Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/179

 FILTER 169 changed into ozone, by which the organic matter of the water is consumed. The magnetic carbide is used in granular form. This filter gained prize medals at the last London and Paris ex hibitions, and its effi ciency was demon strated by the Lancet Sanitary Commission ers report on filters in 18G7. The only other system we shall hero notice is that in which spongy iron is used. This substance is me tallic iron which has been reduced from au oxide without fusion. It is in a spongy or porous state of ex tremely fine division. Its remarkable purify ing action on water was discovered by Professor Gustav Bischof of the Andersonian Univer sity, Glasgow; and ex periments made with his filters by the Royal Commissioners already referred to showed that their power both of re moving organic matter and reducing the hard ness of water even in creased during upwards schof s s P n SJ r Iron Fi of eight months constant use. The general form of the filter is represented in fig. 5. An inner vessel containing tho spongy iron is supported in a case, which, below, contains some prepared sand, a regulator A, and a receptacle C, for filtered water (with tap, not shown). The unfiltered water B is in this form supplied from a bottle which is inverted into the upper part of the inner vessel (a method familiar to chemists). After passing through the spongy iron, the water ascends through an overflow pipe in the direction of the arrows ; the object of this is to keep the spongy iron, when once wet, constantly under water, as otherwise it is too rapidly oxidized. The object of the prepared sand (which is generally in three layers, viz., pyrolusite at the top, then sand, then gravel) is to separate traces of iron retained in solution. The regulator A consists of a tin tube, cemented in the position shown ; it is open at the inner end, which is below the perforated bottom supporting the sand, and closed by a screw cap at its outer end. It has also a small lateral perforation, through which alone the filtered water passes into the reservoir. Should the perforations get choked, the screw cap is removed, and a brush inserted ; on starting at first, too, the cap is un screwed, that the materials may be well washed out with out soiling the lower reservoir. With a ball-cock and con stant supply of water, the inner vessel is dispensed with. The nature of the action of the metal on organic matter is rather obscure. Mr Bischof considers there are both reducing and oxidizing agencies constantly at work, and that the oxides of iron, being present in their nascent state, must be very energetic in their action. Probably ferric hydrate, the last product of oxidation, takes an active part in separation of tho organic matter, transferring oxygen to it. Again, spongy iron is known to be very energetic in precipitating any lead or copper. Its reduc tion of the hardness of water presents some difficulty. This filter, we may add, recently gained the prize medal for general excellence given by the Sanitary Institute of Great Britain, and has been otherwise commended. The subject of cistern filters (of which there are many varieties) need not detain us long here. The arrangement adopted by Lipscombe, shown in fig. G, may be taken as an example. The impure water passes through the inlet PIPE TO SUPPLY BOILER &c. WITH PURE WATER. Fio. 6. Lipscombe s Cistern Filter. HOUSE CISTERN. RED WATE R. K into the chamber of the filter, thence upwards through a plate of porous stone, then through powdered charcoal into the pure water reservoir, whence it may be drawn aff cold by the pure water tap, or hot and pure from the boiler. IX. 22