118 THE CHEMISTRY OF THE FARM. 



what with the diet and condition of the animal ; in summer 

 time the proportion of fluid fats is greater than in winter. 



The sugar contained in milk is known by chemists as 

 lactose. When milk turns sour the lactose is converted 

 into lactic acid ; this acidification of the milk induces the 

 coagulation of the casein, and the milk curdles. The 

 ordinary souring of milk is the work of a ferment, Bacte- 

 riiim lactis; when this ferment is excluded no souring 

 takes place. 



Cow's milk has generally a specific gravity between 

 1*028 and 1*032. As the removal of cream raises the 

 specific gravity, which can be brought back to the normal 

 point by the addition of water, no safe conclusion as to the 

 quality of milk can be based on this indication. 



The composition of cow's milk is affected by various 

 circumstances ; under extreme conditions it may contain 

 from 10 to IG per cent, of dry matter. The milk is poorer 

 when the quantity produced is large, or the diet insuffi- 

 cient, and richer when these conditions are reversed. A 

 cow is generally in full milk from the second to the 

 seventh week after calving; after this period the milk 

 gradually diminishes in quantity, but increases in rich- 

 ness. A separation of cream takes place in the udder ; 

 the milk first drawn is poor in fat, and the richness in- 

 creases as milking proceeds, the last drawn milk containing 

 two or three times as much fat as the first drawn. The 

 milk of old cows is said to be poorer than the milk of 

 young cows. 



The relation of food to the production of milk and 

 butter has been already considered (p. 107). 



Cream. — The fat of milk occurs in the form of globules ; 



