134 BACTERIA IN WATER. 



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 practically 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 River, 

 below Albany. It has become a matter of great importance to 

 know whether freezing does purify such ice and render it safe. 

 The subject has been most carefully investigated, with the following 

 conclusion. Ice does in a measure purify itself in freezing, but not 

 wholly. If typhoid bacilli are in the water, they may be found in 

 the loose snow ice at the top of the frozen layer, but there are very 

 few, if any, in the clear ice below. After the ice has been stored 

 for a while the typhoid bacilli become less and less abundant, and 

 after a few weeks they practically disappear. Even after months 

 of freezing, however, a few may sometimes be found, so that no 

 ice from contaminated water can be guaranteed as absolutely free 

 from them even after six months' storage. But the number that 

 resists this storage is so extremely small that the ice is as pure as 

 filtered water. No cases of typhoid fever have been definitely 

 traced to such a source, though one or two doubtful cases have 

 been so attributed. In general, then, it appears that stored ice 



