Some Physical Properties of Protoplasm 251 



sodium diminishes it, and magnesium has no effect at all." The 

 following series was obtained: 



Ca > Sr > Mg > K > Li > Na . 



This is in keeping with the known dispersing effect of sodium and 

 the aggregating effect of calcium on protoplasm. 



Theories on the elasticity of gels have been numerous, one of the 

 oldest being based on surface tension. It was maintained that the 

 elastic qualities of protoplasm are due to surface forces, those same 

 forces which cause a free liquid droplet to round up into a sphere. 

 This is very unlikely in protoplasm, or in any gel, for the elasticity 

 of a glutinous mass like protoplasm or gelatin is not restricted to 

 the surface. This is obvious when a jelly is torn. Furthermore, the 

 elasticity of protoplasm, measured in terms of extensibility, far 

 exceeds that which could be accounted for on the basis of surface 

 tension. When, as is usually true, the surface layer of protoplasm 

 proves to be more elastic than the interior, this is due not to surface 

 tension, but to a denser knitting of the fibrous molecules at the 

 surface, i. e., to the same structural features which are responsible 

 for elasticity throughout the living substance. 



Change in surface tension entered into a theory of the structure 

 of elastic gels. It was assumed that gelatin is a fine emulsion, and 

 that the physical forces responsible for elastic qualities of gelatin 

 resided in the interface between the dispersion medium and the 

 dispersed phase. By a neat mathematical analysis Hatschek^" 

 demonstrated that surface tension cannot possibly be responsible 

 for the elastic qualities of jellies. If the interfacial tension between 

 two liquids is responsible for the elasticity of the system, then the 

 magnitude of this tension is dependent upon the amount of surface. 

 Consequently, the product of the increase in surface, due to stretch- 

 ing, and a constant representing the degree of interfacial tension and 

 extent of surface, must equal the work done in producing elongation. 

 If stress is plotted against increase in length, the stress, in the 

 theoretical curve, increases until elongation reaches two and a half 

 times the original length, and then decreases, which is experimentally 

 untrue. If the foregoing theoretical curve, plotted on the assump- 

 tion that elastic qualities reside in surface forces, is compared with 

 experimental curves of elongations of gelatin and rubber, the two 

 are found to differ greatly in general character. Obviously, elasticity 



'Rheology 2. 263-270. 1931. 

 '"Trans. Faraday Soc, 12, 17. 1917. 



