CASEIN. 375 



precipitate being again soluble in water. The mere boiling of a 

 solution of casein, under no circumstances,, induces a precipitation. 

 On the other hand, we may be readily led to believe that it is con- 

 verted into a coagulable substance when we have dissolved it in a 

 solution of carbonate of potash, or of nitre to which a little potash 

 has been added ; on neutralising this solution with an acid, a transi- 

 tory precipitate ensues on stirring or shaking the mixture, and if we 

 now boil the fluid, there is formed an abundant thick coagulum ; I 

 have not been able to persuade myself to regard this as a modification 

 of casein coagulable by mere heat (such as sometimes appears to be 

 contained in the milk) but I rather incline to the belief that the 

 acid has converted only a part of the caseate of soda occurring in 

 solution, and of the simple carbonate of soda, into acid salts, and 

 that on the application of heat it is only the acid salts remaining in 

 solution which are decomposed and evolve carbonic acid, while the 

 casein is precipitated. 



From the above observations it follows that casein is not 

 reduced to its coagulated state by the same means as albumen and 

 globulin. We have long been acquainted with the fact that the 

 casein in milk is coagulated by the mucous membrane of the 

 stomach of the calf; our knowledge is, however, by no means clear 

 regarding the peculiar condition under which this coagulation 

 ensues. We have seen that soluble casein, on the evaporation of 

 its solution, is partially transformed into the insoluble modifica- 

 tion ; cases, however, occur, in which the whole of the casein in 

 milk is rendered insoluble by evaporation. Even on prolonged 

 exposure to the air, it is well known that milk coagulates ; the 

 casein thus separated reacts in the same manner as the preci- 

 pitate obtained from a solution of pure casein by means of lactic 

 acid, that is to say, after treating it with carbonate of lime or 

 baryta, it is only slightly soluble in water, most of it having been 

 transformed into the insoluble modification. Simon* and Liebig 

 explain the coagulation of casein by the calf's stomach (rennet) 

 by assuming that the latter primarily acts as a ferment, converting 

 the sugar in the milk into lactic acid, which precipitates the casein; 

 Simon moreover maintains that he has observed that solutions of 

 casein free from milk-sugar are not coagulated by rennet. Certain 

 experiments, instituted by Selmit, are, however, opposed to this 

 view ; he found that alkaline milk could be coagulated by rennet 

 in the course of ten minutes, and that, after the coagulation, it still 

 had a decidedly alkaline reaction ; the same was observed when 



* Fraucnmilch. S. 29. 



t Journ. de Pharm. T. 0, pp. 265-267- 



