BACTERIA IN WATER AND MILK. H3 
First, there is sedimentation or the sinking of 
impurities by reason of their weight. The effect of 
sedimentation can be seen after floods, where the mud 
and dirt is found over the flooded areas. Sedimenta¬ 
tion takes place slowly; so in streams that are flowing 
fast it cannot be relied upon to remove much of the 
impurities. Aeration is another factor. This means 
the mixing of water with air, as takes place, for ex¬ 
ample, in water-falls. It does not destroy the bacteria 
but it removes qbjectionable odors. Sunlight exerts a 
powerful destructive action on the bacteria in water, 
provided the depth of the water is not too great for 
the sunlight to penetrate. Unfortunately, the pene¬ 
trating power of sunlight is not great; so its action is 
limited to the upper layers of the water. The ground 
water is purified by the filtering action of the soil, 
which is very efficient, provided the amount of water 
to be filtered is not too great and it is not required to 
work continuously. 
The ordinary soil bacteria in water are harmless. 
It is only the pathogenic bacteria in the soil from 
human excreta, like the typhoid and dysentery bacilli 
and the cholera- spirilla, that get into the water and 
cause disease. In testing the water to see whether it 
can transmit these diseases or not, it is almost useless 
to look for the disease-producing bacteria themselves, 
because they are extremely difficult to find. The pres¬ 
ence of intestinal bacteria is looked for, particularly 
the colon bacillus, and when they are found in large 
numbers the water is condemned for drinking pur- 
8 
Natural 
methods 
of puri¬ 
fication 
