CASEIN. 381 



[Since the publication of this volume in German, two memoirs 

 on the assumed discovery of casein in the blood have appeared, 

 one by Guillot and Leblanc,* the other by Panum.f G. E. D.] 



This error would be further promoted by a second mode of 

 testing for casein, namely, by its property of being precipitated by 

 acetic acid; this was regarded as a means of distinguishing between 

 casein and albumen ; but if the slight turbidity which affects albu- 

 minous solutions (see p. 333), when they are neutralised or very 

 much diluted with water, occasionally gave rise to a confusion 

 between these substances, this must have occurred far more fre- 

 quently when it was believed that the albumen had been removed by 

 boiling from albuminous fluids; for there then remains, as we 

 have already seen, a little coagulated albumen with soda or potash 

 in solution ; by the addition of acetic acid the albumen is precipi- 

 tated from this solution in precisely the same manner as casein, 

 which is not the case with the unboiled albuminate of potash. 

 Every accurate experimenter must have thus been led (till these 

 facts were ascertained) to believe that he had always found a little 

 casein in the fluid filtered from coagulated albumen. 



The third means of discovering casein is the only one now left 

 us ; and even this, by its incorrect application, has already given 

 rise to false conclusions. We refer to the coagulability of casein by 

 rennet, a test by which some have supposed that they have 

 detected casein in the blood : but in order that the casein may be 

 separated by this means, the rennet must be tolerably fresh, or at 

 all events must not have become putrid, when it is placed in the 

 fluid which is to be examined; the mixture should then digest, 

 at a temperature of 40, for a period not exceeding two hours ; if 

 no coagulum is then formed, we are not justified in assuming that 

 casein exists in the fluid ; for if we allow the rennet to remain for 

 twenty-four hours or longer in the fluid at that temperature, putrefac- 

 tion ensues, with the development of vibriones,and the fluid becomes 

 turbid by the products of putrefaction, but not by coagulated 

 casein. Blood in which, for instance, some chemists fancy that 

 they have thus detected casein, putrefies, on the addition of rennet, 

 after a considerable time, but I have never succeeded in obtaining 

 from it a true coagulum of casein. 



Sulphate of magnesia and chloride of calcium have been 

 recently recommended as very good tests for the presence of casein; 

 the casein separating on boiling in combination with magnesia or 



* Compt, rend. T. 31, p. 585. 



t Arch. f. pathol. Anat. Bd. 3, S 251. 



