258 
IOWA ACADEMY OF SCIENCES. 
third millions to the c.c. If this were the principal cause 
of fluctuation the effluent of November 14th should con- 
tain about 41,000 bacteria to the c.c. other things being 
equal. 
Along with the above the amount of organic matter 
present would have a considerable influence, as it would 
serve as food for the bacteria. Hence fission would be 
more rapid and the number of bacteria to the c.c. 
increased, but no data bearing on this point are at hand. 
The third factor would be the time of taking the 
samples, whether at the beginning or toward the end of 
the discharge. It is presumed that during the period that 
the bed is resting the bacterial life increases; accumulat- 
ing in the interstices between the material of which the 
filter is composed. When the discharge comes on the 
beds the pressure and hence the force being greater at 
that time than at any other, also the number of the bac- 
teria in the interstices being greatest then, might not the 
force of the sewage wash these bacteria free and hence 
through the bed into the effluent? If such be the case the 
number of bacteria to a c.c. would be greatest at the 
beginning of the discharge and least at the end. While I 
have not been able to make experiments to fully elucidate 
this point I feel quite confident from numerous observa- 
tions in taking samples that such may be the case. 
Of course all of these factors and probably others acting 
in unison complicate the problem to such an extent that 
until more data is at hand it will be impossible to accu- 
rately determine the exact amount of variation caused by 
each factor. 
By referring to the tables containing the average num- 
ber of germs per c.c. for each month, of manhole, tank, and 
effluent, it will be observed that there is considerable fluct- 
uation. It will also be noticed that the results for the 
manhole, tank, and effluent decrease on the whole together. 
The month containing the lowest average for the effluent 
is August in 1900 as well as 1899. The largest average for 
the effluent in 1900 is March after which there is a gradual 
decrease until September. Several things must be taken 
