278 
The Work of Acidity in Gheese-mahin fj. 
milk, raising it to the same temperature as that at which the 
previous sample had been maintained, and adding sufficient of 
a solution of lactic acid ' to raise the acidity of the milk to the 
degree above indicated ; curding will then take place. 
As to the exact conditions under which the curd is formed, 
it will be sufficient here to say that it is produced by the separa- 
tion in a compact and insoluble form of a constituent (casein) 
which had previously existed in the milk in a state of what may 
be called diffuse fluidity ; a state, if not of actual solution, of 
diffusion which closely approaches to it, and which may be nearly 
compared with that produced by dissolving a small quantity of 
gelatin in a rather larger amount of water. If we make such a 
solution we shall find that the fluidity of the water is not ap- 
preciably affected ; but by treating the solution with certain 
well-known chemical agents the gelatin can be curdled or ren- 
dered insoluble, precisely in the same way as the casein has 
been in the milk under observation. In the case of the gelatin 
the curd so formed appears in the condition of scattered flocks, 
just as that of milk does if the milk be kept stirred at the time 
when it is coagulating. But, if the gelatin be dissolved in the 
water in larger quantity, by the aid of heat, and the solution 
be allowed to cool, it then, as is well known, i/elatinises — i.e. 
separates in a peculiar condition, iiitermediate between solidity 
and fluidity, in which the whole of the water is retained in a 
state of intimate union with it. 
The union, indeed, in which the gelatin is thus combined 
with the water is so intimate that it cannot be separated from 
it except by drying. The jelly when once formed in this con- 
dition, which can only be done by lowering the temperature of 
the warm solution in the way described, has no property of con- 
tractility by which it can free itself of the water with which' it 
is united. This is a very important point to bear in mind, 
because, as we shall see, it essentially distinguishes the jelly or 
curd of gelatin from the corresponding jelly or curd of casein. 
To appreciate this distinction it is only necessary to raise 
the temperature of the curdled milk to about 90° F. and to 
observe what takes place in it. This observation will be much 
facilitated by scoring it across vertically, at right angles, so as 
to divide it into cubic columns resting on tlie bottom of the 
vessel, just as is done in cheese-making, and by also running the 
knife around the margin of the curd, so as to separate the latter 
from the surface of the vessel. On doing this we shall soon see 
' This aciil niaj' be obtained of any pharmaceutical chemist. It is peTierally 
supplied of such a strength tliat it will recjuire considerable dilution with water 
to adapt it for such experiments as those described in this paper. 
