Relation of Fat to Casein 31 
oily fat, while cotton-seed meal, the seeds of the 
various legumes and wheat bran make a hard fat. 
Constituents other than the fat are not so readily 
affected in this way. 
Of the constituents of milk, the ash and the 
sugar are the least variable, the fat and albumin 
the most variable, while the casein usually bears a 
nearly constant ratio to the fat. The percentage 
of water also varies considerably. The causes of 
the variation of the fat have already been noticed. 
The proportion of albumin is very largely in- 
fluenced by the physical condition of the cow, and 
it has been shown, notably by Van Slyke (see 
Chapter XIII.), that with what may be called normal 
milk,—that is, milk containing from 3 to 4.5 per 
cent of fat,—the proportion of casein rises or falls 
in almost exact ratio with the fat, but when the 
fat rises above this point the casein does not 
follow in the same proportion. 
A notion is prevalent that the percentage of fat 
in the milk is also affected by the age of the cow; 
that duriug the first and second periods of lactation 
the young cow usually gives milk poorer in fat 
than when she is mature. During the years of 
greatest vigor the percentage of fat is supposed to 
be fairly uniform; but in cows of. advanced age ‘it 
may sometimes again fall to a low point. Recently 
some records have been published* that go to show 
that the age of the cow has little, if any, influence 
*Holstein Friesian Herd Book, vols. 18, 14, 15, 16,.17. Cornell University 
Agr. Expt. Station, Bulletin No. 169. 
