IOWA ACADEMY OF SCIENCES. 259 



into account in considering the cause of these fluctuations. 

 One is that during March and April there are greater fluct- 

 uations in temperature, as well as in the humidity of the 

 atmosphere and it is possible that such a condition might 

 favor the rapid multiplication of bacteria. Again, in July 

 and August and the major part of June, college was not in 

 session, hence the sewage was not so strong. In a general 

 way it would appear that the factors considered in connec- 

 tion with the fl actuations of the efiluent are applicable 

 here also. 



One point which is rather interesting is that on different 

 occasions, (June, July, 1900, also December of the same 

 year), the average number of germs to the c.c. in the tank 

 was larger than that of the manhole for the same period. 

 The explanation of this seeming inconsistency seems sim- 

 ple enough after taking into consideration the fact that 

 bacteria increase very rapidly, and that the sewage is 

 allowed to collect in the tank until 20,000 gallons have 

 been accumulated, when it is discharged automatically by 

 a Miller's Automatic Siphon. Now, if the flow in the tank 

 is slow (which is often the case) for any reason, the water 

 stands longer, and hence more time is given the bacteria 

 to multiply. 



It must be borne in mind that the environment in the 

 tank is especially favorable for the rapid production of bac- 

 teria as there is an abundance of organic matter present, 

 while the tank being closed would tend to raise the tem- 

 perature of the sewage rather than to lower it, which 

 would further facilitate the rapid development of germ 

 life. Leone's experiments on the preserving of the Mang- 

 fall water shows very clearly what might be expected from 

 letting sewage accumulate slowly in the tank. It takes 

 some times seventeen to twenty hours and even longer for 

 the tank to fill. Below are given the tabulated results of 

 his experiments together with some similar observations 

 made by Cramer on the water from the Lake of Zurich. 



