ON WATER-FILTERS. 
37 
material in powder of sufficient fineness, a high degree of per- 
fection of mechanical filtration can be reached. The limitation, 
of course, to the fineness of the powder and the thickness of the 
bed is the requirement of a sufficiently rapid filtration. 
Filtration through sand, however, at the best has little, if any, 
effect upon the matters dissolved in water beyond that due to 
the agitation of the water with air by letting it fall in drops 
from the filtering-bed to the receiving vessel. For this reason, 
charcoal in coarse powder has been long used in order to re- 
move certain matters dissolved in water. Charcoal obtained from 
animal matters alone appears to possess the power of removing 
matters from solution in water to any extent. Wood-charcoal 
has been, however, very much used, but with the result, conse- 
quently, of only aiding in mechanically filtering the water. 
There are four kinds of filters we shall now proceed to de- 
scribe as essentially different from the old sand, or sand-and- 
charcoal filter. 
The patent moulded carbon filter consists essentially of a 
block of amalgamated particles of charcoal. This block is 
stated to be formed of animal, vegetable, and mineral charcoal, 
all in fine powder, and incorporated together by means of pitch. 
The mass, after being submitted to enormous pressure in 
moulds, is heated so as to carbonise the pitch. The result is a 
dense porous block. A glass tube is fixed in a hole in the 
block by means of a cork, • and passes to its centre. This glass 
tube then passes, by means of another cork, through the bottom * 
of the chamber containing the carbon block immersed in the 
unfiltered water. By this means all the water passing out of 
this chamber through the glass tube must pass through the 
thickness of the block from its exterior to its centre. Such, at 
least, is the assertion of the proprietors of this filter. This, 
however, is more than open to question ; it is tolerably certain 
that some water must get through holes in the corks, and 
between the block and tube and the tube itself. This filter is 
stated to purify water from organic and other matters in solu- 
tion. We are not able to endorse this statement as regards 
organic matters in solution but to a very limited extent. We 
have found that water, as supplied by one of the Thames water 
companies, loses very little of its colour or smell, and a 
chemical examination has confirmed these observations. As a 
mechanical filter it appears to be very good. The editor of 
the Popular Science Review informs me that he has found a 
moulded carbon filter quite effectual in depriving a natural 
chalybeate water of all ferruginous taste. A sketch of the ar- 
rangement of a moulded carbon filter is shown in fig. 2, which 
represents the glass sideboard filter. A cistern filter is also 
constructed with a moulded carbon block, which, but only to a 
