38 Milk and Its Products. 
degrees of temperature. In no event should the 
temperature of the milk to be tested be more than 10 
degrees warmer or colder than the standard, and it is 
much better if the temperature does not vary from 
the standard more than 5 degrees in either direction. 
The solids in milk are not all of the same 
specific gravity; some are heavier and some lighter 
than water. The fats are lighter, the other solids 
are all heavier. The specific gravity of the milk, 
then, depends not only upon the amount of solids 
present in the milk, but also upon their relative 
proportions. The specific gravity of milk may be 
affected by the addition of any substance to it or 
the abstraction of any of its constituents. Since 
some of the constituents of milk are lighter than 
water, their abstraction in whole or in part would be 
followed by an increase in density. It will be 
readily seen, then, that if a part of the fats are re- 
moved, the specific gravity of the skimmed or partly 
skimmed milk will be heavier than normal, and the 
addition of a certain amount of water or other sub- 
stance lighter than the milk would only serve to 
bring the specific gravity back to the normal point. 
In this way, if it is done skilfully, water may be 
added to milk, and cream abstracted from it, with- 
out affecting the specific gravity as revealed by the 
lactometer, and a very inferior sample of milk might 
pass as perfectly normal if the lactometer alone 
were depended upon for its detection. Mainly for 
this reason the lactometer has been superseded by 
other and more accurate instruments. 
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