168 COMPOUNDS OF THE PROTEINS 



ions, for although it is itself insoluble it will drive carbonic acid out of 

 the sparingly soluble calcium carbonate to form a freely soluble casein- 

 ate of calcium. 



If instead of adding water to a mixture of five c.c. of two per cent, 

 casein in formic acid, and two c.c. of saturated cupric chloride, we add 

 alcohol; no coagulation occurs until the mixture changes in color from 

 green to brown, w r hen a Coagulum of cupric caseinate is produced which 

 redissolves on adding water. 



Similar results are obtained when a 2-molecular solution of Cobalt 

 Chloride is employed instead of a saturated solution of cupric chloride. 

 If to five c.c. of a two per cent, solution of casein in formic acid we add 

 two to three c.c. of this cobalt chloride solution, we obtain a blue- 

 purple mixture. On adding water to this mixture it changes in color 

 from blue-purple, through red-purple to clear pink. Not until a pure 

 pink color is obtained does a precipitate result. If, instead of adding 

 water, we add a considerable volume of alcohol (ten volumes) the mix- 

 ture rather abruptly changes to a clear pale blue, and then, but not 

 before, we obtain a coagulum of cobalt caseinate. 



Electronegative casein (i. e., casein dissolved in alkalies) is not 

 precipitated by the salts of the alkalies, although it is readily precipi- 

 tated by salts of the alkaline earths. Electropositive casein (i. e., 

 casein dissolved in acids) is, however, very readily precipitated by salts, 

 and these precipitates are not soluble upon dilution. Thus if two c.c. 

 of tenth normal hydrochloric acid be added to five cubic centimeters of 

 a one per cent, solution of casein in 0.008 N. potassium hydroxide, a 

 clear, acid solution of casein results. The casein is precipitated from 

 this solution by the addition of four drops of a saturated solution of 

 sodium chloride, or by one drop of a saturated solution of ammonium 

 sulphate. This latter precipitate does not dissolve on diluting the 

 mixture to one-sixteenth. 



Casein Formate affords no exception to the rule that salts of casein 

 with acids are precipitable by relatively small concentrations of neutral 

 salts, but the precipitation will only occur 'in the presence of a sufficiency of 

 water. If to five cubic centimeters of a two per cent, solution of casein 

 in formic acid we add a saturated solution of ammonium sulphate, 

 three cubic centimeters of this solution just suffice to produce a coagu- 

 lum, this becomes more abundant on adding water, and redissolves on 

 adding formic acid. If, however, instead of adding three we add two 

 cubic centimeters of the saturated ammonium sulphate solution, a 

 clear solution is obtained. On adding water to this a precipitate results 

 which redissolves on heating and reappears on cooling. 

 Analogous results may be obtained with Ovomucoid. 

 It is clear, therefore, that protein may be thrown out of solution 

 by electrolytes in two grades of hydration, the one of high, the other of 

 very low hydration. The former process is what we have termed 

 Precipitation, the latter we have defined as Coagulation. At grades of 



