380 PROTEIN-COMPOUNDS. 



solution is allowed to stand for some time in a shallow vessel, 

 when there gradually forms on its surface a layer of fatty matter, 

 which we must remove as completely as possible with a spoon, or 

 else we must decant the subjacent fluid with a syphon. The fluid 

 is now again precipitated with an acid, and the previous steps are 

 repeated. After the casein has been thrice dissolved in carbonate of 

 soda, and the fat as often skimmed off, the last trace of fatty matter 

 may then be easily removed by alcohol and ether, which otherwise is a 

 very difficult task. Casein thus prepared may moreover be rendered 

 entirely free from acid by repeated boiling in water ; so that if, for 

 instance, it has been precipitated with sulphuric acid, chloride of 

 barium does not excite the slightest turbidity when added to its 

 acid solution. Bopp* adopts a modification of Rochleder^s method; 

 he precipitates a solution of casein in carbonate of soda with hydro- 

 chloric acid, and repeatedly washes this precipitate with water con- 

 taining 2^ or 35. of hydrochloric acid ; it is then mixed with pure 

 water, in which it swells and gradually dissolves, especially if the 

 temperature be raised to about 40 ; the solution contains hydro- 

 chlorate of casein, from which the casein must be thrown down by 

 careful neutralisation with an alkali, and the precipitate then 

 washed. 



Tests. It is now ascertained that no reliance is to be placed 

 on certain properties of casein which were formerly regarded as 

 characteristic indications of its presence, and it is unfortunately 

 the case that recent investigations have only shown us the fallacy 

 of our former tests, without affording us better and more certain 

 means of detecting it. There were three especial properties by 

 which it was generally believed that casein might be recognised. 

 In the first place the capability of an animal fluid to form a mem- 

 brane on evaporation, was regarded as the most certain sign of the 

 presence of casein ; we have however already shown (p. 334) that 

 both alkaline albuminates and acid solutions of albumen equally 

 possess this property, and, indeed, that the fluid filtered from 

 ordinary coagulated albumen always contains such an albuminate, 

 and consequently has a tendency to form such a membrane ; the 

 tendency of an albuminous fluid to form a membrane on evapora- 

 tion, is directly proportional to the amount of alkali or albuminate 

 which it contains, and it is this circumstance that has led some 

 very accurate observers to believe that they have found casein in 

 the blood and in fluid exudations, where in reality not a trace of 

 this substance occurs. 



* Ann. d. Ch. u. Pliarm. Bd. 69, S, 16-37. 



