32 MILK [in 



change is effected by bacteria. The class of bacteria 

 effecting it are of common occurrence, and have been 

 found in great numbers on the cow's udder, in the 

 byre, in milk- vessels, etc. They thus obtain easy 

 access to the milk, and the souring of milk through 

 their action follows sooner or later. The following 

 is the composition of milk-sugar : 



Carbon 40 -00 



Hydrogen .... 6'10 



Oxygen . . . . 48 '90 



Water . 5-00 



100-00 



Its chemical formula is C 12 H 22 O n , H 2 0. The con- 

 version of milk-sugar into lactic acid may be ex- 

 plained by the following equation, in which one 

 molecule of milk-sugar is converted into four mole- 

 cules of lactic acid : 



With regard to the bacteria and the conditions 

 under which they develop, further information will 

 be given in dealing with the bacteria of milk. Milk- 

 sugar was discovered in milk as early as the end of 

 the seventeenth century. It is a crystalline body 

 (four-sided prisms) of a white transparent colour, 

 and contains, as has been indicated by the formula, 

 one part of water of crystallisation. It is difficultly 



