The Wm-Ti oj Acidily in Cheese-making. 
279 
the rifts in the curd made by the knife gradually widening and 
the clear whey appearing between the blocks of curd thus 
rendered apparent. This will go on, so long as the temperature 
of the mass is maintained, until the curd, which is at first soft, 
compressible, and easily broken up, becomes firm, resistent, and 
possessed of a considerable degree of cohesiveness or ienacity. 
Now, just as it was evident that the formation of the curd 
was originally due to the pi-esence in the milk of a certain 
degree of acidity, so it is equally clear that the casein of which 
it is composed possesses at the time of separating from the milk 
a certain amount of spontiineoit>< contractilifi/, in virtue of which 
it is able gradually to contract on itself, and in so doing to 
squeeze out the liquid whey. This power of spontaneous con- 
traction which the curd possesses is the very foundation of 
cheese-making, and unless it be thoroughly recognised by the 
cheese-maker, and the conditions under wliich it can be main- 
tained be clearly appreciated, it is impossible for him to acquire 
an intelligent acquaintance with the principles of his art. 
The interest of the phenomenon is the greater because it is 
probably the best illustration which we possess of the lowest 
gi-ade of organisation. In this respect, the jelly of casein, which 
we have already compared with the jelly of common gelatin, 
and have distinguished from it by the possession of this property 
of spontaneous contractility, may be compared, on the other 
hand, with another kind of jelly — viz., that of the clot which 
blood forms outside the vessels in which it circulates in the 
living body, or, after a certain time, inside them, when the 
body cools after death. This coagulation of the blood is caused 
by the gelatinisation of a body (jihrin) closely allied to casein 
in its ultimate composition, but differing from it in possessing a 
still higher degree of self-organising power. Into the very 
interesting problems of the relations exhibited by these three 
bodies — gelatin, casein, and fibrin — to one another, it would 
be out of place to enter here ; they have only been thus 
incidentally referred to in order to impress upon the cheese- 
maker's mind the fact of the spontaneous contractility of the 
curd, and to afford illtistratious which may enable him to 
appreciate the mechanism of the process by which the curd 
gradually assumes the form which he seeks to give it with the 
view of ultimately converting it into cheese. 
In each case the jelly must be looked on in the light of a 
Kponge of gelatinous material, in the interstices of whicli are at 
first retained not only the water in which the spongy fabric 
itself was originally disf^olved, but also any matters which have, 
in common with the gelatinous material, been in solution in it, 
