Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/178

 168 FILTER objection in that the water sent upwards has a tendency to force a passage through certain channels, without being uniformly disseminated in the material ; and the deposit of any filth is excluded from view, and mostly also from smell, instead of being exposed and giving us warning. In passing now to examine more closely some of the ap proved forms of domestic filter at present in use, it should be borne in mind that while any of these filters will doubtless purify water both mechanically and chemically, more or less, it is only on condition of their being properly attended to, and the filtering material renovated at intervals depending on its nature and the nature and amount of impurity in the water. The term &quot; self-cleansing,&quot; applied to some filters, may have a (limited) true sense, but if understood to imply that a filter, let alone, will go on ad infinitum giving pure water, it is quite inapplicable ; solid impurities must accumulate and call for removal. The statement, occasionally made, that a filter is &quot; warranted to remove all impurities&quot; from water is absurd and hardly deserves notice. Absolutely pure water is a thing almost unknown ; careful distillation alone will give an approxima tion to it. Again, the claim that a filter will remove all lime from water is often false ; filtration is capable of re moving only a small quantity of lime. It must be allowed that sundry points in the process of filtration still remain in some obscurity, and it is matter of regret that the action of some common filtering agents has not been so fully cleared up by scientific experiment as others. Still enough has been ascertained probably to guide to the construction of a filter on rational principles. In a large proportion of filters, as already indicated, some form or other of carbon is the chief filtering agent. The well-known filters of Lipscombe are cylindrical-shaped covered vessels of glazed earthenware, in which the filtering medium, a mixture of vegetable and animal charcoal, in granular form, is enclosed between two slabs cemented in the case. The upper (glazed earthenware) slab has a central aperture with raised border, and a small perforated basin immediately below it ; into this is inserted a sponge to arrest the grosser impurities, which is taken out and cleaned at short intervals. The filtered water passes through the lower (and porous) slab to the reservoir below, which communicates above with the outer air by a narrow tube passing up within to the top of the apparatus, and delivers its water through a tap. Charcoal in the form of solid finely porous blocks, which can be conveniently brushed and cleaned externally, is now often moulded for filters. The convenient decanter filter, in which the water passes through the block to a central tube, forms an elegant addition to the sideboard. The annexed figure (1) represents one of these as made by Atkins, who also furnishes earthenware filter vessels having a division across the inside wherein a carbon block is fitted water-tight, which can readily be taken out and cleaned and replaced, or a fresh one sub stituted. Sometimes the block is fitted in a movable pan. Again, in the filter shown in fig. 2 a double filtration is effected, the FlG - 1. Decanter Filter. water passing first through loose charcoal B, then through a charcoal block C, supported as shown. The block in this case is said to last longer without cleaning. The movable and perforated earthenware plate A, which is placed above the charcoal (see fig. 3), allows of easy renewal of the latter. The charcoal used in these filters is chiefly FIG. 3. Plate at A in tig. 2. of vegetable origin. They are found to remove more or less of organic and inorganic matter dissolved in water. It may be useful here to call attention to some of the conclu sions arrived at regarding char coal in the sixth report of the Royal Commission on Rivers Pol lution a short time ago. Fresh animal charcoal has been proved to act powerfully in the removal of organic impurity (considerably more so than vegetable charcoal), as well as of mineral matter. Put, according to Dr Frankland, its reduction of the hardness ceases in about a fortnight, the removal of organic matter continuing even after six months, though to a much less extent, especially if the filter be much used. It was found neces sary to renew the charcoal every six FlG - 2. Filter with double months when used for filtering the comparatively pure water of the London New r River Company, whilst the water of the Thames required re newal of the charcoal every three months. If this be not done, myriads of minute worms, we are told, are developed in the material passing out with the fil tered water. Other statements, of scien tific weight, regarding animal charcoal are more favourable to it, and seem to show that under certain conditions, per haps imperfectly understood, it may give better results. In Major Crease s system, which is adopted in the British army and navy, loose animal charcoal is com pressed between plates by means of a screw, the amount of compression being determined by the degree of impurity in the water to be filtered. The silicated carbon of Mr Dahlke s filters is obtained by mixing animal or vegetable charcoal with the residue of distillation of Boghead coal. By adding a little clay to the latter product, and saturating the whole with oily matter, it can be moulded, after which it is burnt. In one form of the filter, tw r o carbon blocks are sealed into the interior of earthenware vessel, granulated car bon being placed between them ; in another, a double action is obtained by placing a carbon block over the entrance to a second carbon medium. These filters have been highly com mended for their chemical properties, attributed to magnetic oxide of iron present in the medium. We give (fig. 4) a representation of the sili cated carbon siphon filter with its case. Water may be sucked through FIG. 4. Silicated Carbon it from a stream directly into the Siphon Filter. mouth, or passed siphon-wise from one vessel into another. These and similar pocket filters of Atkins s were supplied to the Ashantee expedition. A very powerful filtering medium was discovered and introduced many years ago by Mr Thomas Spencer. It is called magnetic carbide, and consists of protoxide of iron in chemical .combination with carbon. It is obtained by roasting hematite iron-ore with granulated charcoal for twelve to sixteen hours at a dull red heat. Mr Spencer considers the purifying property of the oxide to be due to its pow r er of attracting oxygen to its surface, without the latter being acted upon, the oxygen attracted being then