PURIFICATION OF WATER FOR DRINKING PURPOSES 185 



place. Such improvement is, in fact, most conspicuous 

 in the case of the great natural storage reservoirs con- 

 stituted by lakes. 



This principle of subsidence has not only been 

 employed in the improvement of water intended for 

 domestic and manufacturing purposes, but it has also 

 been abundantly resorted to for the purification of 

 sewage and other refuse liquids. 



Moreover, recognising the observed fact that the 

 improvement in water during subsidence is the more 

 rapid and pronounced the greater the amount of sus- 

 pended matter initially present, the method of purifi- 

 cation by subsidence has been developed by the artificial 

 production of large quantities of suspended matter by 

 the addition of precipitants. Such purification by pre- 

 cipitation has been principally attempted in the case 

 of sewage, but it has also been employed in the shape 

 of the well-known Clark's process for the treatment of 

 hard waters, in which, as has been long ascertained, 

 there is not only a removal of the temporary hardness, 

 but also a marked reduction in the amount of dissolved 

 organic matter. 



With the development of the new methods of bac- 

 teriological water examination, it became obviously a 

 matter of primary importance, therefore, to investigate 

 what was the bacteriological value of these several 

 methods of water purification which had been so long 

 in practical use by water and sewage engineers, and 

 the chemical value of which had already been accu- 

 rately determined. 



These considerations led one of us, as early as 1884, 

 to carry out laboratory experiments on the bacterio- 

 logical value of these subsidence processes in their 

 more important modifications, which investigations 

 were conducted* concurrently with those already de- 



