The Supply and Uses of Water 221 



equivalent to over 2,500,000 gallons an acre a day,which is taken 

 as the best rate of filtration. The operation is not satisfactory 

 for a much higher rate, as it cannot remove many of the dele- 

 terious substances. 



A sand filter depends not so much upon the materials and 

 its mechanical parts as upon bacterial activity. The materials 

 should primarily allow the water to filter through very slowly. 

 Filters freshly installed or cleaned have slight effect on the water. 

 Investigations show that bacteria in water passing through 

 filters just cleaned amount to more than 1800 to each cubic 

 centimeter. Before cleaning there were only forty to a cubic 

 centimeter. On the floor of the filter is formed in time a peculiar 

 gelatin-like substance which is the habitat of beneficial bacteria. 

 Cleaning removes this and largely destroys the bacteria. The 

 mechanical catching of suspended matter is only one part of 

 the function of a filter. The invaluable part is the bacterial 

 destruction of organic matter. 



90. Purification in the home. There are often occasions 

 when water should be purified in the home. The market is 

 well supplied with porous, charcoal, and other ingenious filters. 

 These as may easily be shown are of little value except to 

 remove suspended matter of certain kinds. If the water is 

 suspected of carrying impurities, a simple and effective means 

 of purifying it is by boiling. Disease-causing bacteria and other 

 organisms are killed by boiling. Spores of certain kinds are 

 not so affected ; but these are not considered harmful. Water 

 that is boiled should be allowed to stand before decanting into 

 containers for storage. The peculiar flavor may be remedied 

 by vigorous shaking to aerate it thoroughly, or by cooling. 



SUMMARY 



Cholera, typhoid fever, dysentery, and certain other diseases are 

 often water-borne. 



Many epidemics proven to be due to polluted water show the ne- 

 cessity for purifying water. 



