66 THE NATURE OF LIFE 



butter, to a liquid state resembling cream 

 and vice versa. 



There is no evident reason why proto- 

 plasm cannot contain at the same time 

 jelly-like materials and emulsions. The 

 question must be left to future investiga- 

 tion. It is, however, perfectly clear that 

 the properties of protoplasm are to a 

 large extent determined by the fact that 

 it is largely composed of colloids, so that 

 some biologists speak of organisms as 

 ''colloidal machines." It may be added 

 that it is possible to construct non-living 

 colloidal systems which to a great extent 

 imitate the properties of living matter. 

 Thus protein solutions are affected by 

 heat, heavy metals, and various coagu- 

 lants in much the same way as proto- 

 plasm. 



It is evident, therefore, that combus- 

 tion, as well as other metabolic processes 

 may be regarded as taking place in an 

 aqueous medium. 



In conclusion we may emphasize the 

 fact that destructive metabolism oc- 



