236 STEPHEN DEM. GAGE 



Numbers of bacteria, 50 series. The average numbers of bac- 

 teria and the numbers of acid-producers determined at 20, 40, 

 and 50 on five samples each from 26 different sources are shown 

 in Table 4. In general the numbers of bacteria at 20 and 40 

 and the numbers of acid-producers determined at those tempera- 

 tures, are large or small as the water is polluted or non-polluted, 

 confirming the findings previously discussed under the 30 series. 

 The most significant results are those obtained for the two shallow 

 wells. The number of bacteria determined at 20 is higher in Well 

 No. i than in Well No. 2. Well No. 2 is in a thickly settled communi- 

 ty with vaults and cesspools in close proximity, while Well No. i, sit- 

 uated in an open field upon the top of a hill, is removed from any 

 chance of pollution. Chemical analyses extending over a period 

 of some years indicate that Well No. i is free from pollution, and 

 that Well No. 2 is seriously polluted. The sanitary survey and chemi- 

 cal analyses are confirmed by the bacterial count at 40 and by 

 the numbers of acid-producers developing at 20 and 40, showing 

 that the large numbers of bacteria determined at 20 in Well No. i 

 are of a harmless character. 



The numbers of bacteria and the acid-formers determined at 

 50 C. confirm the results of determinations at 20 and 40, but 

 the distinction between different classes of waters is more marked 

 than by determinations at the lower temperatures. It is noticeable 

 that the 50 bacteria in the effluents from the Contact Filters Nos. 

 175 and 176 were higher than in the regular sewage which was ap- 

 plied to those filters, and the numbers in the effluent of Contact 

 Filter No. 251 were larger than in the septic sewage with which it 

 was operated. On the other hand, the numbers in the effluents 

 from the trickling filters were small, and this type of bacteria were 

 either entirely lacking or present in insignificant numbers in the 

 effluents from the sand filters. The distinction between the river 

 water and the filtered waters is not very well marked, but a class dis- 

 tinction between the river water and the filtered river waters, and the 

 ponds, wells, and springs, is indicated by the entire absence of this 

 type of organisms in the latter class of waters. The occurrence 

 of 50 acid-producing bacteria is also significant, this type of organ- 

 isms being absent from the effluents from three out of four of the 



