336 PHYSICAL PROPERTIES 



The significance of these results and of results which I have 

 obtained with ovomucoid is commented upon in Chap. X. It may 

 further be remarked here that the observed depressions could not 

 have been due to impurities associated with the casein for the 

 following reasons: 



(1) Keeping the concentration of base constant it is evident 

 that increasing the quantity of casein dissolved in it in the pro- 

 portion of 8 to 5 does not alter the observed depression in any 

 appreciable degree, whereas, if this were due to impurities associ- 

 ated with the casein, it should result in a proportionate increase 

 in the observed depression. 



(2) Keeping the concentration of casein constant and increasing 

 the concentration of base bound by it in the proportion of 8 to 5 

 results in a proportionate increase in the observed depression. 



The cryoscopic depression is, therefore, obviously conditioned 

 primarily by the combined alkali. This might be interpreted to 

 indicate that the depression is in reality due to the base and not to 

 the protein. This, however, is not the case, since, as we have seen 

 (Chaps. V and IX), these solutions were respectively neutral to 

 litmus and to phenolphthalein and therefore contained no, or only 

 a trace of, free base. Nor were the depressions due to inorganic 

 ions, since, as we have seen (Chap. VIII), such solutions contain no 

 inorganic ions. As I have explained in Chap. X, the observed 

 depressions must be attributed primarily to protein ions, each 

 equivalent of combined base yielding the same number of protein 

 ions, derived through the splitting of successive N.HOC 

 bonds. 



One fact should be especially commented upon here, and that 

 is that the observed depressions in the two sets of solutions ex- 

 amined stood in direct proportion to the concentration of combined 

 base. If this were so for other concentrations, then at zero con- 

 centration of combined base, if casein were soluble under such 

 conditions, the freezing-point depression due to dissolved casein 

 would be zero. In other words the possibility is indicated that 

 base- and acid-free protein may exert an immeasurably small 

 osmotic pressure. I have elsewhere attributed this to polymeri- 

 zation of the protein as the uncombined protein is set free (104) 

 (106). 



6. The Osmotic Pressure of Proteins in Solution. The 

 direct determination of the osmotic pressure of protein solutions is 



