66 SOAPS AND PROTEINS 



mutually soluble or not. (.Liquid) water and (liquid) oil yield 

 only lyophobic colloids (suspension colloids in the old ter- 

 minology) because the two phases arc mutually insoluble, but 

 (liquid) water and soap (whether liquid or solid) yield lyophilic 

 colloids (emulsion colloids in the old terminology) because their 

 mutual solubility is high. 



The importance of mutual solubility for the understanding 

 of some of the phenomena characteristic of colloids was drawn 

 upon some years ago by W. B. HARDY. 1 HARDY used the concept 

 of mutual solution to explain the physical phenomena encountered 

 in the gelation of protein-water-salt mixtures, but owing to the 

 objection that the phases did not show the constant chemical 

 composition demanded by theory, this important idea seems to 

 have been largely dropped. For reasons which become apparent 

 as we proceed, this objection is not valid, and it seems possible 

 to make a fairly inclusive analysis of what we mean by hydro- 

 philic colloids and the changes in their states, as soon as we add 

 to the concepts of mutual dispersion in degrees coarser than 

 molecular and mutual solubility of the phases a third important 

 point, namely, that of the enormous increase in viscosity observed 

 whenever two liquids or a liquid and a solid, themselves possessed 

 of low viscosity, are subdivided into each other. 



To make the last of these points clear, it is only necessary 

 to introduce the example of WOLFGANG OSTWALD, of water and 

 dry sand, and the observations of J. FRIEDLANDER and V. ROTH- 

 MUND on the viscosity of mutually soluble liquids in the zone 

 of their critical temperature. While dry sand " runs " easily 

 and the viscosity of pure water is relatively low, wet sand may 

 be readily molded and hold its shape. The example of the mutu- 

 ally soluble system phenol/water (which is considered particularly 

 apt in the matter of understanding the colloid behavior of soap/ 

 water systems) is shown in Fig. 47. The bottle on the extreme 

 left contains only phenol (which at 18 C. is a crystalline mass 

 like any " pure " soap at a proper temperature). The succeed- 

 ing bottles contain the same weight of phenol, plus gradually 

 increasing amounts of water. As more and more water is added 

 the phenol fails to crystallize; up to and including the sixth bottle 

 from the left, only." solutions " are obtained, but these are solu- 



1 W. B. HARDY: Jour. Physioi., 24, 158 (1899); Zeitschr. f. physik. Chem., 

 33, 326 (1900). 



