230 STEPHEN DEM. GAGE 



direct comparison, however, was possible between the results obtained 

 at 30 C. and 50 C., and we are forced to make such a comparison 

 indirectly through the counts at 20 C. and 40 C. It should be 

 further borne in mind that, while the series containing the 50 C. 

 counts covered a wide range of samples, only five samples were 

 used from each of the several sources, and these samples were all 

 collected within a limited period of time. For this reason the results 

 of this series, while indicative of the results which might be obtained 

 in practice, should not be taken as conclusive evidence of the value 

 of the 50 counts. On the other hand, the results obtained in the 

 30 series, while covering a smaller range of waters, were obtained 

 from the examination of over 400 samples collected over a period 

 of many months. 



Information bearing on the subject at hand has also been drawn 

 from the routine analyses, as follows: About 80 samples from 15 

 different wells, over 200 samples of sea water from different loca- 

 tions in Boston Harbor, and nearly 1,000 samples of polluted river 

 water upon which bacterial counts at temperatures of 20 and 40 

 have been made, the counts at 40 being on litmus-lactose agar, and 

 including both total colonies and red colonies, determinations of 

 numbers of bacteria and B. coli in over 300 samples of sewage and 

 effluents from sewage filters, and in about 3,700 samples of Merri- 

 mack River water from two sources; from which data we have 

 been able to determine very accurately the average ratio of bacteria 

 to B. coli for these classes of samples, and in the river water results 

 to study the various factors which have caused changes in the num- 

 bers of bacteria and of B. coli, and the ratios between the two. 



Classification of waters included in the investigation. During 

 the various investigations 17 different classes of samples have 

 been examined at one time or another, and it is necessary, for 

 a clear understanding of the tables, that a brief description of these 

 classes of samples should be inserted at this point. 



Lawrence Street sewage is a strong domestic sewage collected directly from one 

 of the large sewers in the city of Lawrence. Regular sewage is sewage from the same 

 source as the foregoing, conveyed to the Experiment Station through about a mile 

 of 2 . 5 inch pipe, and differs from the Lawrence Street sewage mainly in the fact that 

 the grosser particles are very largely broken up by abrasion and by passing through 

 the pump. Station sewage is regular sewage diluted with a small proportion of Mer- 



