142 UNIVERSITY OF WISCONSIN STUDIES 
one set, paraffin oil was added to give anaerobic conditions 
and one was left without the oil to favor aerobic conditions. 
In each set there was one flask which acted as a control. The 
changes in those flasks which were inoculated with bacteria 
were compared with this control, after making allowances for 
difference in composition and errors in methods. 
In eonnection with the dissolved oxygen content, the pre- 
liminary work brought out several interesting facts. It was 
found that the heat of sterilization would in itself drive off 
the greater amount of the dissolved oxygen, but that the flasks 
which were not treated with paraffin oil would pick up oxygen 
rapidly, even at the temperature of the incubator, 37.5° C. 
On the other hand those which had been treated with paraffin 
oil continually lost what little dissolved oxygen there was left. 
There is no doubt but that the unsaturated hydrocarbons of 
the oil extracted the dissolved oxygen, and did it so rapidly 
that at the end of twenty-four hours there was not, for all 
practical purposes, any left. 
The flasks were inoculated twenty-four hours after steriliza- 
tion with 1 ¢. e. of a twenty-four hour bouillon culture of the 
bacteria to be studied and incubated at 37.5° C. for twenty- 
four hours. They were then analysed for free ammonia, or- 
ganic nitrogen, nitrites and nitrates according to the Standard 
Methods for Sewage Analysis. 
Sewage.—The sewage was obtained from the Ward Street 
Pumping Station of the Metropolitan System of Boston, and 
was a typical domestic sewage. The district served by this 
pumping station was mainly residential. There was probably 
a small amount of manufacturing wastes and some storm 
water. Care was taken to obtain the samples at the same time 
each day, so that the sewage would have a similar composition 
throughout the work. 
Bacteria.—The bacteria chosen for the work were not iso- 
lated from the sewage itself. It was thought better to use 
bacteria of known stock, typical of those groups which are 
normally found in sewage. For this reason B. coli, B. cloacae, 
B. pyocyaneus, B. proteus, B. mesentericus var. ruber and 
B. subtilis were selected. All of these have been reported as 
