GELATIN 413 



like structure of definite form, hanging together at certain 

 points, forming a network, and Hardy concluded that the soHd 

 phase consists of a solid solution of water in gelatin, and the 

 liquid phase a solution of gelatin in water ; Wo. Ostwald 

 has put forward the idea of a two-phase liquid-liquid system. 

 Procter postulates the existence of a solid solution of the 

 exterior liquid in the colloid in which both constituents are 

 within the range of the molecular attractions of the mass, 

 and Loeb has extended the idea of Procter, At the present 

 time the idea of a fibrillar structure, as advocated by McBain 

 and his co-workers for soaps, is gaining ground, and is especi- 

 ally supported by Bogue in America and Moeller in Germany. 



The question of the structure of the gel is intimately 

 bound up with that of the constitution of the sol. Reference 

 has already been made to phenomena which indicate the 

 existence of a structure in the sol, and it may be mentioned 

 that various investigators have obtained results showing the 

 existence of torsional stress in dilute sols (Schwedoff, Journ. de 

 Physique, 1889, 8, 341 ; Colin, Comptes Rendits, 1893, 116, 1251 ; 

 Garret, Phil. Mag., 1903, 6, 374 ; Rohloff and Shinjo, Physik. 

 Zeitschr., 1908, 8, 442). The particular structure assigned to 

 gelatin will have to account for the fact that there is a gradual 

 and regular and not a sudden change in properties when the 

 transformation of sol into gel, and vice-versa, takes place. 

 The so-called setting and melting points of gelatin are not 

 specific points as they are with crystalloids ; an appreciable 

 time factor enters into their determination and the two points 

 do not coincide. To quote Sheppard and Sweet {Journ. Ind. 

 Eng. Chem., 192 1, 13, 423), " both the melting point and the 

 setting point are more or less arbitrary conceptions, and their 

 determination depends mainly upon standardised experimental 

 conventions." 



In the solutions of gelatin there seems to be present both 

 the sol and gel forms in some kind of equilibrium. It has long 

 been known that whilst dilute solutions (i per cent.) of pure 

 gelatin would gel at low temperatures (10° C), yet above 

 certain temperatures, roughly estimated at about 35° C, 

 gelation would not take place at any concentration, although 

 exceedingly viscous solutions might be obtained. C. R. 

 Smith (Journ. Amer. Chem. Soc., 1919, 41, 146 ; Journ. Ind. 

 Eng. Chem., 1920, 12, 878) has investigated the mutarotation 

 of gelatin and shown that at temperatures above 33° to 35° C. 

 the specific rotation of gelatin has a practically constant value 

 of about -123°, whilst below 15° C. it is practically constant at 

 about -266°. At all temperatures between 35° and 15° the 

 rotation varies between these limits, and Smith draws the 

 conclusion that in aqueous solution gelatin exists in two 



