40 ELEMENTS OF WATER BACTERIOLOGY. 



require a comparatively small amount of nutriment may 

 multiply at times in a deep well or the basin of a 

 spring. In such a case, however, the appearance of the 

 plates would at once reveal the peculiar conditions, for the 

 colonies would all be of one kind and that distinct from 

 any of the sewage species. Thus Dunham (Dunham, 1889) 

 reports that the mixed water from a series of driven wells 

 gave 2 bacteria per c.c., while another well, situated just 

 like the others, contained 5000, all belonging to a single 

 species common in the air. Plates from polluted water 

 contain, on the other hand, a wide variety of species. 



The process of slow sand nitration for the purification 

 of unprotected surface-water is essentially similar to the 

 action which takes place in nature when rain soaks through 

 the ground to appear in wells and springs; and it is in the 

 examination of the effluent from such municipal plants 

 that the quantitative bacteriological analysis finds, per- 

 haps, its most important application. The chemical 

 changes which occur in the passage of water through 

 sand at a rate of 1,000,000 or 2,000,000 gallons per acre 

 per day are so slight as to be negligible. The bacteria 

 present should, however, suffer a reduction of 98 or 99 per 

 cent, and their numbers are, therefore, used as the standard 

 for measuring the efficiency of such filtration plants. At 

 Lawrence, in 1901, Clark found an average of 3017 bac- 

 teria per cc. in the raw water of the Merrimac River, while 

 the number present in the filtered water was only 26 

 (Massachusetts State Board of Health, 1902). Mechan- 

 ical filtration gives similar results. Fuller at Cincinnati 



