528 THE SECRETIONS. 



cipitated by boiling. It is soluble in alkaline solutions, and is 

 precipitate^ from them by neutralization, but this precipitation 

 is prevented by the presence of alkaline phosphates. It dis- 

 solves in excess of hydrochloric acid, and also, but not so 

 readily, in acetic acid. Milk does not coagulate when it is 

 boiled in a test-tube, but if it is boiled in an evaporating basin, 

 the casein near the surface becomes somewhat dried and forms 

 a scum on the surface; and if this is removed another appears. 

 When milk stands in a warm place, it becomes sour and curdles. 

 If common salt is added to fresh milk, it becomes sour on stand- 

 ing, but does not curdle, for the albumin, separated from the 

 casein by the acid, is kept in solution by the neutral salt. If 

 the solution is boiled, the albumin is coagulated. 



M,,,l,- t' tf,-]>nratin<j Cdnrin. As alkaline phosphates are 

 contained in milk, it must be not merely neutralized but ren- 

 dered distinctly acid, in order to precipitate the casein. The 

 precipitation is not complete unless the milk is diluted. 



Add a little acetic, acid to milk and warm it to 40 C. The 



-cin and the greater part of the fat separates in large flakes. 



Moisten a plaited filter with water, and filter the milk; put the 



filtrate aside, wash the eoagulum thoroughly with water, and 



I remove the fat by exhausting it with a mixture of alcohol and 

 ether in the apparatus described in A pp. 207. Put this solu- 

 tion aside; the remaining coagnlum is casein. 1 



Modi- <if ^I'jHiraltiKf Albumin. Boil the filtrate from which 

 the casein has been precipitated. A precipitate of albumin 

 will be produced. Albumin may also be separated by filtering 

 milk through a porous cell or cone by exhausting the air. A 

 clear fluid will pass through which will not be precipitated by 

 acetic acid, showing that no casein is present, but will be pre- 

 ^ipitated by boiling or by nitric acid. In the acid liquid from 

 which the coagulable albumin has been removed by boiling, a 



jVrecipitatti is produced by Milieu's reagent, although none is 

 occasioned by the addition of nitric acid or mercuric chloride. 

 4} Milk-Sugar. Filter the rest of the fluid in which the albu- 

 min has been coagulated. Shake it with ether to dissolve out 

 the fat; remove the ether with a pipette, and then evaporate 

 the fluid to a thin syrup. The milk-sugar will crystallize out 

 gradually in rhombic prisms. It differs from glucose in its 

 crystalline form (the latter generally occurring in warty crumb- 

 ling masses), in fermenting less readily, and in being insoluble 

 in absolute alcohol. 



Tin- Inorganic Salts of milk are chlorides, sulphates, phos- 



1 Tin- casein in human milk cannot be readily precipitated by hydro- 

 chloric or acetic ncids, and in order to obtain it, magnesium sulphate 

 must be added until the casein is precipitated, and the precipitate must 

 IK- washed with a strong solution of this salt, and then with alcohol and 

 ether. 



