20 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
in which they are present vary very much. The fat shows 
a greater variation than the other solids. In milk rich 
in total solids, the fat content is frequently considerably 
above the average, while the per cent. of solids not fat 
only slightly exceeds the average. On the other hand, 
in thin, poor milk the fat per cent. may be far below 
the average, while the per cent. of solids not fat is not 
very far from the average. Milk from individual cows 
shows a greater variation in composition than different 
samples of market milk, which is a mixture of the milk 
from a number of cows. 
Individual milk may show the following variations 
in the proportion of the different constituents: fat, 2.5 
to 7.5 per cent.; lactose, 4 to 5.8 per cent.; casein, 2 to 5 
per cent.; albumin, 0.39 to 0.95 per cent.; globulin, a 
trace; salts, 0.35 to 1.21 per cent.; water, 83 to 89 per 
cent. 
The fat content shows the greatest range of varia- 
tion and the lactose the least. These variations must be 
taken into consideration in collecting samples of milk 
for certain tests and in judging the results of tests for 
adulteration and skimming. They are due to a number 
of causes. 
Some of these causes, such as the breed, individuality, 
and stage of lactation, are more or less regular and con- 
stant in their operation. As a rule, cows of the Jersey 
and Guernsey breeds give milk richer in fat than Hol- 
steins and Ayrshires, but some individuals of the Hol- 
stein and Ayrshire breeds give milk with a higher fat 
content than some Jerseys and Guernseys. Early in the 
stage of lactation, when the milk flow is most abundant, 
the proportion of solids, especially the fat, is less than it 
is later, when the milk flow has decreased. Instances 
