144 BACTERIA IN WATER 
surprise to be told that stored water is purer and safer than running 
water, but study and experience have shown this to be positively 
the case. 
Springs.—These are thoroughly reliable sources of drinking- 
water if they are properly guarded. The water comes from under- 
ground and has filtered through the soil for unknown distances. 
There may be cases, it is true, where the filtering is through only 
a thin layer of soil, insufficient to purify. But if such cases exist, 
they are very unusual, and examination shows spring-water to be 
free from disease germs, unless carelessly contaminated after the 
water leaves the soil. The spring should be classed with the 
artesian well in this respect, and is the best source of water that 
can be obtained. 
Filtered Water.—The rapidly extending contamination of 
waters by sewage and the growing demand for water have led 
to development of methods of filtering such contaminated water 
in large quantities. ‘This is done by passing it through layers 
of sand which are constructed in such a way as to remove most 
of the bacteria. These filters are in wide use to-day by cities 
that have to depend upon a contaminated supply. The bacteria 
are not wholly removed from the water, but so nearly that prac- 
tically all dangers disappear. Experience has shown that the 
use of filters very greatly reduces the amount of typhoid fever 
in cities dependent upon a contaminated water-supply. It is 
also found that the purified water improves the general health 
of the community, quite apart from the decrease in typhoid fever. 
Ice.—Ice, though not thought of as water, in summer months 
is put into drinking-water to cool it. ‘The ice melts and whatever 
bacteria are in it are liberated and swallowed with the water. It 
has been a belief that freezing purifies water, so that many have 
been perfectly willing to use ice from ponds whose water they 
would not drink. It is a very wide practice to cut the year’s ice 
supply from sewage-contaminated streams, and from places where 
no one would think of drinking the water: e.g., from the Hudson 
