GELATION OF THE NATURAL EMULSOIDS 67 



in a larger value of the factor K in v. Weimarn's formula, or 

 a slower velocity of crystallisation from toluene solution. 



Considering the general case of a substance of very high 

 molecular weight and low diffusivity. Let this be treated 

 with a liquid such that the solubility, L, shall neither be 

 too great in the cold, nor increase too rapidly on heating. 

 Then, on applying heat, the process will be reversible and 

 the Noyes-Nernst formula for the velocity of solution, 



V=—S(L — C) will hold. Since L is not very large and D is 



very small, the velocity of solution will also be very small 

 and the sol stage will persist. Further heating will lead 

 in time to the breaking up of the aggregated particles of 

 the sol and an approach to true solution. If the stage of 

 single molecules is reached this may be regarded as a true 

 solution, though the diffusion constant will be that of a sol. 

 Notwithstanding the considerable size of the molecules the 

 system may be considered homogeneous and v. Weimarn's 

 formula applied. On cooling, with reduction of L, aggregation 



will commence, but the velocity of crystallisation, V~-^S(C—L), 



will be a minimum so that the diminution in L will gradually 

 give a larger and larger P and a still greater value of 



j-. Actually -jj- = — Ky. The velocity of crystallisation will 



■Lt (IJL, JL12 



also increase. And if, when spontaneous crystallisation takes 

 place, N has become sufficiently large, a gel will be formed. 

 Gelation will therefore be dependent on the concentration, solu- 

 bility, and the factor K, which represents the degree of associa- 

 tion and viscosity of the sol. Consequently such a sol, of which 

 the association had been diminished by long heating, would be 

 less likely to gelate on cooling than one which had been freshly 

 solated. 



It would be somewhat remarkable if such substances 

 should not occur, and the reversible emulsoids appear to fulfil 

 these conditions completely. They form sols of considerable 

 concentration, viscosity and aggregation, and have very low 

 diffusion constants, although little is known about their true 

 solubilities. Heating at constant temperature decreases their 

 viscosity while cooling increases it — changes which are now 

 seen to be in complete accordance with Einstein's formula. 



