bonate and sodium chloride. Indeed, the experiments of Mr. Wanklyn with the silicated carbon-filter prove that, by repeated filtration, river water containing a considerable amount of free and albuminoid ammonia may be made as pure as deep spring water.

Of filtering media, animal charcoal, properly washed, is now admitted to be in every way the most efficient. It exerts a chemical as well as mechanical action on organic impurities, and Dr. Frankland is so convinced of its value as a filtering agent, that he recommends its employment on a large scale for the purification of town supplies, in spite of the cost which would be entailed.

Amongst filters which have been specially commended for their efficiency, may be mentioned the cistern filter of the Water Purifying Company, London; Lipscombe's Self-Cleaning Charcoal Filter; the Patent Carbon Block Filter, manufactured by Atkins and Co., London; and the Carbon Cistern Filter, planned by Mr. Finch, of the Holborn Sanitary Works. All of these contain animal eharcoal as the filtering medium, and can be applied to any kind of house cistern. The filtering block of the Silicated Carbon Company consists of 75 per cent of charcoal and 22 of silica, with a little iron oxide and alumina. It is cemented into a vessel which it divides into two chambers, the one containing the filtered and the other the unfiltered water. This filter is found to work very efficiently, and with a little care retains its properties for a long time. The filtering material of the Magnetic Carbide Filter is prepared by heating hæmatite with sawdust. The only objection to it is that it communicates a slight taste of iron to the water. The Patent Moulded Carbon Filter makes an elegant

article for the sideboard. It consists of two glass vessels, the upper containing the filter-block, and the lower, which can be used as a water bottle, the filtered water. Tap-filters, suited for a high or low pressure, can be fitted to the pipes themselves. They contain charcoal or silicated carbon, and would seem to act very well.

A charcoal filter has lately been introduced by Captain Crease, of the Royal Marine Artillery, which, for simplicity of construction, adaptability to different kinds of water and rates of supply, and for efficiency, deserves special notice. It is known as Crease's Patent Tank Filter, and is manufactured by Mr. Bellamy, of the Tank Works, Millwall, London. It is now largely used in the Navy, and is specially suited for large buildings, such as asylums, workhouses, etc. The tank is made of iron, lined with cement, and is divided into three chambers. The two filtering-boxes which it contains are filled with pieces of animal charcoal, or the one may be filled with animal charcoal and the other with sand and gravel. The upper perforated plates of the boxes are movable, so that by means of screws working on rods attached to the fixed under plates, which are also perforated, the filtering media may be lessened or compressed to any extent, according to the degree of impurity of the water. The water descends through one box into a small chamber at the bottom of the tank, which retains any deposit, and then rises through the second box into the reservoir which contains the filtered water (see fig. 7). The whole of the apparatus can be readily unscrewed, taken to pieces, and cleaned out when necessary, the joints being made water-tight by gutta-percha bands. Smaller filters on the same principle have also been patented by Captain Crease.

All filters after a time become clogged up, and have therefore to be taken to pieces and thoroughly cleansed; or, if this cannot be easily done, they may be purified by passing through them a solution of potassium permanganate or Condy's fluid, with the addition of a few drops of strong sulphuric acid, and afterwards two or three gallons of pure or distilled water, acidulated with hydrochloric acid. The charcoal in a filter may also be purified by exposing it for some time to the sun and air, or by heating it in an oven or furnace.

The purification of water without filtration is not carried on in this country on the large scale except by Dr. Clark's process. This consists in adding a certain amount of lime water to a water which contains calcium carbonate rendered soluble by the presence of carbonic acid. Spring waters in the chalk districts are all more or less "hard," and many of them contain such a large amount of calcium carbonate in solution as to be unfit

for washing purposes. rendered "soft" by Clark's process, is let into a tank or reservoir, where it is mixed with a proper proportion of lime water and allowed to settle, the whole of the calcium being precipitated as neutral carbonate. A perfectly clear and wholesome water is thus obtained, well suited for domestic purposes. Calcium carbonate may also be removed by boiling, in which case it is deposited as an incrustation on the inner surface of the kettle or boiler.

Such a water, when it is to be

Aluminous salts have long been used in Eastern countries to purify water, and are found to be very efficacious in removing suspended matters, whether organic or mineral. Organic matters in solution are best treated with potassium permanganate or Condy's red fluid. It readily removes any offensive odour arising from water kept in casks, and oxidises at least a portion of the organic impurities which may be present; but as albumen is only slightly affected by it without the aid of heat, it cannot be regarded as a reliable purifier of water tainted with animal impurities.

Among other purifying agents may be mentioned, distillation, the exposure of water in minute divided currents to the air, the immersion of pieces of charcoal or of iron wire, and the effects of plants and fish. In store reservoirs, the presence of a moderate quantity of living plants exerts a decidedly purifying influence, while the destruction of fish has been followed by an excessive multiplication of the small crustacean animals on which the fish had lived, thereby rendering the water nauseous and impure. The remedy was found in re-stocking the reservoir with fish. (Rankine.)

SECTION V.-EXAMINATION OF WATER.

As an exhaustive analysis of any given sample of water can only be conducted in the laboratory and by a professed chemist, it would be out of place here to attempt to describe any other than easily applied methods for arriving at a reliable estimate of the qualities of a water, and whether it may safely be used.

In addition to a qualitative examination, it is advisable to make inquiries with regard to the source of the water, and to determine the probability, or otherwise, of its pollution. Wells, for example, which have hitherto yielded a good and wholesome water, may become contaminated with the fluids draining away from recent filth-accumulations, from graveyards, from neighbouring cesspools which have become leaky, or from the bursting of some sewer or drain. Reservoirs or cisterns, again, may become so foul, through neglect of cleansing them at stated intervals, that the water-supply is eventually rendered totally unfit for use, and becomes productive of disease; or the cistern-overflow pipe, should it open directly into a drain, may become the channel for the escape of pent-up sewer-gases, which are, in their turn, absorbed by the water stored in the cistern. In small villages, a sudden rise of the subsoil-water, occasioned by heavy rains following a period of drought, may wash into the wells the soakage from middens, cesspools, or open ditches filled with sewage; or the supply of a large town from a river contaminated with sewage may convey impurities to every household through some temporary inefficiency in the filtering process. All these, and numerous other contingencies, have to be borne in mind, more especially as