BACTERIA DEVELOPING AT DIFFERENT TEMPERATURES 231 



rimack River water. Septic sewage: Under "septic sewage" has been included 

 effluents from three septic tanks, all of which treat regular sewage. Strained sewage 

 is regular sewage from which a portion of the suspended matter has been removed 

 by straining through coal. Intermittent sewage filters: Samples from a number of 

 intermittent sand filters operating at low rates with regular sewage or with station 

 sewage are included in the various tables. Trickling filters: Samples from two differ- 

 ent trickling niters operating at high rates with regular sewage are included. Con- 

 tact filters: Samples from three different contact filters have been used, No. 175 oper- 

 ating with regular sewage, No. 176 with strained sewage, and No. 251 with septic 

 sewage. Merrimack River water: Samples from two sources have been used, the 

 source labeled "Intake" including samples of the water as it flows upon the Law- 

 rence city filter. Samples labeled "Canal" are the river water as received at the 

 Experiment Station after passing through the North Canal. The distance from 

 the Experiment Station to the head of the canal is about one mile, and that from the 

 head of the canal to the Intake of the City Filter about one mile, no sewage entering 

 the river or canal between these points. Applied 216: This is canal water which 

 has been treated with alum and settled before being applied to Filter No. 216 

 Water filters: "Filter No. 216" is a mechanical filter operating at a high rate with 

 the above. "Filters Nos. 8, 220," and the "City Filter" are slow sand filters oper- 

 ating at low rates, the city filter receiving Merrimack River water designated "Intake," 

 and the others canal water. "Filters Nos. 243 and 244" are secondary filters oper- 

 ating with water which has already been filtered. Tap water: This is filtered water 

 from the Lawrence city filter, after being stored in the distribution reservoir and 

 passing through the city service mains. Ponds: Many samples from two large ponds, 

 both used for water supply, have been examined. The watersheds of both of these 

 ponds are under sanitary control, but both are used more or less for pleasure pur- 

 poses in the summer, and hence are liable to occasional contamination. Driven 

 wells: The samples were from two series of driven wells less than 50 feet deep used 

 as water supply. Shallow wells: Samples from 15 different wells, the results from 

 both good and polluted wells being averaged together. In tables where results from 

 two different wells are given, "No. i" is of excellent quality, while "No. 2" is badly 

 polluted. Springs: Samples from two different springs, both of good quality, are 

 included. Sea waters include samples collected from a large number of stations 

 during a sanitary survey of Boston Harbor. Most of these samples were more or 

 less polluted with sewage. 



Selective action of different temperatures. In considering the 

 value of counts of total bacteria and of acid-producing bacteria, 

 in addition to the relative numbers obtained by such counts, we 

 should know the selective action of the different temperatures upon 

 the bacterial content of the various classes of waters. We have 

 learned by these experiments that nearly all of the bacteria 

 capable of forming colonies at 20 C. will manifest themselves in 

 four days, this being also true of the acid-producing bacteria. At 

 40 C. and 50 C. nearly all the bacteria and acid-producers capable 

 of developing at these temperatures are shown by a count made after 



