[ 23 ] * H ^M m. 



III. On a peculiar Power possessed by Porous Media {Sand and 

 Charcoal) of removing Matter from Solution in Water, By 

 Henry M. Witt^ F.C.S., Assistant Chemist to the Govern- 

 ment School of Applied Science^. 



VARIOUS methods have been employed at different times 

 for the purification of water for the supply of towns, but 

 none has been found so practically convenient and efficacious as 

 simple filtration through porous media, such as sand ; moreover, 

 charcoal being known to possess a peculiar power of removing 

 organic matters from solution, this substance has been sug- 

 gested and occasionally employed either as a substitute for, or 

 an auxiliary to, ordinary sand filtration. 



The following experiments were undertaken with the view of 

 ascertaining by chemical analysis the more precise nature of the 

 effects produced upon ordinary river-water, such as that of the 

 Thames, by its passage through filters composed of these media 

 respectively, and of comparing their powers ; but it is believed 

 that the results obtained possess an interest extending con- 

 siderably beyond the question to assist in the solution of which 

 they were made. 



Before proceeding to the construction of experimental filters, 

 I availed myself of the kindness of Mr. James Simpson, en- 

 gineer to the Chelsea Waterworks Company, to investigate the 

 results obtained by that Company's system of filtration as carried 

 on up to the present time at Chelsea ; and I have much pleasure 

 in embracing this opportunity of expressing my obligation to 

 this gentleman, and specially also to his son, Mr. James Simp- 

 son, jun., to whom I am greatly indebted for his very able 

 cooperation throughout this inquiry. 



The system of purification adopted by the Chelsea Water- 

 works Company at their works at Chelsea, consisted hitherto 

 (for the supply has by this time commenced from Kingston) in 

 pumping the water up out of the river into subsiding reservoirs, 

 where it remained for six hours ; it was then allowed to run on to 

 the filter beds. These are large beds of sand and gravel, each 

 exposing a filtering surface of about 270 square feet, and the 

 water passes through them at the rate of about 6^ gallons per 

 square foot of filtering surface per hour, making a total quantity 

 of 1687*5 gallons per hour through each filter. 



The filters are composed of the following strata in a descend- 

 ing order : — 



* Communicated by the Author. 



