Experiments on the filtration of water through animal charcoal were made on the New River Company's supply in the year 1866, and they showed that a large proportion of the organic matter was removed from the water. These experiments were afterwards repeated, in 1870, with Thames water supplied in London, which contains a much larger proportion of organic matter, and in this case also the animal charcoal removed a large proportion of the impurity. In continuing the use of the filter with Thames water, however, it became evident that the polluting matter removed from the water was only stored up in the pores of the charcoal, for, after the lapse of a few months, it developed vast numbers of animalcula, which passed out of the filter with the water, rendering the latter more impure than it was before filtration. Prof. Frankland reported in 1874 on these experiments as follows: "Myriads sf minute worms were developed in the animal charcoal, and passed out with the water, when these filters were used for Thames water, and when the charcoal was not renewed at sufficiently short intervals. The property which animal charcoal possesses in a high degree, of favouring the growth of the low forms of organic life, is a serious drawback to its use as a filtering medium for potable waters.

Animal charcoal can only be used with safety for waters of considerable initial purity; and even when so used, it is essential that it should be renovated at frequent intervals, not by mere washing, but by actual ignition in a close vessel. Indeed, sufficiently frequent renovation of the filtering medium is an absolutely essential condition in all filters. Pig. 152 shows Atkins's filter, in which a is the unfiltered and b the filtered water, c being a block of charcoal formed by miring powdered charcoal with pitch or resin, moulding and calcining. The filter is capable of being taken to pieces and can thus be easily and frequently cleaned. The block should on such occasions be scraped, washed, boiled, and baked.

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Fig. 153 illustrates another form of Atkins's, in which powdered charcoal is used, retained between movable perforated earthenware plates.

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Figs. 154, 155, represent Sawyer's filters, in which a is unfiltered water; b, filtered water; c, charcoal hollow cone; d, filtered water tap; c, sediment tap; f, mass of granular charcoal. The most important feature here is the up-ward filtration.

Fig. 154.

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Charcoal, Modified

Several substances have been proposed for combination with carbon to improve its filtering capacity or increase its germ-destroying powers.

Filtering Silicated Carbon

This was one of former, the stoneware receptacle is divided into 2 parts by a diaphragm upon which than is fixed, by a porcelain stay, a silicated carbon block, which entirely closes the apertures in the diaphragm. The upper surface and corners of the filtering block are nonporous, consequently the water has to enter at the edges and follow the coarse indicated by the arrows, before it can reach the clear water compartment below. In cleaning the filter, it is only necessary to unscrew the out, when the block can be lifted out and soaked in boiling water, after which the surface can be scrubbed.

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Fig. 157.

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The 'Army Medical Report' says of filters employing carbon in porous blocks that,"These are powerful filters at first, but they are apt to clog, and require frequent scraping, especially with impure waters. Water filtered through them and stored, shows signs of the formation of low forms of life, but in a less degree than With the loose charcoal. After a time, the purifying power becomes diminished in a marked degree, and water left in contact with the filtering medium is apt to take up impurity again, though perhaps in a less degree than is the case with the loose charcoal." The advantages of combining silica with the carbon are not at first sight apparent.

Maignen combines charcoal with lime to produce a compound which he calls "carbo-calcis." At the same time he employs an asbestos filtering cloth. The arrangement of his filter is shown in Fig. 158. The hollow, conical, perforated frame a is covered with asbestos cloth b; c is a layer of finely powdered carbo-calcis, deposited automatically by being mixed with the first water poured into the filter; d is granular carbo-calcis filling up the space between c and the sides of the containing vessel; e, unfil-tered water; f, filtered Water; g, tube for admitting air to aerate the water and correct the usually vapid flavour of filtered water. This filter has remarkable power; wine passed through it will come out colourless and tasteless. Moreover the cleansing and renewal of the filtering media are simple in the extreme.

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Prof. Bernays, of St. Thomas's Hospital, has taken out a patent for a new filtering material, consisting of charcoal combined with a reduced manganese oxide. The well-known purifying notion of charcoal (animal and vegetable), which in its ordinary state is liable to certain difficulties and objections, is in this invention supplemented and improved by heating it in covered crucibles with 5 to 15 per cent. or more of powdered manganese black oxide (the mineral py-rolusite), together with a very small quantity of some fixed oil, resin, or fat, Having ascertained that the simple admixture of the manganese dioxide with the charcoal without previous heating had no utility as a filtering medium, diminution of the porosity of the charcoal, Prof. Bernays devised the above method with the object of oxidising the hydrogen and other oxidisable impurities of the charcoal, and hence approximating efficacy to platinum black rather than In its ordinary less powerful analogy to spongy platinum. The heating is of course out of contact with air, and the temperature sufficiently high to cause the reduction of the mangnnese dioxide at least to manganous-manganic oxide, which afterwards acts as a carrier of oxygen, and thereby much prolongs the purifying action of the medium.

Another method of obtaining charcoal in combination with mnnganous -manganic ganous chloride (or even manganese residues) and afterwards subject it to a strong heat in closed crucibles. The charcoal prepared in the above manner may be employed in the filtration of Water in layers with sand and other filtering material id the usual manner.

A filtering material which has all the properties of animal charcoal, and is said to give higher results, is magnetic carbide, discovered by Spencer, many years ago, and consists of iron protoxide in chemical combination with carbon. It is considered that the purifying effect is produced by its power of attracting oxygen to its surface without the latter being acted on, the oxygen thus attracted being changed to ozone, by which the organic matter in the water is consumed.

There can be no doubt of the value of this filtering material. Its manufacture is very simple, as it is obtained by roasting hematite iron ore with granulated charcoal for 13 to 16 hours at a dull red heat, and used in a granular form. Another form for making this material is to heat the hematite (iron red oxide) with sawdust in a close vessel. The product is magnetic, and never loses its activity until the pores are choked up. The Southport Water Co. formed their filtering beds of this material, and after years of use it is still giving satisfaction.