CHEMICAL CHANGES IN LIVING MATTER FERMENTS 157 



hitherto proved impossible to obtain any preparation of a ferment which 

 could be regarded as a pure substance. The difficulty in their isolation 

 is increased by the fact that all of them are colloidal or semi-colloidal in 

 character, and present therefore the tendency common to all colloids of 

 adhering to other colloidal matter as well as to surfaces such as those pre- 

 sented by a precipitate. A common method of isolating, or rather obtaining 

 a concentrated preparation of a ferment, is to produce in its solution an inert 

 precipitate such as cholesterin or calcium phosphate. The ferment is 

 carried down on the precipitate and may be obtained in solution on washing 

 the precipitate with water. A further difficulty in their preparation lies 

 in the unstable character of many members of the group. Although they 

 are not coagulated by alcohol, they are nevertheless gradually changed, so 

 that every act of precipitation of a ferment tends to rob it of some of its 

 powers, i.e. of the only characteristic by which we can establish its identity. 

 Of these ferments a large number have already been described as taking 

 part in the ordinary chemical processes of life. So wide is their dominion 

 in cell chemistry that many physiologists have thought that the whole of 

 life is really a continual series of ferment actions. The following list repre- 

 sents some of the ferments whose existence has been definitely established 

 in the animal body. The greater part of them are involved in the processes 

 of digestion in the alimentary canal. The preponderance however of 

 digestive ferments in the list is due to the fact that we know more about 

 digestion than about the other chemical processes taking place within the 

 cells of the body. 





LIST OF FERMENTS. 





