178 PROTOPLASMIC ACTION AND NERVOUS ACTION 



different solubilities of the Na-oleate and the Ca-oleate 

 in the two phases, the former being soluble in water 

 but not in oil, the latter soluble in oil but not in water. 

 The surface-tension conditions at the interface are 

 accordingly oppositely affected by the two salts, with 

 correspondingly opposite effects on the state of disper- 

 sion; at a certain ratio of concentrations the two op- 

 posite effects are balanced. Clowes shows that when 

 equal volumes of oil and alkaline salt solution (mixtures of 

 n/io Na OH and n/io CaCy are shaken together, the 

 effect varies according to the proportions of Na and Ca 

 in the solution; when Na is present in excess, the oil is 

 dispersed as droplets in a continuous water phase, while 

 when Ca is in excess the water is dispersed as droplets 

 in a continuous oil phase. A reversal of phase-relations 

 may thus be accomplished in an emulsion system by 

 changing the salt-content; and this conclusion has 

 highly important biological applications, since it bears 

 closely on the problem of the relations between the lipoid 

 and the aqueous components in the living protoplasmic 

 system. Any system in which the oil (organic solvent 

 or lipoid) phase is the continuous one is permeable to 

 oil-soluble substances, but not to water-soluble sub- 

 stances which are oil-insoluble; and the general corre- 

 spondence of this condition with that observed in living 

 protoplasm suggests the possibility that the external 

 layer of the living plasma membrane consists (at least 

 during the greater part of its existence) of a continuous 

 layer of lipoid material, the continuous condition de- 

 pending on the presence of compounds with properties 

 like those of Ca soaps. The importance of Ca to 

 the semi-permeability and water-resisting properties 



