CASEIN. 487 



taric acid, but is gradually changed by them, and the 

 change takes place the more rapidly, the higher the 

 temperature and the stronger the acid is. Concen- 

 trated hydrochloric acid gives a precipitate in the 

 solution, soluble in an excess. Perfectly neutral solu- 

 tions of serum-albumen coagulate at 72-73. Acids 

 or salts elevate, alkalies lower the temperature required 

 for coagulation. It is not coagulated by shaking with 

 ether. For the preparation of egg-albumen, white of 

 egg is passed through linen, filtered without access of 

 air, and then further purified in the same manner as 

 serum-albumen. In most of its properties it shows a 

 perfect resemblance to serum-albumen; it, however, 

 rotates the plane of polarization somewhat less strongly 

 to the left. With hydrochloric acid it gives a pre- 

 cipitate, which is very difficultly soluble in water and 

 in an excess of hydrochloric acid ; is thoroughly and 

 instantaneously coagulated by alcohol and also when 

 shaken with ether. Concentrated potassa-ley, added 

 to a concentrated solution of either modification of 

 albumen, causes the formation of a transparent, solid 

 jelly of potassium albuminate. 



2. Casein. In milk and the yolk of eggs. In 

 order to prepare it, skimmed milk is mixed with 

 dilute sulphuric acid, the white precipitate, after being 

 filtered off and washed, while still wet, is digested 

 with lead carbonate ; the filtered solution, containing 

 the casein, then evaporated, after the removal of the 

 lead with carbonic anhydride or sulphuretted hydro- 

 gen. Or the milk is diluted and precipitated with 

 acetic acid ; the precipitate washed with water, alco- 

 hol, and ether ; dissolved in very dilute caustic soda ; 

 again precipitated with acetic acid ; and again washed 

 as before. The nature of casein is not yet sufficiently 

 well known. Its solubility in water appears to be de- 

 pendent upon the presence of alkalies. Casein, which 

 is free of alkali, is insoluble in w T ater, and in a solu- 

 tion of common salt, but easily soluble in water con- 

 taining a very little hydrochloric acid or alkali. From 

 this solution it is precipitated (in the absence of alka- 



