290 PHYSICAL PROPERTIES 



This can only be interpreted by supposing that the condition of 

 the bulk of the solution outside the casein particles differs, at 

 any rate for some portion of the time occupied in solution, from 

 the condition of that portion of the solvent which has actually 

 penetrated the protein particles. We know that any substance 

 such as alcohol, which reduces the tension of a solid-water inter- 

 face, tends to become concentrated at such an interface (102). 

 Hence at the surfaces of the casein particles the concentration 

 of alcohol will be greater than in the bulk of the fluid. Suppose 

 that the concentration of alcohol in the bulk of the fluid be such 

 that at equilibrium the proportion of double molecules is 50 

 per cent. If, now, the concentration of alcohol at the surface 

 of the casein particles be such as to lead to the formation of 60 

 per cent of double molecules, then out of every thirty molecules 

 coming off from the interior surfaces of the casein particles 

 eighteen will be of the double type and only twelve of the single 

 type. As these come out into the bulk of the liquid, equilibrium 

 will tend to be re-established, resulting in the formation of sixteen 

 double and sixteen single molecules, with an increase in the total 

 number of molecules to thirty-two. This equilibrium may be 

 supposed to be slowly established, and meanwhile the rate of 

 solution of the casein has fallen very low; that is, the fall in 

 concentration or concentration gradient from within the casein 

 particles to the bulk of the fluid outside is very small. By this 

 time, however, as a result of depolymerization, the molecular 

 concentration of casein in the outer fluid has actually become 

 greater than that in the saturated solution which fills the interior 

 spaces of the casein particles. Thus the concentration gradient 

 has become negative, and as the more concentrated casein solu- 

 tion diffuses back into the less concentrated but nevertheless 

 saturated solution filling the interspaces of the sponge-like par- 

 ticles of casein, the excess of casein must be reprecipitated. When 

 the concentration of alcohol is sufficiently great, so that the 

 molecular condition of the casein is practically the same within 

 and without the particles, a negative concentration gradient can 

 never arise and the curve of solution reassumes the normal form, 

 representing now, however, the curve of solution of double mole- 

 cules instead of single molecules as at lower concentrations of 

 alcohol. 



This interpretation of the above facts finds further confirraa- 



