498 THE POPULAR SCIENCE MONTHLY. 



insoluble. Alcohol added to solid soluble casein renders it opaque, 

 and gives it the appearance of coagulated albumen. The alcohol itself 

 dissolves a little of this. 



The characteristic coagulation of casein, or its conversion from the 

 soluble to the insoluble form, is produced rather mysteriously by ren- 

 net. Acids precipitate it from an aqueous solution, producing an ap- 

 parent coagulation, but it is not a true and complete coagulation like 

 that effected by the rennet, for on neutralizing the acid precipitant 

 with an alkali or metallic oxide the casein again dissolves. Excepting 

 in the cases of acetic and lactic acids (vinegar and the acid of sour 

 milk), which precipitate pure casein, the acid precipitates appear to 

 be a compound of casein with the acids, and the casein is set free in 

 its original state when the acid goes over to the alkali or basic metal- 

 lic oxide. The action of rennet in the coagulation of casein is still a 

 chemical mystery, especially when we consider the smallness of the 

 quantity of coagulating agent required for the rapid and complete con- 

 version. 



A calf has four stomachs, the fourth being that which corresponds 

 to ours, both in structure and functions. It is lined with a membrane, 

 from which are secreted the gastric juice and other fluids concerned in 

 effecting the conversion of food into chyme. A weak infusion made 

 from a small piece of this "mucous membrane" will coagulate the 

 casein of two or three thousand times its own quantity of milk, or the 

 coagulation may be effected by placing a small piece of the stomach 

 (usually salted and dried for the purpose) in the milk, and warming 

 it for a few hours. 



Many theoretical attempts have been made to explain this action 

 of the rennet. Simon and Liebig supposed that it acts primarily as a 

 ferment, converting the sugar of milk into lactic acid, and that this 

 lactic acid coagulates the casein ; but Selmi has shown that alkaline 

 milk may be coagulated by rennet in the course of ten minutes, and 

 that after the coagulation it still has an alkaline reaction. This is the 

 case whether fresh naturally alkaline milk is used, or milk that has 

 been artificially rendered alkaline by the addition of soda. 



Casein, when thoroughly coagulated by rennet, then purified and 

 dried, is a hard and yellowish horn-like substance. It softens and 

 swells in water, but does not dissolve therein, nor in alcohol nor weak 

 acids. Strong mineral acids decompose it. Alkalies dissolve it readi- 

 ly, and, if concentrated, decompose it on the application of heat. 

 When moderately heated, it softens, and may be drawn into threads, 

 and becomes elastic ; at a higher temperature it fuses, swells up, car- 

 bonizes, and develops nearly the same products of distillation as the 

 other protein compounds. 



I have good and sufficient reasons for thus specifying the proper- 

 ties of this constituent of food. I regard it as the most important of 

 all that I have to describe in connection with my subject — ^the science 



