PURIFICATION OF WATER FOR DRINKING PURPOSES 215 



of the soluble organic matter of sewage by precipitation 

 with a proper amount of an iron or aluminium salt, and 

 it seems probable that, in some cases at least, if the pro- 

 cess is carried out with the same care as is required in 

 the purification of sewage by intermittent filtration, a 

 result may be obtained which will effectually prevent a 

 public nuisance.' 



The experiments of all the above investigators, 

 therefore, clearly prove that the most remarkable re- 

 moval of micro-organisms from water may take place 

 by means of precipitation alone. This precipitation 

 may be of three different kinds : 



1. In which the precipitant is added in a solid 

 and insoluble form, and must therefore be practically 

 inert from a chemical point of view. To this class be- 

 long sand, china-clay, infusorial earth, chalk, vegetable 

 and animal charcoal, coke, &c. In the case of the 

 various forms of carbon, more especially in that of 

 animal charcoal, a small chemical effect is doubtless 

 exerted 011 some of the constituents of the water ; there 

 is, however, no evidence that they have any bactericidal 

 power, but rather the reverse. It will be noticed that 

 there is the greatest diversity in the bacterial purifica- 

 tion effected by the substances belonging to this class, 

 the more porous and slowly subsiding substances carry- 

 ing with them a much larger proportion of the suspended 

 bacteria than those w^hich are smooth, impervious, com- 

 pact, and therefore rapidly subsiding. 



This class of precipitation is particularly interesting, 

 inasmuch as it is of the kind which takes place during 

 the storage of turbid water in large reservoirs or lakes. 

 As would be expected, of course, the bacterial im- 

 provement is the more striking the larger the quantity 

 of material added, as in this way the points of attraction 

 for the bacteria are correspondingly multiplied. 



