BACTERIA IN WATER. 635 
and numerous observations show that the number of bacteria in river 
water is greatly increased in the vicinity of and below the mouths 
of city sewers. 
We conclude from the experimental data recorded that water 
containing less than 100 bacteria to the cubic centimetre is presum- 
ably from a deep source and uncontaminated by surface drainage, 
and that it will usually be safe to recommend such water for drink- 
ing purposes, unless it contains injurious mineral substances. 
Water that contains more than 500 bacteria to the cubic centimetre, 
although it may in many cases be harmless, is to be looked upon 
with some suspicion, and water containing 1,000 or more bacteria is 
presumably contaminated by sewage or surface drainage and should 
be rejected or filtered before it is used for drinking purposes. But, 
as heretofore stated, the danger does not depend directly upon the 
number of bacteria present, but upon contamination with pathogenic 
species which are liable to be present in surface water and sewage. 
In swallowing a glassful of pure spring water a number of bacteria 
from the buccal cavity are washed away and carried into the stomach, 
which, if enumerated, would doubtless far exceed in numbers those 
found in the most impure river water. 
The number of bacteria does not depend alone upon the amount 
of organic pabulum contained in a water, and cannot be depended 
upon in forming an estimate of this; for, as has been shown by 
Bolton, certain water bacteria multiply abundantly in water con- 
taining comparatively little organic matter, while other species fail 
to grow unless the quantity is greater. In a water containing con- 
siderable nutrient material the water bacteria may be restrained in 
their development by other species present until the amount of pabu- 
lum is reduced so that these no longer thrive, when the common 
water bacteria will take the precedence, and an enumeration may 
show a greater number of colonies than at first. But, in general, 
water rich in organic material contains a greater number of bacteria 
and a greater variety of species than that which is comparatively 
pure. 
That certain bacteria may multiply in water which has been 
carefully distilled has been shown by Bolton and others. Two com- 
mon water bacteria—Micrococcus aquatilis and Bacillus erythrospo- 
rus—multiplied abundantly in doubly distilled water, and when 
this water was again sterilized and re-inoculated with one of these 
species the same abundant increase occurred. This was repeated six 
times with the same result (Bolton). Computing the number of 
these water bacteria in ten cubic centimetres of distilled water at 
twenty millions, and estimating their specific gravity at one, and the 
diameter of the individual cells at one s, the total weight of the entire 
