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 connection 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 c. c. 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 



