140 BACTERIA IN WATER 
poses has had to be abandoned because it has become so polluted 
with sewage as to be no longer safe. It is thus a matter of prime 
necessity to find some delicate means of determining whether water 
is sewage contaminated; for while sometimes the contamination is 
so great as to be evident, in most cases, especially in wells, it can- 
not 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 type of bacterium called B. coli, that is a common in- 
habitant 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 deter- 
mine whether or not it is present ina 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. colz is found means, then, that the 
water is probably unsafe, since sewage contamination may at any 
time infest it with typhoid germs. While B. cold itself is harmless, 
its presence indicates the certainty of danger. 
PURITY OF DRINKING-WATER FROM DIFFERENT SOURCES 
Recognizing sewage contamination as the great source of dan- 
ger in drinking-water, we may classify waters as pure or safe in 
proportion 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 up- 
on the number of people contributing to produce it and upon the 
size of thestream. The only safe position to hold, however, is that 
