52 WATER 



topographical examination showed no possibility of human con- 

 tamination the following may be quoted : 



B. coli 



Source 1 organisms per c.c. group organisms in 



0'5 2 10 40 C.C. 



- - + + 



- - - + 



- + 



It is obvious that for this class of waters bacteriological 

 analyses must be interpreted with considerable caution and only 

 in the light of all the topographical and other data available. 

 While the addition of animal excreta is not of anything like the 

 same potential harmfulness as human excreta, yet in considerable 

 amount it is unsatisfactory and should be prevented. It is there- 

 fore justifiable to look adversely upon water supplies of this 

 character which contain a large number of B. coli and possibly 

 other indicator organisms. Each case requires careful considera- 

 tion but in general the presence of * excretal ' B. coli in I c.c. or 

 less of such waters points to heavy and undesirable pollution 

 while if less numerous special topographical investigation is 

 required, 



Deep water supplies. 



Waters of this class deep well or spring are derived from 

 rain which has filtered through a considerable depth of soil and 

 which has usually taken a considerable time over the process. 

 The deeper layers of soil are germ free and in its passage the 

 bacilli are filtered out of the water. 



1 For a detailed consideration of the bacterial content of upland surface waters, 

 especially in relation to topographical findings, see Savage, 1902, Journ. of Hygiene y 

 IT, p. 320. 



