6 ORGANIC CONSTITUENTS. 



Vegetable casein is cliiefly found in the seeds of peas^ beans, 

 lentils, and similar leguminous seeds. Like vegetable albumen, 

 it is soluble in water, but differs from it in this, that its solution 

 is not coagulated by heat. When the solution is heated or 

 CAi'aporated, a skin forms on its sm'face, and the addition of an 

 acid causes a coagulum just as in animal milk. 



" The chemical analysis of these three substances has led to 

 the very interesting result that they contain the same organic 

 elements united in the same proportion by weight ; and what is 

 still more remarkable that they are identical in composition Avith 

 the chief constituents of blood, animal fibrin and albumen. 



They all three dissolve in concentrated muriatic acid, with the 

 same deep purple colour, and even in their physical characters 

 animal fibrin and albumen are in no respect diflerent from ve- 

 getable fibrin and albumen. It is especially to be noticed, that 

 by the phrase identity of composition we do not here imply 

 mere similarity, but that, even in regard to the presence and re- 

 lative amount of sulphur, phosphorus, and phosphate of lime, no 

 difference can be observed.^' ^ 



When animal or vegetable albumen, fibrin, or casein is to be 

 used for the extraction of protein in a state of purity, the fol- 

 lowing steps are to be taken. The selected substance is suc- 

 cessively washed with water, alcohol, and ether, for the purpose 

 of remoiing extractive matter, fat, and soluble salts. It is then 

 treated with dilute hydrochloric acid, Avhich extracts the phos- 

 phate of lime and any other insoluble salts that may happen to 

 be present. We now dissolve it in a moderately strong solution 

 of caustic potash, and keep the solution for some time at a tem- 

 perature of 120°, whereby the sulphur and phosphorus that are 

 present form phosphate of potash and sulphuret of potassium. 



The protein is then to be thrown down from the solution, after 

 filtration, by acetic acid, which must be added only in very slight 

 excess, as otherwise the precipitate would be redissolved. It 

 must then be collected on a filter and carefully washed till every 

 trace of acetate of potash is removed. 



In this state it occurs in the form of grayish white gelatinous 

 flocks, Avhich, when dried, become hard and yellow, and give an 

 ainber-coloured powder. It is insoluble in Avater, alcohol, and 



' Liel)ig's Animal Clicinistiy, translated liy Gregory; p. 17. 



