EDWIN JOSEPH COHN 719 



Loeb's observation suggests that the casein that persists at a neutral 

 reaction is largely combined with divalent bases. We have accord- 

 ingly removed and discarded this precipitate, either by centrifugation 

 (in a Sharpless centrifuge at nearly 30,000 revolutions a minute) or 

 by filtration through filter paper pulp. The casein in the filtrate or 

 in the centrifugate was then reprecipitated at its isoelectric point by 

 the addition of hydrochloric acid, washed, and again dissolved by 

 sodium hydroxide. It has been our experience that the casein 

 completely dissolved the second time at a neutral reaction. The 

 casein in this clear neutral solution was finally precipitated and purified 

 at the isoelectric point in the manner that has already been described. 



Casein has generally been supposed to be nearly completely in- 

 soluble. Laqueur and Sackur (49), it is true, observed that a small 

 amount of their casein preparations always dissolved in water, but 

 decided that this amount was negligible. Our experiments upon 

 different preparations show that casein is no less soluble than the 

 globulins that have thus far been prepared in states of comparable 

 purity. 



As in the cases of serum globulin and tuberin,- the first measure- 

 ments of the solubility of precipitated casein were high. In this case 

 also this can be explained by assuming that casein forms readily 

 soluble compounds. The nature of these compounds and the extent 

 to which they can exist in the neighborhood of the isoelectric point 

 will be considered in another communication. The solubility of 

 two preparations of uncombined casein has been determined. The 

 results of thirty-two nitrogen determinations made upon the casein 

 dissolved in fivedifferent flasks, containing different amounts of pre- 

 cipitate, on 3 different days, agree within the errors of measurement. 

 Since the earlier work, though not nearly so accurate, is in fair agree- 

 ment with these later determinations, the average may be taken as 

 the most probable solubility of casein, and yields the result that 

 0.11 gm. of casein dissolves in 1 liter of water at 25°C. 



Many of the measurements that are reported were made by Miss 

 Jessie L. Hendry. It gives me pleasure to express my indebtedness 

 for her invaluable aid. 



