THEORY OF RESPIRATION. 267 



Arterial blood, when drawn from the body, is 

 soon altered ; its florid colour becomes dark red. 

 The florid blood, which owes its colour to the glo- 

 bules, becomes dark by the action of carbonic acid, 

 and this change of colour affects the globules, for 

 florid blood absorbs a number of gases which do not 

 dissolve in the fluid part of the blood when sepa- 

 rated from the globules. It is evident, therefore, that 

 the globules have the poicer of eomhining with gases. 



The globules of the blood change their colour in 

 different 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 colour can- 

 not, 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 car- 

 bonic acid. The same thing takes place in nitrous 

 oxide. It is clear that they have here undergone 

 no decomposition, and, consequently, they possess 

 the power of combining with gases, lehile the coin- 

 pound they form tvith carbonic acid is destroyed hy 

 Ocvygen. When left to themselves, out of the body, 

 the compound formed with oxygen again becomes 

 dark, but does not recover its florid colour a second 

 time by the action of oxygen. 



The globules of the blood contain a compound of 

 iron. From the never-fkiling presence of iron in 

 red blood, we must conclude, that it is unquestion- 



