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water of Glasgow 4.5°. Of Edinburgh about 5o. Newcastle-upon-Tyne Company's water, nearly 5o. Thames water near Mortlake had 14-2° hardness, while the average of many trials upon Thames water, after conveyance through pipes, gave only 11.8°. The inference, therefore, is, that it had lost 2.4° of its original hardness during its passage and exposure.

35. The outline of Dr. Clark's process may be gathered from the following abridged extract from the specification of his patent:-" Chalk forms the bulk of the chemical impurity that the process will separate from water, and is the material whence the ingredient for effecting the separation will be obtained. In water, chalk is almost, or altogether insoluble, but it may be rendered soluble by either of two processes of a very opposite kind. When burned, as in a kiln, chalk loses weight. If dry and pure, only 9 oz. will remain out of 16 oz.; these nine will be soluble in water, but require 40 gallons for entire solution. Burnt chalk is called quicklime, and water holding quicklime in solution is called lime-water, and is clear and colourless. The 7 oz. lost by burning the 16 consist of carbonic acid gas, which, dissolved under compression by water, forms soda-water. The other mode of rendering chalk soluble in water is nearly the reverse. In the former mode, one pound of pure chalk becomes dissolved in water, in consequence of losing 7 oz. of carbonic acid. To dissolve in the second mode, not only must the pound of chalk not lose the 7 oz. of carbonic acid, but it must combine with 7 additional ounces of that acid. In such a state of combination, chalk exists in the waters of London, dissolved, invisible, and colourless like salt in water. A pound of chalk dissolved in 560 gallons of water by 7 ounces of carbonic acid, would form a solution not sensibly different, in ordinary use, from

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the filtered water of the Thames, in the average state of that river. Chalk, which chemists call carbonate of lime, becomes bicarbonate of lime when dissolved in water by carbonic acid. Any lime-water may be mixed with another, and any solution of bicarbonate of lime with another, without any change being produced. But, if lime-water be mixed with a solution of bicarbonate of lime, the mixture acquires the appearance of whitewash, and chalk is precipitated, leaving the water above perfectly clear. This operation will be understood by supposing 1 lb. of chalk, after being burned to 9 oz. of quicklime, to be dissolved, and form 40 gallons of limewater; that another pound is dissolved by 7 oz. of extra carbonic acid, so as to form 560 gallons of a solution of bicarbonate of lime, and that the two solutions are mixed, making up 600 gallons. The 9 oz. of quicklime from the 1 lb. of chalk unite with the 7 extra ounces of carbonic acid that hold the other pound of chalk in solution. These 9 ounces of quicklime and 7 ounces of carbonic acid form 16 oz., or 1 lb. of chalk, which, being insoluble in water, becomes visible at the same time that the other pound of chalk, being deprived of the extra 7 oz. of carbonic acid that kept it in solution, reappears. Both pounds of chalk will be found at the bottom of the subsidence. The 600 gallons of water will remain above, clear and colourless, without holding in solution any sensible quantity either of quicklime or of bicarbonate of lime."

36. All the methods of mere mechanical clearing of water are one or other of two processes, viz., settling, or subsidence, and filtration. The first of these processes is of a negative character, consisting simply in letting the water remain for a considerable period in an undisturbed condition. It is well known that, if a quantity of water, having particles of any foreign mat

ters of greater specific gravity than water floating or diffused within it, be allowed to continue in a quiescent state for a sufficient length of time, these particles will subside to the bottom of the water, which is thus left comparatively clear and limpid. In order to accomplish this purpose on a great scale, reservoirs are constructed, in which the water is accumulated and permitted to remain, and from which it is delivered as required. Such reservoirs are termed subsiding or settling reservoirs.

37. The East London Water Company, which draws the water from the River Lea, near Lea Bridge, and supplies the eastern part of the metropolis and suburbs, has 20 acres of settling reservoirs. The arrangement is this: The water is introduced through a canal, two miles long, into a wide canal, or small reservoir, at the end of which there are two sets of gates, each of which opens a communication with a separate reservoir. The water is admitted into both of these reservoirs, but drawn from only one of them at a time, the other remaining closed. Thus the water remains for one day in each reservoir alternately, while, in time of floods, it may be shut off altogether from these reservoirs for four or five days.

38. The value of all merely settling reservoirs can be derived only by drawing the water from the upper part of them. It is evident that, while the subsidence is going on, the whole bulk of the water is clarified only in proportion to its distance from the bed; and thus, the lower down that the point of exit is situated, the less clear must be the water that passes away.

39. To make the principle of subsidence fully effective, it is likewise necessary that the water should remain for some period, probably 24 hours at least, entirely undisturbed. If any motion is permitted, the

subsidence is interrupted, if not arrested. The reservoir should therefore be filled, and then totally closed both to ingress and egress. At the expiration of 24 hours, the upper part of the water should be gently drawn off. If the extent of supply will admit, the lower portion of the water may afterwards be let off for manufacturing or inferior purposes, or allowed to mingle with another fresh portion. If both the supply and the discharge be conducted at a sufficiently slow rate, and enough time be allowed for the quiet completion of the subsidence, the bulk of the water will always maintain a high degree of mechanical clearness, and the intermixture of the water remaining after each drawing-off with the in-coming water, will not involve any material loss of time in the process.

40. The process of filtration is effected by providing a bed of easily permeable materials, in which the water deposits the solid particles which it held in suspension, and finds its way to the lower bed in a comparatively clear state. The filtering materials employed in large filters are sand and gravel of various degrees of fineness, pebbles and shells. These latter, by their calcareous properties, act chemically on the water to a trifling extent, or while they retain free carbonic acid; the other materials admit the passage of the water, but prevent that of such solid particles as are larger than the interstices between the particles of the materials. The filtered water is collected in brick tunnels, constructed in the lower filtering stratum, and having apertures in the joints to admit the water. Fig. 3 is a section of a filtering reservoir as constructed for the Chelsea Water Company. In this reservoir the water comes in contact first with a bed of fine sand, a, which arrests the mechanical impurities. It thence passes through the strata, b, of coarse sand, c, of pebbles and

shells, and, d, of fine gravel, into the lowest bed e, which consists of large gravel, lying upon a firm foundation of clay, 18 inches thick, and having the brick culverts, fff, built within it. The clay bottom must, of course, be rendered sufficiently compact to resist the passage of the water; and, if no clay be found, it will be necessary to form an artificial bed for the purpose. The collecting tunnels are here constructed of blocks of brickwork in cement, and partly open-jointed. They are three feet in diameter, and two half-bricks in thickness. The water is admitted to the filtering-bed at nine places, the ends of the supply-pipes being fitted with curved boards to diffuse the water, and prevent any disturbance of the upper stratum of sand. The quantity filtered in this bed, which is 240 feet long, and 180 feet wide, is 72 gallons per superficial foot of the filtering-bed daily, according to the demand. The undulating surface of the bed allows parts of it to be drained when necessary, without removing the water from the adjacent hollows. It is found that the sediment penetrates only from six to nine inches in depth, and the removal of one inch in thickness of the fine sand, every fortnight, is found sufficient to secure the proper action of the apparatus. Air-drains are provided to admit the escape of the condensed air which may collect in the tunnels. It has been found that it is necessary, in all cases, to remove the old sand before introducing fresh sand; otherwise a film is formed on the original sand which will resist the passage of the


41. The first and current expense of this system of filtration is estimated by Mr. James Simpson, the engineer of the Chelsea Water-Works, to be as follows:

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