THE MECHANISTIC CONCEPTION 19 



uted to the access of the oxygen of the air during 

 thousands of years. But this action is so slight that 

 it has no real bearing on the oxidations here in 

 question. Equally inactive is the oxidized haemo- 

 globin of the blood, known as oxyhsemoglobin. It 

 has long been clear that to account for the animal 

 oxidations we must seek for oxidizing agents in 

 some cell constituents which get their oxygen from 

 the oxyhsemoglobin of the blood. The difficulty 

 of investigating these oxidizing substances is very 

 great and their exact identity still remains un- 

 revealed. Yet there are many signs that these sub- 

 stances have the nature of unstable peroxides. 

 It is known to chemists that many reactive sub- 

 stances, such as benzaldehyde, combine with the 

 ordinary oxygen of the air to form peroxides. These 

 peroxides can give up part of their oxygen and so 

 effect oxidations which otherwise would not occur. 

 It is reasonable to suppose that the oxidizing enzymes 

 or oxydases of the animal tissues are similar in type 

 to the peroxides just mentioned. 



If this hypothesis were true, one would expect to 

 find a similarity between the types of oxidative change 

 brought about by tissue oxidases and the types in- 

 duced by artificial peroxides. In the course of exper- 

 imental studies carried on by Dakin, a comparison has 

 been made between the action of the simplest arti- 

 ficial peroxide hydrogen peroxide and the action 

 of the living animal tissues. This comparison reveals 

 a singularly close analogy in the two cases. 



