276 PHYSICAL PROPERTIES 



will hold no more of the caseinate. Addition of alkali to the 

 mixture results in the passage of more casein into solution; 

 addition of acid in the throwing of uncombined casein out of 

 solution. Apparently caseinates, of the alkalies at all events, 

 are indefinitely soluble in water, although in practice we are 

 limited in the preparation of strong solutions, not only of the 

 caseinates but of other proteins, by the viscous and glairy nature 

 of such solutions, and the consequent difficulty of ensuring their 

 homogeneous character. Corresponding to this fact we find 

 that it is very difficult to prepare the caseinates of the alkalies 

 in the form of dry powders. I have precipitated lithium casein- 

 ate from 90 per cent alcohol by the addition of ether and en- 

 deavored to dry this product by washing with ether and standing 

 over sulphuric acid at 30 degrees. The result is merely a soft 

 gelatinous mass; on prolonged standing over sulphuric acid 

 (a matter of weeks) it becomes horny; at no definite time can 

 we state that the product has become anhydrous; it simply 

 exhibits a continual change in consistency with the continual 

 withdrawal of water, just as gelatin does. On the other hand, 

 proteins which are insoluble in water and certain other proteins 

 and protein salts are readily rendered anhydrous by treatment 

 with alcohol and can be prepared through this means in the 

 form of dry powders (Cf. Chap. II). The affinity of a pro- 

 tein for water, therefore, is a function which varies, not only 

 with the character of the protein, but with the character of any 

 base or acid which may be combined with it. In all cases it 

 would appear to be of considerable magnitude, however, the 

 difficulty which histologists experience in thoroughly dehy- 

 drating tissues, for example, being eloquent testimony of this 

 fact. 



It has been shown by Noyes and Whitney (61) (10) that under 

 conditions involving no chemical interaction (except the pos- 

 sible formation of "solvates" (Cf. Chap. VI)), the rate of solution 

 of a crystalloid in water is at each moment proportional to the 

 difference between its concentration at that moment and its 

 concentration at saturation of the solvent. They conclude from 

 this that at the boundary of the crystals and the solution there 

 exists a film of solution, which is always saturated and, for a 

 constant rate of stirring, of constant thickness. The velocity 

 of solution is thus determined by the rate of diffusion of the 



