PRECIPITATION AND COAGULATION 129 



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 add- 

 ing water, we add a considerable volume of alcohol (10 volumes) 

 the mixture rather abruptly changes to a clear pale blue and 

 then, but not before, we obtain a coagulum of cobalt caseinate. 



Electronegative casein is not precipitated by the salts of the 

 alkalies, though it is readily precipitated 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 2 cc. of N/IO HCl be 

 added to 5 cc. of a 1 per cent solution of casein in 0.008 N KOH, 

 a clear acid solution of casein results. The casein is precipitated 

 from this 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 is no exception to the other salts of casein with 

 acids, but the precipitation will only occur in the presence of a 

 sufficiency of water. If to 5 cc. of a 2 per cent solution of casein 

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

 3 cc. of this solution just suffice to produce a coagulum; this 

 becomes more abundant on adding water, and redissolves on 

 adding formic acid. If, however, instead of adding 3, we add 

 2 cc. of the saturated ammonium sulphate solution, a clear solu- 

 tion 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 solu- 

 tion by electrolytes in two grades of hj'-dration, 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 hydration intermediate between these 

 extremes the protein may be soluble. Dehydration, partial or 

 complete, leading to resolution or to coagulation, may be induced 

 by heat, by non-electrolytes possessing an affinity for water or 

 by electrolytes.* 



The importance of a high degree of dehydration in the pro- 



* See also Chick and Martin (10). 



