PROPERTIES OF GELATINE AND GLUE 209 



tension of gels, and which therefore hinder imbibition. The 

 effect on swelling is as follows : 



Sodium-sulphate > tartrate > citrate > acetate ; > alcohol > 

 glucose > cane sugar ; (water) chlorides-potassium < sodium 

 < ammonium ; < sodium-chlorate < nitrate < bromide < 

 iodide < thiocyanate < urea. 



As the amount of compression will depend upon the 

 amount of substance, we expect and find that the effect 

 is usually additive, and that suitable mixtures of substances 

 having an effect in the opposite sense will produce no change. 



The interpretation of lyotrope influence is of course 

 somewhat speculative, but considered as a surface pheno- 

 menon, the surface specific of the molecules and ions of the 

 lyotrope substance must be one of the factors involved. 

 One naturally also connects the effect with solubility and 

 the tendency to form hydrates in solution, the zones of com- 

 pression being zones of orientation and of electro-chemical 

 attraction. The hydrate theory of solution again affords 

 an instructive commentary. The fact that, broadly speaking, 

 the polyvalent anions and the monovalent anions also group 

 themselves together, suggests that electrical forces are at 

 work, and the order of effect of monovalent anions almost 

 suggests that what are called " residual valencies " are in 

 operation. It is difficult to resist the conclusion that in the 

 lyotrope influence, in the crystallizing of salts, and in the 

 formation of a gel, we have zones of compression and orienta- 

 tion which are manifestations of the same forces surface 

 and electrical ; the chief differences in the case of gelatine 

 being that the zones are larger and that the electrical effect 

 is perhaps of less definite magnitude. 



However these things may be, the fact of water com- 

 pression determines the rigidity of the gel, and the changes in 

 this compression of the continuous phase determine the 

 surface tension resultant which hinders swelling, and which 

 is one of the two main factors fixing both the rate at which 

 gelatine swells in water, and the final volume attained by 

 the gel. 



M 



