20 ORGANIC CONSTITUENTS. 



a. Evaporate milk to dryness in the water-bath ; triturate 

 the sohd residue and treat it with boiHng ether, as long as it 

 gives off any butter. When this ceases to be the case, remove 

 the butter, and evaporate off the ether ; dissolve the residue in 

 water, and filter. On the addition of alcohol to the clear filtered 

 fluid, the casein is separated and thrown down. In order to 

 remove any sugar of milk that may be entangled with the 

 casein, the precipitate may be redissolved in water, and again 

 thrown down by alcohol ; if it be now collected and dissolved in 

 water, it affords a tolerably pure solution of casein. 



b. Casein may also be obtained by the addition of sulphuric 

 (or any other) acid. Sulphate of casein is precipitated, which 

 must be carefully washed in water, freed in the ordinary manner 

 from butter, &c. and then digested with carbonate of lime. By 

 careful and, if necessary, repeated filtration we obtain a clear 

 solution, Avhich however is not free from lime. 



A solution of casein prejjared according to either of these 

 methods is possessed of little flavour; on the application of 

 warmth it evolves a milky odour, and during evaporation it 

 becomes covered with a skin or film, which on being removed 

 is speedily renewed. This skin is due to the action of oxygen, 

 for it does not form in an atmosphere of carbonic acid. 



By a continuance of the evaporation we ultimatel}^ obtain a 

 residue of dry casein. It appears as a brittle yellow substance. 

 It does not admit of being perfectly dissolved in water, in con- 

 sequence of a portion of it having assumed an insoluble condi- 

 tion during evaporation. 



According to Mulderi casein is represented by the formula 

 C,ooH3,,N,„0,,, SorlOPi: + S. 



The action of milk in the nutrition of young animals proves 

 that casein is capable of being converted into albumen, and 

 fibrin ; while the production of milk in an animal fed on albu- 

 men or fibrin shows that these substances may be reconverted 

 into casein. 



The alkalies exert a similar solvent power over casein as over 

 protein and its other modifications. The metallic salts also 

 form similar double compounds. It differs from albumen, in 

 being precipitated by all acids. The latter reagents must be 



' See Appendix I, Note 9. 



