256 IOWA ACADEMY OF SCIENCES. 



It will be noticed that the percentage of gas producers 

 is highest in the manhole, and low^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. prodigiosns, 

 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 



