52 Elements of Water Bacteriology. 



case, however, the appearance of the plates at once reveal, 

 the peculiar conditions, for the colonies are 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 tjie others, contained 

 5000, all belonging to a single species common in the air. 

 Except in such peculiar cases as this, high numbers in a 

 ground-water mean contamination. 



The process of slow sand nitration for the purification 

 of unprotected surface-waters 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 furnish the best standard for 

 measuring the efficiency of such filtration plants. At 

 Lawrence, in 1905, Clark found an average of 12,700 bac- 

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

 the number present in the filtered water was only 70 

 (Massachusetts State Board of Health, 1906). Where 

 the number of bacteria in the applied water is smaller it 

 is difficult to obtain so high a percentage efficiency. At 



