INTRODUCTORY. 9 



the same time the corresponding alkaline salt. Thus when 

 the fat of milk is digested with potash or soda, it furnishes 

 glycerine, and, at the same time, the oleate, palmitate, 

 stearate, and butyrate of potash or soda. The fat of milk is 

 hard at winter temperatures, and soft at summer temperatures 

 (its fusing point lying, in fact, at such temperatures as are 

 reached in summer). Fat is distinguished from the other 

 solid constituents of milk by being soluble in ether. 



Caseine. This is the nitrogenous constituent of milk. In 

 regard to this portion of the milk, the remark should first be 

 made that it is not perfectly homogeneous that is to say, 

 there are at least two- distinct chemical substances comprised 

 by the nitrogenous portion of milk. 



There is caseine proper, and also albumen that is to say, 

 a certain proportion of the nitrogenous substance is coagu- 

 lated on boiling milk, but the major part of the nitrogenous 

 substance is not coagulated on boiling. Whether the portion 

 of nitrogenous substance which is not coagulated on boiling 

 is itself homogeneous, is even a matter of some doubt. In a 

 corresponding case that of flour we know that the nitro- 

 genous constituent is a very complex mixture, and that, under 

 the name of gluten, a whole tribe of substances are compre- 

 hended. Under the name of caseine it will be convenient to 

 designate the entire nitrogenous constituents of milk ; just 

 as, under the name gluten, the entire nitrogenous portion of 

 flour is comprehended. Like albumen, caseine exists under 

 two modifications it is either soluble or insoluble. In the 

 former of these states it exists in fresh milk ; in the latter, 

 after the milk has " turned." 



It used to be believed thai the soluble variety of caseine 

 was in reality a salt caseine, whersin caseine played the part 

 of acid, and the alkali naturally present in milk-ash played 

 the part of base. The coagulation or curdling of milk was 

 explained on the supposition that lactic acid, generated by 



