BACTERIOLOGICAL STUDY OF WATER 641 



favorable temperature; probably the antagonistic influence 

 of more hardy saprophytic bacteria, particularly the so- 

 called "water-bacteria," and of more highly organized 

 water-plants; the effect of precipitation and of sedimenta- 

 tion; and, of great importance, the disinfecting action of 

 direct sunlight. 



Though the positive demonstration of typhoid bacilli in 

 drinking-water by bacteriological methods is of extreme 

 rarity, it must not be concluded that bacteriological analyses 

 of suspicious waters shed no light upon the existence of 

 pollution and the suitability or non-suitability of the water 

 for drinking-purposes. 



In the normal intestinal tract of human beings and 

 domestic animals, as well as associated with the specific 

 disease-producing bacillus in the intestines of typhoid- 

 fever patients, is an organism that is frequently found in 

 polluted drinking-waters, and whose presence is indicative 

 of pollution by either normal or diseased intestinal con- 

 tents; and though efforts may result in failure to detect 

 the specific bacillus of typhoid-fever, the finding of the 

 other organism, bacillus coli, justifies one in concluding 

 that the water under consideration has been polluted by 

 intestinal evacuations from either human beings or animals. 

 Waters so exposed as to be liable to such pollution should 

 never be considered as other than a continuous source of 

 danger to those using them. 



Another point to be remembered is in connection with 

 chlorine as an indicator of contamination by human excre- 

 ment. It is commonly taught that an excessive amount 

 of chlorine in water points to contamination by human 

 excreta. This may or may not be true, according to cir- 

 cumstances. A high proportion of this element in a sample 

 41 



