20 PROTEIDS. 



Casein may also be separated from milk by precipitation with an excess of 

 sodium chloride 1 ' or magnesium sulphate 2 . The latter procedure 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 is a fine, snow-white 

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

 (4 6 grm.) leaves scarcely a trace of ash. It is practically insoluble in 

 water, but is soluble in alkalis, carbonates and phosphates of the alkalis, 

 lime- and baryta-water. From these solutions it may be precipitated 

 by excess of neutral salts such as sodium chloride, and by dilute acids, 

 in which it is again soluble if any excess of acid is present. Its reactions 

 thus correspond closely to those of acid- and alkali-albumin, but as will 

 be presently shewn it is in many ways perfectly distinct from these 

 substances. Solutions of pure casein are not coagulated by boiling, 

 but if heated to 130 150 in sealed tubes a coagulation is obtained. 



When acids are added to diluted milk to effect the precipitation of casein no 

 precipitate is obtained until the solution has a distinctly acid reaction; this has 

 usually been attributed to the presence in milk of potassium phosphate 3 . Ham- 

 marsten has however shewn 4 that the same holds good for solutions of casein free 

 from this salt. 



When prepared from milk by magnesium sulphate (see below), freed 

 by ether from fats, and dissolved in water, casein possesses a specific 

 rotatory power (a) D = 80; in dilute alkaline solutions, of 76; in 

 strong alkaline solutions, of 91; in very dilute solutions, of 87 5 . 



Although purified casein leaves no ash-residue on ignition Hammar- 

 sten found that it contained a constant and fairly large amount of 

 phosphorus, as a mean '847 p.c. From this fact and its behaviour 

 towards sodium chloride in dilute solutions, he regards casein as 

 being a nucleo-albumin 6 (see below). This view corresponds with the 

 results previously obtained by Lubavin 7 , who found that a phosphorised 

 (nucleiri) constituent of casein is separated out as an insoluble residue 

 during the digestion of casein with gastric juice. 



According to the views of many authors 8 milk contains not one casein only but 

 at least two forms of proteid which pass under the one name. Hammarsten 9 has 



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



2 Hoppe-Seyler, Hdbch. d. phys.-path. chem. Anal. Aufl. iv. (1875), S. 241. 



3 Kiihne, Lehrb. d. physiol. Chem. 1868, S. 565. 



4 Maly's Ber. vn. S. 162. 



5 Hoppe-Seyler, Hdbch. (Ed. v.) p. 286. 



6 Maly's Ber. iv. (1874), S. 153. 



? Hoppe-Seyler 's Med.-chem. Untersuch. Hf. iv. (1871), S. 463. 



8 Millon u. Commaille, Zt. f. Chem. 1865, S. 641. Compt. Rend. T. i. (1865), 

 pp. 118, 859, T. n. p. 221. Selmi, Ber. d. d. chem. GeselL Bd. vn. (1874), S. 1463. 

 Danilewsky u. Kadenhausen. See Maly's Ber. Bd. x. (1880), S. 186. Zt. f. physiol. 

 Chem. Bd. vn. (1883), S. 427. Struve, Jn. f. prakt. Chem. (2) Bd. xxix. S. 71. 



9 Maly's Ber. Bd. v. (1875), S. 119, Bd. vi. (1876), S. 13. Zt. f. physiol. Chem. 

 Bd. vii. (1883), S. 227. 



