38 THE SOLUBLE CONSTITUENTS. 



Among the other substances present in traces in milk the 

 following have been described : Urea, hypoxanthine and other 

 nitrogeneous basic substances, a colouring-matter, odorous sub- 

 stances and alcohol (described by Bechamp, but certainly not 

 ordinarily present). 



The Gases of Milk. It is extremely probable that the gases 

 of milk are derived from the air by absorption during and after 

 milking. Carbon dioxide and but little oxygen being present 

 in the milk as it comes from the cow, while oxygen, nitrogen 

 (probably argon), and carbon dioxide are present in fresh milk. 

 As the milk is kept the amount of oxygen first increases and 

 then decreases and that of the carbon dioxide increases ; this 

 is probably due to aerobic micro-organisms, which absorb the 

 oxygen and give out carbon dioxide. 



The gases of milk may also include products of decomposition ; 

 thus in decomposed milk, volatile sulphur compounds of evil 

 odour are present. If such, as is probably the case in dirty 

 surroundings, were present during milking they would be absorbed 

 to some extent by the milk. 



The gases have no practical importance. 



Milk is sometimes charged with carbon dioxide under high 

 pressure to form an effervescing drink. In this case, and in 

 koumiss and kephir, products of fermentation of milk, the carbon 

 dioxide is an important constituent. 



