PURIFICATION OF WATER FOR DRINKING PURPOSES 169 



number of bacteria in the filtered sewage at different 

 periods of the flow, results which are in complete har- 

 mon}^ with observations which have been made by one 

 of us during the purification of sewage on the large 

 scale in various places. 



Experiments on a small scale with various filtering 

 materials. — Although sand is the material almost in- 

 variably employed for the filtration of water on the 

 large scale, yet we know that in domestic filters 'other 

 substances are extensively used ; and it was with a 

 view of ascertaining the efficiency of different sub- 

 stances as regards their retention of microbes that the 

 following series of experiments were conducted by one 

 of us.^ 



The materials investigated were ferruginous and 

 very finely grained greensand, silver sand, animal- 

 charcoal, iron sponge, brick-dust, coke, vegetable and 

 animal charcoal, and powdered glass ; they were all 

 used in a very fine state of division, the filters, which 

 were all 6 inches in depth, being composed of particles 

 which had passed through a sieve of forty meshes to 

 the linear inch. In all cases the materials were steri- 

 lised before use. The results are embodied in the 

 table on following page. 



It will be seen that vegetable-carbon, whether in 

 the form of charcoal or of coke, offers a very strong 

 barrier to the passage of microbes. This material has 

 been generally regarded as of but little value for water 

 purification, owing, as in the case of sand, to its chemi- 

 ■cal inactiAdty ; but as biological filters these substances 

 occupy a high place, and owing to their cheapness 

 should prove of great service in the purification of 



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

 Hoy. Soc. Proc, 1885. Also Proceedings Institution of Civil Engineers, 

 1886. 



