IOWA ACADEMY OF SCIENCES. 255 



making parallel cultures the relative number of gas pro- 

 ducers present in a c.c. of sewage. 



A method which has given excellent results is as fol- 

 lows: Take a tube of ordinary agar, melt and pour in a 

 petri dish, after it has cooled to such a degree that it is 

 just liquid, add one-tenth cc. of sewage and immediately 

 turn it around rapidly in order to secure equal distribu- 

 tion of the sewage; then, after it has been cooled so far as 

 to become solid, add another tube of melted agar, care 

 being taken that it is not too hot, after which, without 

 stirring, set it away to develop. This last agar forms a 

 layer containing no germs, if the work has been properly 

 done. The anaerobic gas producers working in the lower 

 portion produce gas, which appears in the agar as minute 

 air bubbles. 



The effluent of July 12, 1900, after standing one week, 

 showed 25 gas producers in the plate, and as one-tenth 

 c.c. of sewage from the effluent was used in making the 

 culture, there would be 250 gas producers to the c.c. of 

 effluent. The number of germs counted from a parallel 

 culture was 2,400, which means that approximately ten 

 per cent of the total number of germs were gas producers, 

 the above result being obtained from the sample of efflu- 

 ent taken from the west bed. The temperature of the air 

 and sewage being 72° Fahrenheit. A similar culture from 

 the tank on the same date showed 118 gas producers in the 

 plate, making 114, 130 gas producers to the c.c. of sewage, 

 or about 33| per cent of the germs in the tank at that time 

 were gas producers, the temperature of the sewage in the 

 tank being 62°. The total number of germs for the c.c. 

 being 342,400. 



The manhole sample taken July 12th and examined July 

 17th, shows a still greater percentage, there being 104,030 

 gas producers to the c.c, or 43 per cent of the germs in the 

 raw sewage at that time were gas producers. The tem- 

 perature of the raw sewage was 68°, the total number of 

 germs to the c.c. on an ager culture being 242,400. Other 

 cultures were made in the same manner, with approxi- 

 mately the same results. 



