suppose, therefore,, that 
the presence of this gas accelerates and in- 
creases tlie change, which would have taken 
place upon the blood without any external 
agent. 
9th.. If arterial blood is left in contact with 
oxygen gas, it gradually assumes the same 
dark colour which it would have acquired in 
vacuo, or in contact with hydrogen ; and 
after this change oxygen can no longer re- 
store its scarlet colour. It is therefore only 
upon a part of. the blood that the oxygen 
RESPIRATION. 
acts; and after this part Iras undergone the 
change which occasions the dark colour, the 
blood loses the power of being affected by 
oxygen. 
10th. Mr. Hassenfratz poured into venous 
blood a quantity of oxynuiriatic acid ; the 
blood was instantly decomposed, and assum- 
ed a deep and almost black colour. When 
he poured common muriatic acid into blood, 
the colour was not altered. Now oxymu- 
riatic acid has the property of giving out its 
oxygen readily ; consequently the black co- 
lour was owing to the instant combination of 
a part of the blood with oxygen. 
Such are the phenomena produced upon 
the blood by the different gases out of the 
body ; but the science is not far enough ad- 
vanced at present to be able to explain them 
in a satisfactory manner. The obvious 
changes produced on the blood in the lungs 
by respiration, are the florid red colour, and 
the disappearing of the chyle. 
3. That carbonic acid is emitted from the 
lungs during expiration, has been fully ascer- 
tained; but whether it is formed in the lungs, 
according to the theory of Lavoisier, by the 
combination of the oxygen of the air with 
carbon emitted by the blood, or is emitted 
ready-formed from the blood at the same 
time that the air is absorbed, is not so ob- 
vious ; but the latter opinion is more proba- 
ble, and indeed follows from the supposi- 
tion that air is absorbed without decompo- 
sition. 
4. It is much more reasonable to conclude 
that the watery vapour which exhales from 
the lungs along with the air expired, has been 
emitted from the blood, or from the vessels 
of the lungs, than to suppose with Lavoisier, 
that it is formed in the lungs by the combi- 
nation of the oxygen of the air with hydrogen 
emitted from the blood. 
From the preceding enumeration of facts, 
we may conclude that the following changes 
are produced by respiration : The blood, as 
it passes through the lungs, absorbs a portion 
j of air, and carries it along with it through 
i the blood-vessels. During the circulation 
i this air is gradually decomposed by the 
, blood, its oxygen and part of its azote enter- 
! ing into new combinations, while at the same 
time a portion of azote, of carbonic acid, 
and water, is evolved. When the blood re- 
turns to the lungs, it absorbs a new dose of 
air, and at the same time lets go the azotic- 
gas, carbonic acid gas, and watery vapour, 
which had been formed during the circula- 
tion. The same changes are again repeated, 
and the same substances emitted, every time 
the blood comes to the lungs. 
It is probable that, during a considerable 
part of the day, there is a constant influx of 
chyle into the blood ; and we are certain that 
lymph is constantly flowing into it. Now it 
appears, from the most accurate observa- 
tions hitherto made, that neither chyle nor 
lymph contains fibrina, which forms a very 
conspicuous part of the blood. This fibrina 
is employed to supply the waste of the 
muscles ; the most active parts of the body, 
and therefore, in all probability, requiring 
the most frequent supply. Nor can it be 
doubted that it is employed tor other useful 
purposes. The quantity of fibrina in the 
blood, then, must be constantly diminishing, 
and therefore new fabrina must be constant- 
ly formed. But the only substances out of 
s;3 
which it can be formed are the clple and 
lymph, neither of which contains it. There 
must, therefore, he a continual decomposi- 
tion of the chile and lymph going or. in the 
blood-vessels, and a continual new formation 
of fibrina. Other substances also may be 
formed ; but we are certain that this must be. 
formed there, because it does not exist pre- 
viously. Now, one great end of respiration 
must undoubtedly lie, to assist this decom- 
position of cln le, and complete formation of 
blood. 
It follows, from the experiments of Four- 
croy, that fibrina contains more azote, and 
less hydrogen and carbon, than any of the 
other ingredients of. the blood, and conse- 
quently aiso than any of the ingredients of 
the chyle. In what manner the chyle, or a 
part of it,, is converted into fibrina, it is im- 
possible to say : we are not sufficiently ac- 
quainted with the subject to be able to ex- 
plain the process. But we can see at least, 
that carbon and hydrogen must be abstracted 
from that part of the chyle which is to be- 
converted into fibrina ; and we know, that 
these substances are actually thrown out by 
respiration. We may conclude, then, that one 
use of the air absorbed is to abstract a quan- 
tity of carbon and hydrogen from a part of 
the chyle bv compound affinity, in such 
proportions- that the remainder becomes- 
fibrina : therefore one end of respiration is 
to form fibrina. Doubtless the other in- 
gredients of the blood are also new-modified, 
though we know too little of the subject to 
throw any light upon it. 
But the complete formation of blood is not 
the only advantage gained by respiration 
the temperature of all animals depends upon, 
it. It has been long known, that those ani- 
mals which do not breathe have a tem- 
perature but very little superior to the 
medium in which they live. This is the 
case with fishes and many insects. Man, 
on the contrary, and quadrupeds which 
breathe, have a temperature considerably 
higher than the atmosphere : that of. mail: 
is 98°. Birds, who breathe in proportion a 
still greater quantity of air than man, have 
a temperature equal to 103° or 104°. It has 
been proved, that? the temperature of all ani- 
mals is proportional to the quantity of air 
which they breathe in a given time: 
These facts are sufficient to demonstrate, 
that the heat of animals depends upon re- 
spiration. But it was not till Dr. Black’s 
doctrine of latent heat became known to the- 
world, that any explanation of the cause of 
the temperature of breathing animals was 
attempted. That illustrious philosopher, 
whose discoveries form the basis upon which 
all the scientific part of chemistry has been 
reared, saw at once the light which his doc- 
trine of latent heat threw upon this- part of 
physiology, and he applied it very early to 
explain the temperature of animals. 
According to him, part of the- latent heat 
of the air inspired become s-sensible ; and of 
course the temperature of the lungs, and the 
blood that passes through them,, must be 
raised : and the blood, thus heated, com- 
municates its heat to the whole body. This- 
opinion was ingenious, but it was liable to 
an unanswerable objection : for if it was. 
true, the temperature of the body ought to- 
be greatest in the lungs, and to diminish gra- 
dually as- the distance from, the lungs iire 
