GELATINIZATION AND COAGULATION 301 



tion of this fact, that there can be no sharp separation into phases 

 within an agar, and, presumably, within a gelatin jelly. They 

 believe that a sharp distinction should be drawn between the 

 coagulation of a protein by dehydrating agents and the gelatin- 

 ization of its solution, since in the former case a sharp separation 

 into two phases occurs while in the latter it does not. The 

 experimental basis of this objection to Hardy's view is, however, 

 not altogether a sufficient one, since if any two parts of a chemical 

 system, which are of more than molecular thickness, are sepa- 

 rated by a surface, they constitute, in the sense of the phase-rule, 

 separate phases. The observation of Pauli and Rona shows, 

 however, that, at least in agar jellies, if separate phases exist 

 they are not of definite or constant composition. 



The fact that in protein jellies which do consist of two phases 

 the phases are not of constant composition is also very clearly 

 revealed by Hardy's own results, of which a description follows: 



The manner in which the structure of an insoluble gel is built 

 up can, according to Hardy, be readily observed in the ternary 

 mixture alcohol, gelatin, and water. If 13.5 grams of gelatin 

 are mixed with 50 cc. of water and 50 cc. of absolute alcohol, 

 a mixture is formed which is optically homogeneous at 17 to 20 

 degrees but which separates into two phases at temperatures 

 below this. "As the temperature falls below the limit a clouding 

 occurs which I find to be due to the appearance of fluid droplets 

 which gradually increase in size until they measure 3 ju. On 

 cooling further, these fluid droplets become solid and they begin 

 to adhere to one another.* The framework is therefore an open 

 structure which holds the fluid phase in its interstices." " When 

 once formed the phases have considerable stability. If the drop- 

 lets are composed of a solid solution one may, by the addition 

 of water, cause them to increase to relatively vast dimensions 

 without their being destroyed; as they increase in size their 

 refractive index approximates more and more to that of the 

 external phase until t ley are finally lost sight of. The addition 

 of alcohol, however, once more brings them into view and causes 

 them to shrink. Owing to this stability, once a configuration 

 has been established, one has to far overstep the conditions of 

 its formation in order to destroy it. This would account for 



* The formation of similar droplets has been observed by Pauli and Rona 

 (72) in the coagulation of gelatin by salts. 



