L'O PROTEIDS. 



The product thus obtained is very pure, but there is a consider- 

 able loss of material during the washing of the gelatinous lumps, 

 owing to the solubility of the substance in the alkali which is 

 being removed. The pure substance itself is also slightly solu- 

 ble in water. 



4. Casein. 1 



This is the well-known proteid existing charactejiatically_in 

 milk and in no other fluid or secretion of the body.' 2 



It has recently been proposed to call this proteid 'caseinogen ' and 

 to use the name casein for the product of its decomposition_the clot 

 or curd, which is formed by the action of renntn^ugon_Jt. This 

 nomenclature would have the advantage of indicating a relationship 

 between the two proteids similar to that between fibrin and fibri- 

 nogen, rayosin and myosinogen (Halliburton). 



Preparation? Fresh milk is diluted with 4 volumes of jlis- 

 tilled water and acidulated with acetic acid until the diluted 

 imTk contains from -075 to O'l p.c. of the acid. If the milk has 

 been diluted with ordinary tap-water rather more acid must be 

 added. I The precipitated casein is now washed two or three times 

 by decantation with water, as rapidly as possible* dissolved in the 

 least quantity of dilute caustic soda which suffices for its solution, 

 and filtered through a series of filters until the filtrate is quite 

 clear and only faintly opalescent. This filtrate is then somewhat 

 diluted, the casein again precipitated by the careful addition of 

 acetic acid, and the whole process of washing, solution, and repre- 

 cipitation carried out a second time. The final product is now 

 freed as far as possible from water, worked up into an emulsion 

 with 97 p.c. alcohol, collected on a filter, washed with alcohol, 

 finally with ether, dried by exposure to the air, and finally in vacuo 

 over sulphuric acid. 



Casein may also be separated from inilk by precipitation with an 

 excess of sodium chloride * or magnesium" sulphate. 5 I'he latter pro- 

 cedure is chiefly of use for the preparation of casein from human milk, 

 from which it can scarcely be precipitated by means of acids. 



Pure casein as obtained by the above method i& a fine, snow- 

 white powder, which on ignition of even large quantities of the 



1 Our knowledge of the chemistry and properties of casein are based chiefly upon 

 the researches of Hammarsten. His papers were mostly published originally in 

 Swedish or Latin, but are fully abstracted by himself in Maly's Jahresbericht d. Thier- 

 chem., to which reference will in each case be made. 



2 For methods of conducting a complete analysis of milk see Pfeiffer, Die Anali/se 

 der Milch, Wiesbaden, 1887. 



3 Hammarsten, Maly's Bericht. Bd. vn. (1877), S. 159. 



4 Hammarsten, Maly's Ber. Bd. iv. (1874), S. 135. 



5 Hoppe-Seyler, Hd'dch. d. phys.-path. chem. Anal. Aufl. iv. (1875), S. 241. 



