1 86 Elements of Water Bacteriology. 



bacteria is of subsidiary importance. Rideal (1906) has 

 recently commended a somewhat less extensive series o'f 

 tests, including aerobic and anaerobic counts, both at 20 

 and 37 degrees, with the determination of the number of 

 liquefiers and the number of spore formers. The results 

 attained do not seem to warrant any such elaborate pro- 

 cedure. As far as the authors are aware, the determination 

 of liquefying bacteria, anaerobic bacteria and thermo- 

 philic bacteria does not add any information of material 

 importance to that obtained from the total count. Some 

 test for specific sewage organisms is of course desirable. 

 Here again, however, the determination of B. sporogenes 

 and sewage streptococci tells the observer little more than 

 can be learned from the routine use of the colon test. In 

 the United States the practice of sewage bacteriologists is 

 crystallizing around the total count and the estimation of 

 B. coli. In the absence of evidence as to the specific 

 value of other data, the routine control of filter plants 

 may well be limited to these two determinations. 



The total count of bacteria should be made, as in the 

 case of waters, on gelatin at 20 degrees. Determinations 

 carried out in duplicate on agar at 37 degrees add addi- 

 tional information of considerable value. The ratio of 

 the 37-degree count to the 20-degree count varies with 

 different sewages. At Boston the body temperature count 

 is 70 to 80 per cent of the total count; at Lawrence it 

 appears to be proportionately much lower (Gage, 1906). 

 In using either medium, it is well to add lactose and lit- 



