130 BACTERIA IN WATER. 



contaminated; for while sometimes the contamination is so great as to 

 be evident, in most cases, especially in wells, it cannot be detected 

 by ordinary examinations. The chemical analysis of water gives no 

 sure indication, and the determination of the number of bacteria 

 alone is only suggestive. It chances, however, that there is a 

 species of bacterium called B. coli, that is a common inhabitant of 

 the human intestine, but is rarely found free in nature or inhabiting 

 pure waters. This B. coli is so abundant in feces that it is practically 

 sure to be found in all sewage-contaminated waters, while it 

 is not found, at least to any great extent, in water free from sewage 

 contamination. Since this bacillus is fairly easy to recognize by 

 bacteriological methods, it is not difficult to determine whether 

 or not it is present in a sample of water. Hence this bacterium 

 becomes a test for sewage pollution. A sample of water showing 

 the presence of B. coli is almost surely contaminated by sewage, 

 while water free from it is not thus polluted. The report from a 

 bacteriologist that B. coli is found means, then, that the water is 

 unsafe, since sewage contamination may at any time infest it with 

 typhoid germs. While B. coli itself is harmless, its presence indi- 

 cates the certainty of danger. 



PURITY OF DRINKING-WATER FROM DIFFERENT SOURCES. 



Recognizing sewage contamination as the great source of danger 

 in drinking-water, we may classify waters as pure or safe in propor- 

 tion to their freedom from such contamination. 



Water from Streams. Ordinary streams are the most likely 

 to be sewage contaminated. They constitute the drainage system 

 of the land, receiving sewage from towns, villages, and cities. The 

 amount of sewage, and hence the extent of the danger, depends upon 

 the number of people contributing to produce it and upon the size 

 of the stream. The only safe position to hold, however, is that 

 all streams upon whose banks are human habitations are polluted 

 and unsafe for drinking. The question of the purification of such 

 water will be noticed later. 



Wells. Next to running streams, wells are the most dangerous 

 source of drinking-water. The extent of the danger depends upon 



