81 
only a slight degree of sweetness, though of nearly the same 
composition as cane sugar, and of the same use in the animal 
economy. Ordinary milk is found to contain about 2.8 per 
cent, of sugar, which is equal in value, as respiratory food, to 
1.4 lb. of butter; hence 100 lbs. of milk would be equivalent 
to 5.9 lbs. of fat, or to 11.8 lbs. of starch or sugar, in 
supporting respiration, and keeping up the heat of the body. 
This milk sugar is a compound exceedingly liable to 
undergo decomposition, particularly if placed in contact 
with a body already in a state of change. The gastric 
juice of the stomach, or the infusion of rennet, which con- 
tains the principle of the gastric juice, converts it into 
lactic acid with great rapidity. The same change also takes 
place spontaneously in milk, if kept for a long time in warm 
weather, as we observe in the process of souring; and in 
this case, the other constituent of the milk — the caseine — 
becomes an insoluble coagulum. It would appear that the 
caseine in milk is held in solution by a small portion of free 
alkali, as we find distinct traces of caustic soda when new. 
A quantity of acid, sufficient to neutralize this alkali, always 
produces coagulation; and it is in this manner that the 
caseine is rendered insoluble in the stomach, or by the 
spontaneous formation of lactic acid in the milk. When 
the curds produced from milk are drained, well washed, 
and dried, they form the compound called caseine, of 
which good milk contains about seven per cent. This 
substance differs in composition from the two just described, 
in containing nitrogen as one of its constituents. It con- 
sists in 100 parts of 
Carbon 55.74 
Hydrogen 6.83 
Nitrogen 16.14 
Oxygen 21.29 
VOL. ill. 
F 
100.00 
