126 SCIENTIFIC REPORTS OF THE AGRICULTURAL RESEARCH 



This series of plates showed the very great variation in this 

 respect which occurs in river water from day to day accord- 

 ing to the incidence of the rainfall; immediately after rain 

 large numbers of bacteria, derived no doubt from surface 

 wash,- appear in the river water ; after 24 hours or less this 

 extra supply may have completely disappeared but in the 

 meantime the character of the fermentation may be entirely 

 changed owing to the presence of a relatively enormous 

 number of bacteria in the steeping vat. It is a serious 

 defect in the design of the Pusa factory that no provision 

 exists for settling or filtration of the water, so that in a 

 larger number of cases the extreme dirtiness of the water 

 used introduced excessive numbers of bacteria of all sorts, 

 the activities of which so far complicated the results as to 

 make the latter of little more than negative value. Thus 

 it will easily be realized that when endeavouring to com- 

 pare an artificially inoculated vat with an uninoculated 

 control, the existence in the water of both of a natural bac- 

 terial population already up to the limit of its capacity 

 would inevitably reduce the artificially induced difference 

 in bacterial content to a negligible quantity. A natural 

 criticism immediately suggests itself; would it be practi- 

 cable on a factory scale to remove any such excess of bac- 

 teria in order to make room for artificially introduced 

 species? It may be said at once that the importance of 

 this point was realized very early in the course of the 

 experiments, and our aim has been to find some practicable 

 means of overcoming the difficulty. 



In order to understand fully the points involved it 

 must be realized that so far as indigo manufacture is con- 

 cerned we may divide bacteria into three classes : — 



(1) active hydrolyzers, 



(2) inert so far as indican is concerned, and 



(3) active destroyers either of indican or indoxyl or 



both. 



The artificial cultures isolated at Pusa have been selected 

 for their positive and negative virtues, i.e., they are active 



