Clark Process of Purification 85 



water purification based either on the formation of 

 precipitates which subsequently carry down the sus- 

 pended matter, or on the direct action of the subsiding 

 particles in carrying down the bacteria. One of these, 

 known as the Clark process, involves the use of lime- 

 water and is intended primarily for the softening of 

 hard waters. The lime-water added reacts with the 

 dissolved bicarbonate of lime. The precipitate thus 

 formed gradually subsides, carrying with it, as in the 

 case of alum, the particles in suspension. 



The efficiency of this method was investigated and 

 it was found that the subsidence of the precipitated 

 matter in open tanks was so rapid as to render the water 

 fit for distribution in three hours. The bacteriological 

 examination of samples of softened and unsoftened 

 water gave the following results: 



Bacteria in 

 one cc. 



Unsoftened water 322 



Water after softening and two days' subsidence drawn 



from the main service pipes 4 



Reduction in the number of bacteria, 99 per cent. 



Purification by finely divided solids. — Still other 

 methods of water purification that have been proposed 

 are based on the direct removal of the bacteria by agi- 

 tating the water with finely divided solids. Spongy 

 iron, animal and vegetable charcoal, coke, ground 

 limestone, chalk, infusorial earth, and the like, have 

 shown a more or less high efficiency in this respect. 

 Spongy iron, charcoal and coke are particularly adapted 

 for this purpose because of their structure. 



