32 CHEMICAL CONSTITUENTS : ALBUMEN, CASEIN. 



their bodies have been completely dried up in the first 

 instance. No trace of organization can be detected in 

 coagulated albumen, which seems to be composed only of a 

 mass of granules ; and in this respect it differs in an im- 

 portant degree from fibrin as we shall presently see. 

 Albumen may also be made to coagulate readily by the action 

 of acids, especially the nitric (aqua-fortis) ; so that a very 

 small quantity of it may be detected in water, by the tur- 

 bidity produced by adding to it a drop or two of nitric acid, 

 and then heating it. Now, when thus coagulated, albumen 

 cannot be dissolved again by any ordinary process ; but its 

 solution may be accomplished by rubbing it in a mortar with 

 a caustic alkali, potass or soda. From this solution it may be 

 precipitated again on the addition of an acid in sufficient 

 quantity to neutralise the alkali. Albumen is distinguished, 

 then, by its peculiar property of coagulating on the applica- 

 tion of heat, or on being treated with certain acids. 



15. Nearly allied to albumen is the substance termed 

 Casein, which replaces it in milk ; and this is specially 

 worthy of notice here, because it is the sole form in which 

 the young Mammal receives albuminous nourishment during 

 the period of suckling, in which it draws its sustenance from 

 its parent. Like albumen, this substance may exist in two 

 forms, the soluble, and the insoluble or coagulated ; and the 

 presence of a small quantity of free alkali seems essential to 

 its continuance in the soluble form. Casein differs from 

 albumen, however, in this, that it does not coagulate by 

 heat, and that it is precipitated from its solution by organic 

 acids, such as the acetic and lactic, which have no coagulating 

 action on albumen. It is further remarkable for the facility 

 with which its coagulation is effected by the contact of 

 certain animal membranes ; as we see when a small piece of 

 rennet (which is the dried stomach of the calf) is put into a 

 large pan of milk in the process of cheese-making, the ' curd* 

 which then separates being composed of casein entangling the 

 oily particles of the milk. In the coagulated state, casein 

 differs but very little from albumen, and is readily converted 

 into it by the gastric fluid. It is remarkable for its power of 

 dissolving the earthy phosphates, as much as 6 per cent, of 

 phosphate of lime being usually obtainable from it ; and it is 

 in this combination, that the large quantity of bone-earth 



