376 PROTEIiN-COMPOUNDS. 



milk, artificially rendered alkaline by the addition of soda, was 

 exposed to the action of rennet. Conversely, casein dissolved in 

 an excess of acetic or oxalic acid, coagulated, like the alkaline solu- 

 tion, at a temperature of from 50 to 56. The true cause of coa- 

 gulation is still entirely unknown. It appears, however, from the 

 observations of Scherer*, that casein cannot coagulate in the form 

 of a membrane, unless in the presence of oxygen. 



From the large number of individual facts which we have 

 mentioned in relation to casein, it may be inferred that our know- 

 ledge of this substance is still very defective ; for otherwise we 

 could have embraced in a few paragraphs the most essential points 

 in relation to this body ; our difficulties are increased by the pro- 

 bability that casein is not to be regarded as a simple organic body, 

 but as a mixture of at least two different substances. Mulderf 

 and SchlossbergerJ have especially directed attention to this cir- 

 cumstance. If freshly washed casein be digested for a couple of 

 days with dilute hydrochloric acid, it is found to be perfectly dis- 

 solved ; by neutralisation with carbonate of ammonia there is pre- 

 cipitated from this fluid a white, viscid body, difficult to separate 

 by filtration ; but in the neutralised fluid there still remains in 

 solution another substance which may be thrown down by an 

 excess of hydrochloric acid ; and the hydrochloric acid even now 

 holds in solution a protein-like body. The first of these bodies 

 was found by Schlossberger to contain sulphur, and the second to 

 be free from that element. 



Here, however, it might be supposed that the prolonged diges- 

 tion of the original casein with the dilute hydrochloric acid had 

 decomposed it into several substances. Another and an earlier 

 experiment of Mulder, however, supports the view that casein con- 

 sists of several substances. To milk which had been as thoroughly 

 as possible freed from butter- globules by chloride of sodium, Mulder 

 added dilute hydrochloric acid, which yielded the ordinary preci- 

 pitate ; there remained, however, in solution, a similar body, 

 which was not precipitated till this mixture was boiled. 



It is very difficult to arrive at a definite opinion on this point; 

 for any one repeating the experiments on casein which have been 

 described by different authors, will find that all the statements 

 regarding this substance confirm one another to a certain degree, 

 but that on often repeating the same experiment differences present 

 themselves which thus explain the discrepancies in the statements of 



* Ann. d. Ch. u. Pharm. Bd. 40, S. 36. 



** Berzelius Jahresbr. Bd. 26, S. 910. 



t Ann. d. Ch. u. Pharm. Bd. 58, S. 92-95. 



