II. 
‘BACTERIA IN WATER. 
THE water of the ocean, of lakes, ponds, and running streams 
necessarily contains bacteria, as they are constantly being carried 
into it by currents of air passing over the neighboring land surfaces, 
and by rain water which washes suspended microérganisms from 
the atmosphere ; and, as such water contains more or less organic 
material in solution, many of the saprophytic bacteria multiply in it 
abundantly. It is only in the water of springs and wells which 
comes from the deeper strata of the soil that they are absent. The 
number and variety of species present in water from any given 
source will depend upon conditions relating to the amount of organic 
pabulum, the temperature, the depth of the water, the fact of its 
being in motion or at rest, its pollution from various sources, etc. 
The comparatively pure water of lakes and running streams contains 
a considerable number of bacteria which find their normal habitat 
in such waters and which multiply abundantly in them, notwith- 
standing the small quantity of organic matter and salts which they 
contain. The water of stagnant, shallow pools, and of sluggish 
streams into which sewage is discharged, contains a far greater 
number and a greater variety of species. 
The study of these bacteria in water has received much attention 
on account of the sanitary questions involved, relating to the use of 
water from various sources for drinking purposes. In the present 
section we shall first give an account of the methods of bacteriologi- 
cal water analysis, and then a condensed statement of results ob- 
tained in the very numerous investigations which have been made. 
A very important point to be kept in view is the fact that a great 
increase in the number of bacteria present, in samples of water col- 
lected for investigation, is likely to occur if these samples are kept 
for some time. A water which, for example, contains only two 
hundred to three hundred bacteria per cubic centimetre when the ex- 
amination is made at once, may contain several thousand at the end 
of twenty-four hours, and at the end of the second or third day 
twenty thousand or more may be present in the same quantity. 
