84 Bacteria in Relation to Country Life 



by methods other than sedimentation and sand-filtration. 

 Some of these involve the removal of the bacteria by 

 chemical substances added to the water, although the 

 process by which this is effected may be, in itself, largely 

 mechanical. 



The alum method. One of the methods in question 

 is based on the addition to the water of alum at the rate 

 of J to J grain per gallon. In the presence of the car- 

 bonate in the water, the alum is decomposed with 

 the formation of a jelly-like mass, which gradually 

 subsides and carries down with it the suspended parti- 

 cles, including the bacteria. It appears, moreover, that 

 not only are the bacteria carried down by the alum, 

 but that the latter exercises a deleterious effect on their 

 subsequent redistribution in the upper layers of the 

 liquid. The alum possesses the further advantage of 

 combining with the organic substances dissolved in the 

 water and of decolorizing it. 



After being clarified by the precipitated alum, the 

 water is made to pass through layers of sand. Thus it 

 receives the benefit of careful filtration. When the 

 water is soft and deficient in carbonates for the decom- 

 position of the alum, the filtering material is made up 

 of a mixture of sand and crushed marble. In the prac- 

 cal application of the method care is taken, of course, 

 not to add an excess of alum lest a portion of it be carried 

 over into the purified water. It is asserted that water 

 purification involving the use of alum is both efficient 

 and economical, particularly on account of the saving 

 of space that results from its use. 



The Clark process. There are other methods of 



