12 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



the properties of the proteins of which the pro- 

 toplasm mainly consists. The proteins are sub- 

 stances of a colloid nature, that is to say, substances 

 which do not permeate animal membranes at all 

 or only with the utmost sluggishness. The colloid 

 character of the proteins imparts to the living pro- 

 toplasm a high degree of physical stability and 

 enables unlike chemical processes to be carried on 

 side by side in the living cell, without risk of inter- 

 ference between the minute foci of action. If we 

 study the living protoplasm with the microscope, 

 we can see features of structure which speak for the 

 view first expressed by Biitschli that protoplasm 

 has a foamlike structure. There are reasons for 

 believing that there is a close physical analogy 

 between foams and emulsions, and the colloids lend 

 themselves readily to existence in both these states 

 and to the solid and liquid conditions on which 

 emulsions and foams depend. An extremely sig- 

 nificant feature of the colloid property of undergoing 

 minute subdivision is the fact that the very extended 

 surface so generated is a powerful aid to chemical 

 activity. The intense chemical activities of living 

 protoplasm are doubtless made possible by this 

 accelerating action of widely extended yet contig- 

 uous surface. Chemistry furnishes us with a gross 

 analogy which may be cited in this connection. If 

 we pass ammonia gas through a porcelain tube heated 

 to 1000 C., there occurs a moderate decomposition 

 of the gas. But if we repeat the experiment, using 



