256 
IOWA ACADEMY OF SCIENCES. 
It will be noticed that the percentage of gas producers 
is highest in the manhole, and low T est in the effluent, 
while the number in the tank lies between, which would 
seem to -show that the gas producers are destroyed while 
the sewage is passing through the tank and filter bed, 
which is very desiraable, in view of the fact that gas pro- 
ducing species, while not actually condemned as patho- 
genic, are to be regarded with suspicion. 
The primary object of bacteriological analysis of sew- 
age is to determine the number of germs present per c.c. 
in the sewage at the different stages of its purification. 
By such data the efficiency of the beds and other parts of 
the system may be readily determined. 
The number of germs present per c.c. determine the 
relative purity of the water, but far more important from 
a sanitary standpoint, is the kind of germs present. 
But little attention has been given to the determination 
of species, except incidentally. Bacillus cloacea , B. coli- 
communis, and some others were determined by Dr. Pam- 
mel and 0. J. Fay, while I have run out B. prodigiosus u 
B. mutabalis, and several other species. 
Bacillus prodigiosus does not occur in the sewage to any 
considerable extent, it having been found up to date only 
three times; once in the tank on June 19th, and twice in 
the effluent, once on the 22nd of June in the east effluent, 
and once on the 27th of June in the west effluent. At 
no time was more than one colony found on the plates 
in any of the above cultures. Its appearance at that time 
is both significant and interesting; significant in showing 
the efficiency of the beds but two colonies having been 
found one coming from each bed, at an interval of five 
days from each other, which would seem to indicate that 
the germs were present in very small quantities and that 
the beds are about equal from the standpoint of efficiency. 
It is interesting from the fact that it presumably found 
its way into the sewage by washing petri dishes contin- , 
ually in a sink in the laboratory which empties into the 
sewer. The original culture having been obtained at 
Marshalltown about the middle of March, 1900. This one 
