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The water is then allowed to subside in a small tank, and is eventually filtered through filters covered with asbestos cloth, the basis of the filter being similar to that of the " Filtre Rapide.” A part of the carbonate of lime and magnesia deposited from the water adheres to the filtering surface, and the softened water filters through it. The apparatus may be seen at work in this Exhibition.

The Process of Messrs. Gaillet and Huet.- In this process, which was patented in February, 1883, the patentees make use of certain known agents, the patent itself applying to the apparatus used for the purpose of producing the results after the chemicals have been applied. The agents they propose are lime and caustic soda. Whenever the water contains organic matter, they use salt of alumina or iron in addition. Iron, however, is not recommended in any case where the water is required for washing purposes. The apparatus consists, virtually, of a series of tanks in duplicate, in which the chemicals are mixed, and these enter a vertical pipe in proper proportion to the water to be softened, and which communicates with the bottom of an upright chamber divided by a series of sloping shelves, through which the water gradually works upwards in a zigzag path. These shelves slope in one direction, and are of V shape, so that as the deposit takes place it accumulates at one point, at which there is an opening ordinarily closed by a tap, and when any tap is open the deposit on the sloping shelf communicating with it is washed out. The apparatus appears to be extremely simple in its design, but its efficiency has yet to be tested, although it is at work at Messrs. Duncan's, Victoria Docks, where the water is reduced from 24 degrees to 6 degrees and the Thames water is reduced from 16 degrees to 2 degrees.

Purification by Distilling.--It is not necessary to devote any large amount of attention to the question of purifying water by the process of distillation. The process is one which has been used from remote periods in order to produce absolutely pure water, and during the last forty years very great improvements have been adopted in order to bring this process into more general application in connection with the purposes of water supply. The difficulty of obtaining absolutely pure water is practically exemplified by this process, for in attaining this result, unless the water is distilled some two or three times, and every time a large proportion of the residue is discarded, pure water cannot be obtained. In the case, however, of water distilled for dietetic purposes, it is not necessary to carry out the process to the extent required in procuring water for some chemical purposes. It has generally been considered that distilled water lacks aëration, and on this account it has been strongly recommended that it should be filtered. The great improvements in the process of distillation are due to Dr. Normandy, whose first patent, taken out in 1851, has been improved upon by many subsequent patents. The process has been adopted with the greatest possible advantage in many of our ocean steamers, and the preservation of the health of the crews and passengers visiting countries liable to the ravages of epidemic disease is, in a great measure, due to the use of this process. It is generally believed by many high sanitary authorities that if this system were adopted at malarious stations, one of the largest channels by which infection is disseminated would be effectually closed. Dr. Macnamara states that on our ocean steamers, “as a general rule, condensed seawater is employed for drinking purposes, which, although it may not be always very palatable, must obviously be free from all chance of choleraic contamination ;” and this is one of the great safeguards to Europe against the spread of cholera.

Softening by Exposure.—The exposure to the air of water containing salts which are held in solution by carbonic acid causes a loss of carbonic acid. Water of deep wells which has been in contact with chalk and other rocks often contains free carbonic acid by exposure, especially under the inequalities of diurnal temperature, the original charge of ground air is got rid of, and pure atmospheric air takes its place. On exposure to air, hard waters are especially liable to develop vegetable growth. A few days' exposure of

very hard water in the summer time will soon develop green confervoid growth, and so soon as this growth takes place, carbonic acid is rapidly used up by it, so that the bicarbonates in the water are soon converted into simple carbonates, and are precipitated. Water, therefore, exposed to air undergoes a chemical metamorphosis; the bicarbonates of lime and magnesia are converted into carbonates, and are precipitated, and it is in this way that exposure assists in softening water.

Softening by Freezing.–Pliny, speaking of the quality of water and of the controversy going on in his time amongst physicians as to the use of water, says that some people preferred rain-water above all others, because it is the lightest. He also says that some prefer "snow-water before that which cometh down in showers; and the water of ice dissolved before the other of melted snow," and he goes on to say that the rain, snow, and ice are all lighter than those which spring out of the earth, and ice amongst the rest far lighter than any water in proportion. Ice taken from hard or other impure waters, if found to be perfectly crystalline and free from air bubbles, will produce, on melting, a water as soft as that of distilled water. If, however, the ice contains air bubbles or cavities of any description, such water will not be entirely pure. Some years ago the author made an extensive series of experiments upon the degree of purity which might be arrived at by freezing water, when it was shown that the act of freezing may be carried to such an extent as to produce, in the remaining water, a precipitation of the salts in solution ; but ice frozen upon very superficial water was found liable to have the impurities frozen in it which adhered to the under sides of the ice, and which became embedded in it by subsequent freezing ; but water which has been largely deprived of air by boiling or exposure, upon being frozen, if perfectly crystalline, will produce absolutely pure water. Several patents have been taken out with a view to freezing sca-water so as to furnish a supply of fresh water

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on board ship, but such processes will not compete, from an economical point of view, with the process of distillation.

Geological.The geological formations which furnish water of a quality suitable to be softened are those of the dolomitic or magnesian limestone, which gives great hardness to water, for while salts of lime render water hard and troublesome, in washing, those of magnesia cause the water to curdle, and render it considerably more disagreeable for washing and ablution. The mountain limestone, which is ordinarily of an impermeable nature, does not yield water of such a hard quality as those of the magnesian limestone. The waters of the colite and chalk are chiefly hard from what has been termed temporary hardness, that is due to the presence of bicarbonates of lime and magnesia in the water, which may be got rid of by boiling, or by the lime process. The waters of the new red sandstone and Permean beds vary considerably in hardness; many of them have a considerable permanent degree of hardness, but there are none of them which may not be softened to a great extent by the adoption of the lime process, while this process, in combination with the other alkaline earths, such as soda, when the water is not intended to be used for dietetic and washing purposes, will still further reduce the hardness of these waters.

The surface wells of the country, usually sunk in drift covering various geological formations, furnish water of various degrees of hardness. Scarcely any such wells yield a soft water, and in most instances, when these wells are sunk in populous places, in addition to their natural hardness, the waters are highly polluted, and such waters ought never to be used for dietetic purposes, unless they are first boiled.

ON THE DETECTION OF SEWAGE

CONTAMINATION BY THE USE OF
THE MICROSCOPE, AND ON THE
PURIFYING ACTION OF MINUTE
ANIMALS AND PLANTS.

By H. C. SORBY, LL.D., F.R.S.

By studying with the microscope the solid matters deposited from the water of a river, the previous contamination with sewage can usually be detected without any considerable difficulty. If the amount be serious, the characteristic particles of human excrement can easily be seen ; and even if it is small, and has been carried a long way by the current, it can usually be recognised by means of the hairs of oats derived mainly from the droppings of horses, which resist decomposition for a long time, and are not consumed as food by minute animals. I, however, do not propose to enter into detail in connection with this part of my subject, but specially desire to call attention to the connection between the number of minute animals and plants, and the character of the water in which they live, and also to their influence in removing organic impurities.

For some time past I have been carefully ascertaining the number per gallon of different samples of river and seawater, of the various small animals which are large enough not to pass through a sieve, the meshes of which are about zboth part of an inch in diameter. The amount of water used varies from ten gallons downwards, according to the number present. By the arrangements used there is no important difficulty in carrying out the whole method in a satisfactory manner. I confine my remarks entirely to general mean results.

The chief animals met with in fresh water are various entomostraca, rotifera, and the worm-like larvæ of insects.

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