40 
POPULAR SCIENCE REVIEW. 
certain that all the water passes through the pores of the mate- 
rial. Fig. 1 serves to give some idea of this arrangement, 
only that the filtering-slab appears there as much too thick. 
The slab is prepared from the coke of the Torbane Hill 
mineral or coal, and is represented by the proprietors of the 
patent (Dahlke’s) to owe its activity to the silica as well as the 
carbon it contains. 
Thames water, as supplied by the water companies, passes 
through these filters with the loss of most of the colour due to 
organic matters, and quite free from sensible particles of matter 
in suspension. Its discoloration is not so completely effected 
as it is by the animal-charcoal siphon-filter last described. A 
chemical examination indicates a partial removal only of organic 
matter, but then this is of the more offensive and least oxidised 
portion. 
The last filter we shall give some account of is the magnetic 
carbide purifying filter, patented by Mr. Thomas Spencer. 
This filter, represented by fig. 1, is formed of a layer of 
“ magnetic carbide,” contained between two perforated stone- 
ware partitions. A thin layer of sand is usually placed next 
the lower partition, to increase the mechanical filtering effects 
of the filter ; and, when the water to be filtered is unusually 
turbid, a sponge is placed above the filtering material, through 
which all the water is first strained, so that the larger quantity 
of the suspended matters may be removed, and so kept from 
choking the pores of the layer of filtering material. This 
material, called by Mr. Spencer “magnetic carbide,” consists 
essentially of black or magnetic oxide of iron, but having 
also some carbon combined with it, according to the patentee. 
It is prepared by heating haematite or red oxide of iron with saw- 
dust in close vessels. The product is magnetic. Mr. Spencer 
claims for this material the property of never losing its activity 
so long as its pores do not become choked up. The same can- 
not be said of animal charcoal, which, after a time, loses much 
of its purifying power. We have been able to judge of the 
effects of a domestic filter that had been years in use, and its 
action seemed to be undiminished. . As the action of the mag- 
netic carbide stands comparison with that of fresh animal char- 
coal, its permanence of action places it before all other filtering 
materials. Spencer’s filters appear to be little, if at all, inferior 
to the animal- charcoal filters freshly mounted. Water passed 
through them loses all but a very small quantity of its organic 
matters. As mechanical filters they seem to be also excellent. 
The magnetic carbide was employed in making the filtering- 
beds of the water company at Southport, and after years of use 
is still, we understand, giving every satisfaction. From its 
mineral character it is suited for this purpose, whereas charcoal 
