IN THE VICTORIAN ERA 9 



probability due to the evolution by the bacteria 

 of certain volatile chemical substances which either 

 directly or indirectly enter into reaction with the 

 sensitive film. Similar phenomena have been 

 discovered in regard to many metals as well as 

 organic substances, but this is the first observation 

 which has been recorded of the action of living 

 structures on sensitive films in the dark. 



We have already referred to the important 

 services which Pasteur has rendered by distinguish- 

 ing between different varieties of yeast, and separ- 

 ating them out according to their functions and 

 properties pioneer work which has been followed 

 up by and borne such splendid fruit in the hands of 

 the renowned Danish investigator, Emil Christian 

 Hansen of Copenhagen. This work of isolating 

 out individual varieties of micro-organisms has 

 been not only pursued with the energy familiar 

 to all in the case of bacteria associated with 

 disease, but has been pursued in various other, 

 though perhaps less well known, directions. 



A great deal of activity has lately been ex- 

 hibited in so-called dairy bacteriology, and a long 

 list has already been compiled of milk, cheese, 

 and butter microbes; and agricultural authorities, 

 even in this country, are slowly awakening to the 

 fact that, in order to compete on modern lines 

 with foreign da^iry produce, dairy schools must 



