352 PHILOSOPHY OF ZOOLOGY. 
led to adopt the second opinion, and to conclude, that, in 
the process of aeration, the blood parts with a portion of 
carbon, and that its carbon combines with the oxygen of 
the air inspired, and passes into the state of carbonic acid. 
The changes which the blood itself undergoes, support the 
conclusion. 
In man and the red-blooded animals, the pulmonic or 
venous blood changes its colowr m the lungs, and passes 
from a dark toa florid red. That this effect 1s produced 
by the formation of the carbonic acid, is demonstrated from 
the circumstance, that pulmonic blood exposed to the air 
of the atmosphere acquires the florid colour, that part of its 
oxygen disappears, and an equivalent bulk of carbonic acid 
occupies its place. When the blood is exposed to azotic 
gas, no change takes place, but when oxygen is substituted, 
the red colour appears, a portion of the gas is consumed, 
and a similar quantity of carbonic acid formed. 
But not only is the colour of the blood changed, but 
likewise its density. The specific gravity of venous blood 
obtained from a sheep, was found, by Dr Joun Davy, to 
be 1051, while the aerated or arterial blood was 1049. It 
would perhaps be rash to found any reasoning with regard 
to the proof that carbon is emitted, from this change of den- 
sity in the blood, owing to the sufficiency of the experi- 
ments which have been performed, to serve as standards of 
comparison; at the same time, it would be easy to deter- 
mine the difference in density between the venous and 
aerated blood, if the change consists merely in the emission 
of carbonic acid, and the absorption of a corresponding 
bulk of oxygen. 
Besides the change of colour and density experienced by 
the blood in the process of aeration, an alteration likewise 
takes place it its specific heat. Dr Craw¥orp found the 
specific heat of venous blood (water 1.000) to be 0.8928, 
