THEORY OF RESPIRATION. 255 



Arterial blood, when drawn from the body, is soon 

 altered ; its florid color becomes dark red. The florid 

 blood, which owes its color to the globules, becomes 

 dark by the action of carbonic acid, and this change of 

 color affects the globules, for florid blood absorbs a 

 number of gases which do not dissolve in the fluid part 

 of the blood when separated from the globules. It is 

 evident, therefore, that the globules have the power of 

 combining with gases. 



The globules of the blood change their color in dif- 

 ferent gases : and this change may be owing either to a 

 combination or to a decomposition. 



Sulphuretted hydrogen turns them blackish green and 

 finally black ; and the original red color cannot, in this 

 case, be restored by contact with oxygen. Here a 

 decomposition has obviously taken place. 



The globules darkened by carbonic acid become 

 again florid in oxygen, with disengagement of carbonic 

 acid. The same thing takes place in nitrous oxide. It 

 is clear that they have here undergone no decomposi- 

 tion, and, consequently, they possess the power of 

 combining with gases, while the compound they form 

 with carbonic acid is destroyed by oxygen. When left 

 to themselves, out of the body, the compound formed 

 with oxygen again becomes dark, but does not recover 

 its florid color a second time by the action of oxygen. 



The globules of the blood contain a compound of 

 iron. From the never-failing presence of iron in red 

 blood, we must conclude, that it is unquestionably neces- 



