m] THE CONSTITUENTS OF MILK 27 



freely exposed to the air, and in the presence of 

 sunlight, this decomposition goes on more quickly. 

 It may be here pointed out that not merely are 

 certain fatty acids set free, but actually new acids, 

 such as formic acid, are formed by the absorption of 

 oxygen. In this process of decomposition the fat 

 assumes a white colour. The conditions which 

 influence the percentage of fat in milk will be 

 discussed in the next Chapter, under the wider 

 question of the conditions which influence the com- 

 position of milk. 



The Albuminoids of Milk. The albuminoids of 

 milk, from a chemical point of view, are the most 

 interesting of the milk constituents. More discussion 

 has taken place with regard to their nature than with 

 regard to the nature of all the other constituents of 

 milk. Some, like the eminent French investigator 

 Duclaux, have held that there is only one albuminoid 

 substance in milk. This theory, however, seems to 

 be at variance with the behaviour of milk when 

 treated with certain reagents ; and there can be little 

 doubt that .there are several albuminoids in milk. 

 Of these, casein, or what has been called caseous 

 matter, or caseinogen, to distinguish it from pure 

 casein, is the most abundant, and forms about 80 per 

 cent of the total nitrogenous matter. The other 

 albuminoids are albumin, which is next abund- 

 ant in amount to casein ; lactoprotein, which has 



