vi] THE BACTERIA OF MILK 85 



reaction. It completely ceases as soon as the milk 

 has become coagulated, or, to speak more correctly, 

 as soon as the caseous matter has become coagulated. 

 It was on this account formerly thought that the 

 cause of blueness in milk was connected with the 

 casein, and that it was produced by some chemical 

 ferment which acted upon the casein. We now 

 know, however, that blueness in milk is due to the 

 action of micro - organisms. The history of this 

 discovery is an interesting one, and much research 

 has been devoted to its elucidation. The organism 

 chiefly implicated in the action has been called the 

 bacillus cyanogenus. d It cannot produce a blue colour 

 apart from milk, and seems to act on the casein and 

 to have no effect on the milk-sugar. Indeed, it 

 would seem that the explanation of blue milk is a 

 double one, and that it is the joint work of the 

 above-mentioned bacillus in conjunction with the 

 lactic organism, since it has been found that if the 

 bacillus cyanogenus be inoculated into sterilised 

 .jnilk, it does not produce a blue colour, this being 

 "only produced by the addition of a little acid/ or by 

 inoculating the milk with lactic organisms. It thus 

 seems to be an example of what is known as 

 symbiosis, that is, the association of several species 

 of micro-organisms, working together to produce 

 some special effect. We have further examples of 

 such " feeding " associations, in other departments of 



