216 MICRO-ORaANISMS IN AVATEH 



2. In which the precipitant is soluble and more or 

 less chemically active, but in which, through entering 

 into reaction with constituents of the water, it is so 

 rapidly conA^erted into an insoluble form that little or 

 no directly bactericidal effect can be exerted. 



To this class of precipitants belongs lime when used 

 in suitable proportions for the softening of water con- 

 taining ' temporary hardness ' in Clark's Process, but 

 not when employed in excess, as in the latter case, of 

 course, a directly bactericidal effect will beproducedby 

 the caustic lime remaining in solution. 



Alum again, when used in excessively minute quan- 

 tities, may be regarded as belonging to this class of 

 precipitants. 



The precipitates in these cases, being actually gener- 

 ated within the water containing bacteria, are extremely 

 successful in entangling the latter and dragging them 

 to the bottom. Moreover, in this respect the bulky 

 gelatinous precipitate of alumina is naturally more 

 effective than the pulverulent or even crystalline pre- 

 cipitate of carbonate of lime. 



3. In which the precipitant is soluble, chemically 

 active, and added in such quantity that a direct bacteri- 

 cidal effect is produced in addition to the mechanical 

 one of precipitation. This kind of treatment cannot as 

 a rule be employed for the purification of drinking 

 water, but is abundantly made use of in the chemical 

 treatment of sewage. 



