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 
