THE ACTION OF CERTAIN BACTERIA ON SEWAGE 147 
colt established itself so late in the incubation period that 
the little chemical effect it might have had was not apparent. 
B. coli and B. proteus.—In pure culture both of these 
bacteria had a slight and irregular chemical action. However, 
when grown together, the changes in composition were regu- 
lar and consistent. In both the aerobic and anaerobic flasks 
there was an increase in the free ammonia and a decrease in 
the organic nitrogen. No changes were noted in either the 
nitrites or nitrates. 
B. coli and B. subtilis—In the aerobic flasks the changes 
were those of a pure culture of B. subtilis, 1. e., 
an increase in the free ammonia and a corresponding de- 
crease in the organic nitrogen. Except in one case where there 
was a slight increase in the nitrites, neither the nitrites or 
nitrates showed any change. 
As I have noted, neither of the bacteria has a regular 
chemical action at 37.5° C. under aerobic conditions. How- 
ever, when grown together there is an increase in the free 
ammonia and a decided decrease in the organic nitrogen. 
This increase and decrease balanced each other. 
Neither the nitrites nor nitrates were affected in any way. 
Just what the explanation for this condition is, I am unable 
to say at the present time, as B. cola was completely overgrown. 
B. subtilis and B. cloacae.—In the aerobic flasks, in which 
B. subtilis markedly overgrew B. cloacae, the chemical 
changes were typical of B. subtilis. The free ammonia was in- 
creased with corresponding decrease in the organic nitrogen. 
The nitrates were decreased with an increase in the nitrites. 
In the anaerobic flasks where the overgrowth of B. subtilis 
was not apparent, the chemical changes were in the main those 
of B. cloacae, differing from those of the pure culture in the 
tendency towards increasing the nitrites and nitrates. 
B. subtilis and B. mesentericus var. ruber.-—In the aerobic 
flasks the action was irregular, resembling that of B. mesen- 
tericus var. ruber. Both the nitrites and nitrates were in- 
creased. 
In the anaerobic flasks the free ammonia was increased 
with a corresponding decrease in the organic nitrogen and 
