352 PHILOSOPHY OF ZOOLOGY. 



Jet! to adopt the second opinion, and to conclude, thai, 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 colour in the lungs, and passes 

 from a dark to a florid red. That this effect is 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 JOHN 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 insufficiency 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 CRAWFORD found the 

 specific heat of venous blood (water 1.000) to be 0.8928, 



