114 
BACTERIOLOGY. 
Artificial 
methods of 
purification 
poses: first, because drinking-water should not con¬ 
tain substances excreted from the intestines of man or 
animals, and, secondly, water that does contain such 
substances is constantly open to infection with bacteria 
that produce disease. 
Nowadays practically all surface waters are con¬ 
taminated with human sewage. To render these 
waters safe for drinking purposes in cities, the natural 
process of water purification cannot be relied upon, and 
artificial methods, based on filtration, are employed. 
The water may be made to percolate through beds 
made of fine gravel and covered with a thick layer of 
fine sand. The dirt and slime in the water cling to 
the small particles of the sand, and only the water free 
from its impurities is permitted to pass through. 
About 90 per cent, of the bacteria in water can be 
removed by sand filtration. In mechanical filtration, 
a chemical substance like alum is added to the water 
in sufficient quantity to coagulate the solid and ex¬ 
traneous materials, which sink and carry the bacteria 
with them. In the home, water may be rendered pure 
by filtration through porcelain filters, and, where these 
are not available, by boiling. The flat taste of boiled 
water may be removed by passing the water from one 
container to another so that air may be mixed with it. 
