Various means of compressing carbon into solid porous masses have been patented, and filters are made in which the water passes through blocks of this compressed carbon. Most of these are well adapted for the purpose, but their asserted superiority over filters composed of layers of sand and charcoal is doubtful. A very elegant and convenient portable filter for soldiers, travelers, and others who may require to drink from turbid ponds and rivers, was constructed of Ransoine's filtering stone, and is also made of the compressed carbon. A. small cylinder of the stone or carbon is connected with a flexible India-rubber tube in such a manner that the cylinder may be immersed in a river, the mouth applied to a mouth-piece at the other end of the tube, and the water drawn through the filtering cylinder.
The filtration of water on a large scale will be treated of under WATER-SUPPLY.
Some very interesting experiments were made by Mr. H. M. Witt, to ascertain whether soluble matter, such as common salt, is in any degree removed from water by filtration. Theoretically, it has been assumed that this is impossible, since the filter only acts mechanically in stopping suspended particles; but the results of Mr. Witt's experiments show that from 5 to 15 per cent of the soluble salts were separated by sand-filters such as above described. This is a curious and interesting subject, well worthy of further investigation. Another most important matter, on which a series of accurate experiments is required, is to ascertain to what extent soluble organic matter may be decomposed by filtration, especially by charcoal filters, and to ascertain how long charcoal and other porous matter retains its property of acting on organic matter in watery solution. The power of dry charcoal in decomposing organic matter in a gaseous state is well established (see below), and it is also well known that fresh char coal acts powerfully upon organic matter in solutions, but the extent to which this power is retained in the charcoal of a filter in continuous action has not been satisfac torily ascertained. This is of the highest importance, as it sometimes happens that water of brilliant transparency, and most pleasant to drink, on account of the carbonic acid it contains, is charged with such an amount of poisonous organic matter as to render its use as a daily beverage very dangerous. Charcoal obtained from burning bones is still more efficacious than charcoal from wood. A filter of animal charcoal will render London porter colorless. Loam and clay have similar properties. Prof. Way found that putrid urine and sewer water, when passed through clay, dropped from the filter colorless and inoffensive.
When a liquid contains mucilaginous or other matter having viscous properties, there is considerable difficulty in filtering it, as the pores of the medium become filled up and made water-tight. Special filters are therefore required for sirups, oils, etc. Such liquids as ale, beer, etc., would be exceedingly difficult to filter, and therefore they are clarified by the processes described under FINING. Oil is usually passed through long bags made of twilled cotton cloth (Canton flannel). These are commonly 4 to 8 ft. long, and 12 to 15 in. in diameter, and are inclosed in coarse canvas bags, 8 or 10 in. in diameter, and thus the inner filtering-bag is corrugated or creased, and a large
surface in proportion to its size is thus presented. Sirups are filtered on a small scale by confectioners, etc., by passing them through conical flannel bags, and on a large scale in the creased bag filter just described. Thick sirups have to be diluted or clarified with white of egg, to collect the sediment into masses, and then they may be filtered through a coarse cloth strainer. Vegetable juices generally require to be treated in this manner.
The simple laboratory filter has to be modified when strong acid or alkaline solu tions, or substances which are decomposed by organic matter, require filtration. Pure silicious sand, a plug of asbestos, pounded glass, or clean charcoal, are used for this purpose. Bottger recommends gun-cotton as a filter for such purposes. He has used it for concentrated nitric acid, fuming sulphuric acid, chromic acid, permanganate of potash, and concentrated solutions of potash and aqua regia. He says that properly prepared gun-cotton is only attacked at ordinary temperatures by acetic ether.
Filtering paper for laboratory purposes requires to be freed from inorganic impuri ties that are soluble in acids, etc.; • this is effected by washing the paper with hydro chloric acid, or, when thick, with nitric and hydrochloric acid, and removing the acid by washing thoroughly with distilled water.
When a considerable quantity of liquid has to pass through a filter, it is sometimes desirable that it should be made to feed itself. In the laboratory, this is done by invert ing a flask filled with the liquid over the filtering funnel, the mouth of the flask just touching the surface of the liquid when at the desired height in the funnel. As soon as it sinks below this, air enters the flask, and some liquid falls into the funnel. On a large scale, self-acting filters are fed by the common contrivance of a ball-cock and supply-pipe.
Air-Einem—The extraordinary powers of charcoal in disinfecting the gaseous prod ucts evolved from decomposing animal and vegetable matter, have made availa ble by Dr. Stenhouse in constructing an apparatus for purifying air that is made to pass through it. A. suitable cage, containing charcoal in small fragments, is fitted to the opening from which the deleterious gases issue, and is found to render them perfectly inodorous, and probably innocuous. The first application of this was made in 1851, when a charcoal air-filter was fitted up in the justice-room of the mansion house, London, the window of which opens above a large urinal, the smell of which was very offensive in the room. The filter at once destroyed the nuisance, and the charcoal has been found to last many years without the need of renewal. 103 of such filters have been applied to the outlets of the sewers of one district of the city of London, and no bad smell is observable where they are placed, and no obstruction offered to the ventila tion of the sewers. They have been applied with like results in two or three county towns. The subject is fully treated by Dr. Stenhouse in a letter to the lord mayor, published by Churchill (London). Charcoal respirators are small air-filters of the same kind applied to the mouth. See RESPIRATOR.