190 MICRO-OKGAOTSMS IN AVATER 



ment in the Missouri river at St. Louis, at certain seasons 

 ■of the year, amounts to 1"8 per cent, of the bulk of the 

 water. About 94*5 per cent. of this sediment is deposited 

 in the setthng basins in twenty-four hours, during 

 ordinary states of the river ; but for two months in the 

 spring of each year no convenient length of time for 

 settling will clarify the water. That in thus endeavour- 

 ing to remove the coarser suspended particles an im- 

 mense bacterial improvement is brought about in these 

 river waters there can be no doubt ; unfortunately no 

 bacteriological investigations have been made in this 

 direction, but from the experiments made by one of 

 us on the London reservoirs and elsewhere it is obvious 

 that the reduction in the number of micro-organisms 

 present must be very important. This factor of sub- 

 sidence comes into play in more than one industrial 

 process for the purification of water on the large scale. 



Clark's Process. — The most important of these 

 methods of water-purification is that well known as 

 Clark's Process, in which the 'temporary hardness' or 

 soluble bicarbonate of lime present in water is con- 

 verted into insoluble normal carbonate of lime by the 

 addition of a suitable proportion of lime-water, with 

 the result that both the lime originally present as 

 bicarbonate as well as that added in the form of lime- 

 water are precipitated. The bacteriological effect of 

 this process has been investigated by one of us, both 

 on the laboratory scale (see p. 196) ,as well as in the 

 form in which it is carried out at waterworks and for 

 industrial purposes. 



Thus, at the Colne Valley Water-works, Herts, the 

 hard water derived from deep-wells sunk into the chalk 

 is mixed with the requisite proportion of clear lime- 

 water, and then allowed to settle in open tanks. The 

 subsidence of the precipitated carbonate of lime is so 



