DIRECT METHOD OF PRECIPITATION 69 



the same relative proportion of base to protein as the pre-existing 

 compound in aqueous solution. Thus Van Slyke and Hart (52) 

 employed alcohol to precipitate calcium caseinate, and they 

 found that the precipitate from solutions neutral to litmus con- 

 tained exactly the quantity of calcium (uncombined with hydro- 

 chloric acid) which corresponds with the quantity of calcium 

 hydrate which casein will neutralize to litmus while in aqueous 

 solution. Similarly the precipitate from solutions neutral to 

 phenolphthalein contained the calculated quantity of calcium. 

 As we shall see in considering the electrochemical phenomena 

 which accompany the coagulation of the caseinates by alcohol, 

 coagulation of these salts through the addition of alcohol to their 

 aqueous solutions is preceded by a profound decrease in their 

 degree of dissociation (43); hence the caseinate is precipitated 

 in combination with the proportion of base which was bound 

 up by it in its aqueous solution, because, immediately preceding 

 precipitation, the combined base is bound up in an undissociated 

 molecule. 



It has also been shown that serum-globulin is precipitated 

 from serum by alcohol in the form of a salt with a base, and not 

 in the form of the free protein, since the mixed proteins which 

 may thus be precipitated are completely soluble in distilled water, 

 and this solution yields a precipitate on passing CO2 through it 

 (42). 



It would not be safe to assume, however, without special in- 

 vestigation, that alcohol precipitates all protein salts without 

 alteration of the relative proportion of base or acid to protein 

 which subsists in these compounds as they exist in aqueous 

 solution. So far this can only be positively affirmed for the 

 "neutral" and "basic" caseinates, that is, the caseinates the 

 solutions of which are respectively neutral to litmus and to 

 phenolphthalein. 



The influence of other coagulating agents upon the composition 

 of the protein salts which they precipitate is even more uncertain 

 than that of alcohol, and in one case at all events, namely that 

 of the precipitation of sodium caseinate from solutions containing 

 a considerable excess of sodium hydrate by means of ammonium 

 sulphate, we are in possession of definite evidence that the act 

 of coagulation is accompanied by a change in the composition 

 of the protein salts. 



