32 CHEMICAL CONSTITUENTS *-ALBUMEN, CASEIN. 
their bodies have been completely dried np 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—ns 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 
