60 
Composition of Chceie. 
Scientific and practical writers on milk have stated that the 
casein is held in solution by a small quantity of alkali ; that 
when in warm weather milk curdles, lactic acid, which is 
always found in sour milk, is formed from a portion of the sugar 
of milk ; and this lactic acid, by neutralizing the alkali which 
holds the casein in solution, causes its separation from the milk. 
Rennet is supposed to act as a ferment, which rapidly converts 
some of the sugar of milk into lactic acid. Whether, therefore, 
milk coagulates spontaneously after some length of time, or more 
rapidly on the addition of rennet, in either case the separation of 
the curd is supposed to be due to the removal of the free alkali 
by lactic acid. 
This theory, however, is not quite consistent with facts. The 
casein in milk cannot be said to be held in solution by free 
alkali ; for, although it is true that milk often has a slightly alka- 
line reaction, it is likewise a fact that sometimes perfectly fresh 
milk is slightly acid. We might as well say, therefore, that the 
casein is held in solution by a little free acid, as by free alkali. 
Newly drawn milk, again, is often perfectly neutral ; but, 
whether milk be neutral, or alkaline, or acid, the casein exists 
in it in a state of solution, which cannot, therefore, depend 
on an alkaline reaction. We all know that milk, when it 
turns sour, curdles very readily. It is not the fact that a good 
deal of acid curdles milk which I dispute, but the assumption 
that the casein in milk is held in solution by free alkali. The 
action of rennet upon milk, then, is not such as has been 
hitherto represented by all chemists who have treated of this 
subject. Like many other animal matters which act as ferments, 
rennet, it is true, rapidly induces the milk to turn sour ; but free 
lactic-acid, I find, makes its appearance in milk after the curd 
has separated, and not simultaneously with the precipitation of 
the curd. Perfectly fresh and neutral milk, on the addition of 
rennet, coagulates, but the whey is perfectly neutral. I have even 
purposely made milk alkaline, and yet succeeded in separating 
the curd by rennet ; and, what is more, obtained a whey which 
had an alkaline reaction. 
What may be the precise mode in which rennet acts upon 
milk, I do not presume to explain. I believe it to be an action 
sui f/eneris, which as yet is only known by its effects. We at 
present are even unacquainted with the precise chemical cha- 
racter and the composition of the active principle in rennet, and 
have not even a name for it. Finding the effect of rennet upon 
milk to be different from that which I expected, I made a number 
of experiments, which may here find a place. 
\st Experiment. — To a pint of new milk, slightly alkaline to 
