THE STABILITY OF PROTEIN SOLUTIONS 253 



4. When some of the water of a gelatin chloride or Na gelatin- 

 ate solution is replaced by ethyl alcohol, the mechanism which 

 keeps the gelatin in solution is not changed, but when we continue 

 increasing the relative amount of alcohol in the solution a critical 

 point is reached where the amount of salt required for the 

 precipitation changes abruptly. 1 We must conclude that the 

 mechanism by which the gelatin is held in solution changes at or 

 near this critical alcohol concentration. It is possible to show 

 that when the amount of alcohol exceeds the critical limit the 

 forces guaranteeing the stability of the solution of gelatin in the 

 alcohol-water mixture are the forces resulting from a Donnan 

 equilibrium. 



We will first show that such a critical point exists for the ratio 

 water: alcohol. Ten per cent solutions of gelatin chloride of 

 pH 3.0 or of Na gelatinate of pH 10.0 were prepared. Five 

 cubic centimeters of such a solution were first warmed to liquefy 

 the gelatin and then while still warm they were diluted with 45 c.c. 

 of a mixture of alcohol and water; the relative quantity of alcohol 

 and water in the 45 c.c. varying. Ten cubic centimeters of these 

 1 per cent solutions of gelatin chloride or Na gelatinate in water- 

 alcohol were titrated with a solution of a neutral salt, (NH 4 ) 2 SO 4 , 

 NaCl, and CaCl 2 , at 20C. until precipitation occurred. It was 

 noticeable that while it was not possible to precipitate the gelatin 

 at all with 2% M CaCl 2 or 5 M NaCl and only with compara- 

 tively high concentrations of (NH 4 ) 2 SO4 as long as the concentra- 

 tion of alcohol did not exceed a certain critical value, when this 

 critical limit was exceeded traces of these salts sufficed for pre- 

 cipitation. This is illustrated in Tables XLVI and XLVII. 

 When the solution contained no alcohol, i.e., when 45 c.c. of H 2 O 

 were added to 5 c.c. of the 10 per cent solution of gelatin chloride 

 of pH 3.0, 7.1 c.c. of 2 M (NH 4 ) 2 SO 4 were required to cause pre- 

 cipitation (Table XLVI) in 10 c.c. of the 1 per cent gelatin chloride 

 solution, and the quantity of (NH 4 ) 2 SO4 required increased at 

 first the more H 2 O was replaced by alcohol. When the 45 c.c. of 

 liquid added to the 5 c.c. of 10 per cent gelatin solution consisted 

 of 18.75 c.c. of water and 26.25 c.c. of ethyl alcohol, 17.8 c.c. of 2 M 

 (NH 4 ) 2 SO 4 were required to cause precipitation in 10 c.c. of the 

 gelatin-alcohol-water mixture, but if now the proportion of 



1 The rest of this chapter is based on experiments not yet published. 



