158 On the Composition and Properties of Drinking -Water, 
in water, however, are not diminished by sand filtration, or 
only in a very slight degree. The necessity for this process may 
be greatly diminished by the use of subsiding reservoirs, which, 
moreover, have the advantage of exposing the remaining water 
for a length of time to the oxidising influence of atmospheric 
oxygen, whereby it is deprived of some objectionable colour- 
ing matter. But filtration cannot be entirely superseded, being 
indispensable as the concluding operation of purification, to 
remove accidental impurities which may find access to the water, 
as well as fine particles of clay after remaining for a long time 
in suspension. 
For household purposes, turbid and slightly-coloured water 
may be made bright and almost colourless by the use of the 
tank or hand-filters, which are now supplied in all sizes by the 
London Water-purifying Company, Strand ; by Messrs. Atkins, 
Fleet Street ; Mr. Lipscombe, Temple Bar ; and other makers of 
water-filters. In most of the tank and hand-filters advantage is 
taken of the well-known property of animal charcoal to remove 
colouring matters. Vegetable or animal charcoal, moreover, 
retains effectually every trace of lead which a water may contain, 
either in solution or in a suspended state, and thus tank or hand- 
filters, in which charcoal is employed as a purifying agent, afford 
the greatest security against danger arising from the presence of 
lead poison. 
In Spencer's Magnetic Carbide Filter, the purifying agent em- 
ployed is magnetic oxide of iron mixed with carbon. Mr. Thomas 
Spencer prepares this material by using Cumberland hematite 
iron-ore with a certain proportion of carbon, and heating the mix- 
ture to a dull red heat in retorts for 24 hours. The porous magnetic 
oxide produced is mixed with coarse sand when used for filtering 
water, and it removes effectually all organic impurities in a state 
of putrescence, and any traces of lead that may be present. 
The most recent invention in water-filters has been made by 
Professor Bischoff, who employs spongy iron as a purifying 
agent. Bischoff has experimentally investigated the properties 
of spongy iron, and, amongst other particulars, found that the 
organic nitrogen and albuminoid ammonia in water are always 
much reduced in quantity by filtration through spongy iron, 
which also diminishes the amount of organic carbon. Filtration 
through spongy iron thus appears capable of decomposing organic 
matter. It further removes entirely every trace of lead, and, con- 
sequently, is a valuable purifying agent for water. 
Domestic water-filters, on Professor Bischoff's plan, are made 
by Messrs. Murray & Co., of the Caledonian Pottery, Ruther- 
glen. The spongy iron through which the water is filtered is 
contained in a stoneware vessel, with a slightly curved bottom. 
