PURIFICATION OF WATER FOR DRINKING PURPOSES 193 



series of investigations on this important subject we 

 are indebted to the Massachusetts Board of Health. 

 A detailed description of their experiments and of the 

 results arrived at is given at the close of the present 

 chapter (see p. 205). 



We shall in the next instance pass on to a description 

 of experiments made on the small scale with a view 

 of ascertaining the effect of the subsidence of solid par- 

 ticles of different kinds in water containing bacteria 

 in suspension. It was by means of these laboratory ex- 

 periments in fact that the remarkable removal of micro- 

 organisms by subsidence and entanglement in solid 

 particles was first brought to light and demonstrated. 



PURIFICATION OF WATER BY AGITATION WITH SOLID 

 PARTICLES AND SUBSEQUENT SUBSIDENCE 



Experiments on a small scale. The first experiments 

 of this kind were made by one of us in 1885. 1 In 

 these investigations water containing micro-organisms 

 was shaken up for a definite length of time with a given 

 quantity of finely-divided material (made to pass 

 through a sieve of forty meshes to the linear inch and 

 subsequently sterilised). The water was then allowed 

 to subside, and the clarified liquid submitted to examin- 

 ation as soon as possible after complete subsidence 

 had taken place, as it appeared probable that, if the 

 organisms were simply carried to the bottom by the 

 subsiding particles without suffering any injury, they 

 would rapidly again become distributed through the 

 upper layers of water by multiplication. This redistri- 

 bution of the micro-organisms would of course to a 

 certain extent depend upon the character of the par- 

 ticular va-rieties in the water, e.g. whether they were 



1 ' The Removal of Micro-organisms from Water,' Percy Frankland. 

 Proc. Roy. Soc. 1885. Proceedings Institution of Civil Engineers, 1886. 



O 



