304 PHYSICAL PROPERTIES 



have undergone no measure of coagulation also possess the type 

 of structure outlined by Hardy, but that owing to its fineness the 

 details of this structure are not visible. 



The existence of a structure in jellies formed by the solution 

 of gelatin in water is also objectively demonstrated by the ob- 

 servation of Liesegang (51) that when silver nitrate diffuses 

 into gelatin which is impregnated with potassium bichromate, 

 the precipitation of insoluble silver bichromate does not occur 

 indifferently in all parts of the area of diffusion but in concentric 

 circles. It has also been shown by Rohonyi (91) that when 

 thin films of gelatin are frozen the ice crystals are formed in 

 concentric rings. It is difficult to clearly conceive any mecha- 

 nism which would permit this in a perfectly homogeneous medium. 

 The theory that crystallization is inhibited by the gelatin until 

 a certain degree of super-saturation is attained might account 

 for failure of precipitation or crystallization at certain points, 

 but, provided the jelly is strictly homogeneous and structureless 

 it fails to account for its appearance at other points. That the 

 distinction between gelation and coagulation is merely a dis- 

 tinction of degree and not of kind has been shown by Buglia (12). 



The experiments of Hardy show that on adding water to the 

 system alcohol-water-gelatin, the gelatin-rich phase progressively 

 imbibes water until it passes by a series of insensible transitions 

 into a solution of gelatin. Having regard to this fact and to the 

 probability, which will be indicated in the following chapter, that 

 the structure of a jelly of uncoagulated protein is merely the con- 

 tinuation of a structure which pre-exists in the solutions which 

 become gelatinized, it appears highly probable that in uncoagulated 

 protein jellies the structure is of molecular dimensions, so that the 

 constituents of the jelly are not separated from one another by any 

 definite interface. This conception does not in the least militate 

 against the view that the structure of a coagulated or partially 

 coagulated jelly merely results from the coarsening of a pre-exist- 

 ing molecular structure. It is probable that the coagulation of a 

 protein solution, without passing through the intermediate stage 

 of gelatinization, is similarly accompanied and possibly (from 

 a mechanical standpoint) accomplished by the coarsening of a 

 pre-existing molecular structure. 



4. The Coagulation of Proteins by Heat, Light and Hydro- 

 static Pressure. — Several aspects of the phenomenon of heat- 



