PROTOPLASMIC STRUCTURE 71 



so concentrated — with more than 90 per cent of oil as a 

 disperse phase in some of the emulsions prepared by 

 Pickering^ — that the whole mass has a jelly-like con- 

 sistency. The differences of opinion as to whether 

 living protoplasm belongs to the ''sol" or ''gel" type 

 are thus seen to be unimportant, since transitions between 

 these states are continuous, and in fact many forms of pro- 

 toplasm exhibit a liquid consistency at one stage (or under 

 certain conditions) and a solid consistency at another.^ 



In a foam of air and soap solution the individual 

 bubbles do not coalesce, although the intervening films 

 may be extremely thin; evidently the structural stability 

 of the system as a whole is determined by the properties 

 of the films. If we break down these films, mechanically 

 or otherwise, the whole foam structure collapses. The 

 stability of emulsions of oil in aqueous media is similarly 

 conditioned; in this case the coalescence of the separate 

 oil droplets is prevented by interfacial films of soap or 

 other material, and such an emulsion can be also de- 

 stroyed (de-emulsified) by altering the material compos- 

 ing the films, e.g., by adding strong acid if soap is the 

 emulsifying material. Such facts lead to the general 

 question of the conditions determining the stability of a 

 foam structure or emulsion. 



Two chief conditions for the persistence of an air 

 foam are that the layer of solution separating the 

 adjacent bubbles should have (i) a low surface-tension 

 and (2) a high viscosity. A low surface-tension is 

 favorable because the tangentially acting forces tending 



^ Pickering, Journal of the Chemical Society, XCI (1907), 2001. 



* Cf . Bayliss' recent paper in Proceedings of the Royal Society, B, 

 XCI (1920), 196. 



