THE STABILITY OF PROTEIN SOLUTIONS 255 



alcohol to water was shifted only slightly in favor of alcohol, 

 namely 17.5 c.c. of water and 27.5 c.c. of alcohol, 0.04 c.c. instead of 

 17.8 c.c. of 2 M (NH 4 ) 2 SO 4 sufficed for precipitation (Table XL VI). 



In the case of NaCl the drop was still more striking. When 

 the 45 c.c. added to the 5 c.c. of 10 per cent gelatin solution con- 

 sisted of 8.75 c.c. of water and 36.25 c.c. of alcohol, it was 

 impossible to cause precipitation in 10 c.c. of the gelatin chloride- 

 alcohol-water mixture with any amount of 5 M NaCl. When, 

 however, the proportion of alcohol was only slightly increased, 

 namely 7.5 c.c. of water and 37.5 c.c. of alcohol, 0.2 c.c. of NaCl 

 sufficed for precipitation. In the case of CaCl2 the critical point 

 was reached when the ratio was 5 c.c. of water and 40 c.c. of 

 alcohol. 



The existence of the critical point can equally well be demon- 

 strated in the case of Na gelatinate as is shown in Table XL VII. 



What interests us is the following fact. The mechanism by 

 which gelatin chloride of pH 3.0 and Na gelatinate of pH 10.0 

 are kept in solution is not altered as long as not too much of the 

 water is replaced by alcohol, since in this case (NH 4 ) 2 SO 4 is 

 always a better precipitant than CaCl 2 for both gelatin chloride 

 and Na gelatinate. When, however, the relative amount of 

 alcohol exceeds a certain critical point, the mechanism by which 

 the particles of gelatin are held in solution changes abruptly as is 

 indicated by two facts; namely, first that the concentration of the 

 salt required for precipitation becomes suddenly very small, and 

 second, that the efficient ion has now the opposite sign of charge to 

 that of the colloidal particle. Thus, in the case of gelatin chloride 

 (Table XL VI), the critical points for CaCl 2 and NaCl are close 

 together while the critical point for (NH 4 ) 2 SO 4 is at a much lower 

 concentration of alcohol. In this case the gelatin ion has a 

 positive charge and the precipitating ion on the alcohol side of 

 the critical point is the anion. In Table XL VII the critical 

 points for (NH 4 ) 2 SO 4 and for NaCl are close together while the 

 critical point for CaCl 2 lies at a much lower concentration of 

 alcohol. In this case the colloidal ion is negatively charged and 

 the precipitating ion of the salt is the cation. The valency effect 

 will be demonstrated more strikingly in a later part of this 

 chapter. 



5. If the precipitating effect of low concentrations of neutral 



