40 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 dii*ection. 

 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 gravitj^ of the milk, 

 then, depends not only upon the amount of solids 

 present in the milk, but also upon theii' 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 skilfullj', 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 this reason the lactometer has been superseded bj^ 

 other and more accurate instruments. 



