28 Mr. H. M. Witt on the Power possessed by Porous Media 



is robbed of almost all its acid, and the vinegar does not pass 

 through unchanged until the sand has become well charged 

 with acid. Potato brandy diluted with water, and filtered 

 through quartz sand, yields at first pure water, then a mixture 

 of water and alcohol deprived of its fusel oil, and lastly, the 

 original mixture unaltered : wood shavings also deprive vinegar 

 of nearly all its acid ; charcoal acts still more powerfully*/' 



Now it is possible that these being organic compounds they 

 may be oxidized, even into carbonic acid and water, under these 

 circumstances; but whatever difference of opinion may arise 

 respecting the caiLse, this cannot affect the simple experimental 

 fact. 



It will naturally be asked, if sand possess this power, Are not 

 the salts removed from the water accumulated in the sand, and 

 would not this throw light upon the mode of formation of sand- 

 stone conglomerate rocks ? To enable me to reply to this ques- 

 tion, I have in my possession samples of sand, before use, and 

 also after employment in the middle of the filter at Chelsea for 

 no less than twenty years, and their analyses will form the sub- 



i'ect of a future communication ; pressure of other duties having 

 litherto prevented me. from the completion of this important 

 point in the inquiry. 



But, as before mentioned, the preceding are not the only ex- 

 periments which I have made on this subject : the following were 

 performed at Kingston, where the water is far purer, more par- 

 ticularly for the purpose of ascertaining the effects of charcoal 

 and comparing its powers with those of sand. 



A.t the time these experiments were in progress, the Company's 

 filter-beds at Kingston, now just completed, were unfinished, 

 and consequently, in order to ascertain the effects both of sand 

 and charcoal on the comparatively pure water of Kingston, it 

 became necessary to construct smaller experimental filters. 



The earlier experiments were upon the effects of charcoal, and 

 several preliminary trials were made for the purpose of ascertain- 

 ing the form of filter and rate of filtration which would nearest 

 approximate to the operation as carried on on the large scale. 



In the first of these experiments, to which attention may be 

 drawn, the water was pumped from the river into a cistern of 

 slate, where a deposition of the greater portion of the suspended 

 matter took place ; it was then conducted into the upper part of 

 the right-hand compartment of a nearly cubical wooden cistern, 

 this right-hand compartment being separated from the one on 

 the left by a porous diaphragm, the walls of which were of wire- 

 gauze, and the interior filled with fragments of oak charcoal, 

 about the size of a hazel nut to a walnut : this charcoal dia- 

 * Wagenmann, Poggendorff's Annalen, xxiv. 600. 



