THE ACTION OF CERTAIN BACTERIA ON SEWAGE 145 
aerobic conditions at room temperature, both the nitrites and 
nitrates were increased, while the organie nitrogen was de- 
creased. These changes did not balance, leaving a large defi- 
cit in the organic nitrogen which has not been satisfactorily 
accounted for. 
B. subtilis—This micro-organism gave really consistent 
results. They were obtained under aerobic conditions and 
confined themselves to the free ammonia and the organic 
nitrogen. There was in all of the experiments an increase in 
the free ammonia with a decrease in the organic nitrogen that 
practically balanced it. This activity was relatively very 
great, amounting to as much as eight parts per million 
at 37.5° C. and two parts per million at room temperature. 
In regard to the nitrites. and nitrates, the tendency was more 
often to decrease than to increase them, but this action was 
not constant. 
Under anaerobic conditions the action of the bacteria was 
regular at 37.5° C., both the free ammonia and the organic 
nitrogen being decidedly decreased with an increase in the 
nitrites and nitrates, the latter being very decided. At room 
temperature the action was not so marked and in general 
followed that of the aerobic conditions, showing an increase 
in the free ammonia and a decrease in the organic nitrogen. 
From the results which were obtained, I believe that this 
micro-organism and the entire group of allied micro-organisms 
will bear much careful study in their relation to the bacterial 
processes in the purification of sewage. 
The action of mixed cultures.—I have already pointed 
out that in mixed cultures B. coli was completely overgrown 
in the anaerobic flasks, and way overgrown by all of the bac- 
teria in the aerobic flasks, with the exception of B. pyocyaneus. 
Also that B. subtilis and B. proteus (vulgarus) overgrew the 
other bacteria in both sets of flasks, When grown together they 
about balanced each other. 
The fact that the chemical action very closely followed 
the predominating bacteria was extremely intcresting. The 
action was in certain cases, as I will point out later, very dif- 
ferent in the aerobic and anaerobic flasks with the same 
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