366 ON THE EXTERNAL CAUSES OF DISEASE. 
ounce of venous blood, and that the expired air contains about 
three and a half per cent. But the amount varies much ; for if 
we examine the air expired a short time after a full meal, we 
shall find twice as much carbonic acid gas in the blood that 
there is when the stomach is empty. 
> From what has been stated, then, it appears reasonable to 
conclude, that the formation of carbonic acid gas takes place 
over the whole body. This theory also admirably explains the 
manner in which animal heat is formed ; for wherever this gas 
is produced, there likewise must heat be generated. For to 
suppose that animal heat is maintained by the combustion arising 
from the union of oxygen and carbon in the lungs, would be to 
suppose these organs a furnace, where the refuse and stubble of 
the system w'ere consumed, and, by the same means, with a com¬ 
mendable economy, animal heat produced. 
The experiments of Dr. Stevens* have thrown some light on 
this subject. His views accord with those who reject the idea 
of the formation of carbonic acid as taking place wholly in the 
lungs. One of the most striking facts which the Doctor has 
brought into notice, is the powerful attraction which exists be¬ 
tween oxygen and carbonic acid. It was so fully admitted 
amongst chemists that carbon in carbonic acid is united with its 
maximum dose of oxygen, that the idea of attraction between 
carbonic acid and oxygen, w^as rejected by every one : the fact, 
however, is clearly proved by the experiments of Dr. Stevens. 
If a receiver filled with carbonic acid, and closed by a piece of 
bladder firmly tied over it, is exposed to the atmospheric air, 
the carbonic acid, notwithstanding its superior specific gravity, 
rapidly escapes, and does so without the exchange of an equiva¬ 
lent portion of atmospheric air ; the bladder is, consequently, 
forcibly depressed into the receiver. If the converse of this ex¬ 
periment be tried, and the receiver, containing atmospheric air, 
be tied over with a piece of bladder or thin leather, and then 
be immersed in carbonic acid, this gas will so abundantly pene¬ 
trate the membrane, and enter the receiver, as to endanger its 
bursting. 
There are many phenomena connected with the blood, that 
Dr. Stevens has noticed, which it wmuld be impossible to notice 
here; but in order to understand the application of the attrac¬ 
tion which we have just pointed out between oxygen and carbo¬ 
nic acid, it will be necessary to be aware of the following facts :— 
Ail acids impart' a dark colour to the blood. With respect to 
most acids, this colour remains, although the added acid be 
* Stevens on the Blood. 
