32 DYNAMICS OF LIVING MATTER 



Biitschli * has expressed a view concerning the structure of living 

 protoplasm which is more probably correct than the assumption of the 

 net structure. According to him, living protoplasm has the structure 

 of a microscopic emulsion. His view is shared by E. B. Wilson and 

 other authors, while it is accepted, but not unconditionally, by Hardy 

 and Pauli. Biitschli's conception, however, seems to harmonize with 

 a great many facts, and therefore we may discuss some features of the 

 theory of emulsions. Emulsion and foam are, according to Quincke, 

 only different names for the same (diphasic) physical system. An 

 emulsion like milk consists of a large number of spherical droplets of 

 fat, which are distributed in a watery liquid. f We speak of a foam in 

 the case of an emulsion of a gas in a liquid. The foam in a soap solu- 

 tion consists of spherical masses of air which are distributed in a watery 

 liquid. Foams and emulsions have therefore the same physical struc- 

 ture. The peculiarity of emulsions and foams which interests us in 

 this connection is their durability, and the theory of foams is concerned 

 with this side of the problem. According to Lord Rayleigh, an ab- 

 solutely pure liquid cannot form a durable foam.J When a gas bubble 

 rises in pure water it is surrounded by a liquid film which, however, 

 does not possess any durability, and therefore bursts. If, however, 

 the water is contaminated by another substance, these liquid films 

 become more durable. This influence upon the durability varies with 

 the nature of the contaminating substance. It is known that the addi- 

 tion of small quantities of colloidal material, e.g. soap, saponin, a solu- 

 tion of gelatine, is capable of making the liquid films very durable. 



This influence of the contamination is due to its effect upo'n surface 

 tension. Experiments have shown that those substances which make 

 these liquid films more durable decrease the surface tension which ex- 

 ists at the limit between water and the emulsified substance. In con- 

 sequence of this latter fact, these substances have a tendency to collect 

 at the common surface between water and air, or whatever the emulsi- 

 fied substance may be. The surface acts like a trap on such particles, 

 inasmuch as it requires an outside force to bring them back to the 

 interior, if they have once collected at the surface. The consequence 

 is that a film of this contaminating material is formed at the surface, 

 between the water and the oil droplets, or whatever the substance held in 

 emulsion may be. In the case of oil emulsions in water, the contami- 

 nating substance is a trace of soap formed through the hydrolysis of fat 



* Biitschli, Untersuchungcn uber tnicroscopische Schaume und das Protoplasma, Leipzig, 

 1892. 



1 Quincke, Pfiuger's Archiv, Vol. 19, p. 129, 1879. 



| Lord Rayleigh, Scientific Papers, Vol. 3, p. 351, 1902. 



F. G. Donnan, Ze itsch. fur physikal. Chemie, Vol. 31, p. 42, 1899. 



