BACTERIA IN SOIL, AIR, WATER, AND MILK 199 
group, dysentery and cholera. Most other harmful bac¬ 
teria are short-lived in water, but those mentioned thrive 
there, and have often caused severe epidemics. 
Should water become contaminated with these germs, 
it may be purified by boiling and distillation. Where a 
large body of water must be purified, science has devised 
the filters and the action of chemicals. With the use of 
these agents from 95 to 98 per cent of the bacteria is 
destroyed. 
Many cities now filter their water supply. The water 
is pooled in large reservoirs. The bottom of these reser¬ 
voirs is composed of a layer of coarse and a layer of fine 
gravel, on top of this is a strata of coarse and fine sand. 
The water percolates through these at a certain rate, 
as it does so the impurities and bacteria are deposited. 
After a certain time the deposit must be removed. 
There are many kinds of domestic filters, although only 
a few can be recommended for permanent use. Many, 
particularly the cheaper kind, are actually harmful. It 
is impossible to clean them properly, so that the bac¬ 
teria they hold back breed there. The better and more 
expensive types, to be efficient, need constant care and 
should be cleaned and sterilized at regular intervals. 
When the germ content of water is unknown, during 
Certain seasons and epidemics, the use of boiled water 
for drinking purposes should be encouraged. All bac¬ 
teria are destroyed by this process. The flat, insipid 
taste resulting may be partially overcome by shaking the 
water in open vessels, aerating it. 
Distillation is the process of driving steam into a cool 
jar where it condenses; the water resulting is not only 
free from bacteria but all mineral. It is used largely 
in the laboratory, but is not recommended for drinking 
purposes. When taken into the body in large quanti- 
