552

HISTORY OK ENGINEERING.

Rook I.

pumping required, and the cost of cleansing is lessened materially.' The return of the waterupward cannot, however, entirely remove all that has been previously lodged, nor can itserve the purpose of so effectually cleansing as to obviate the necessity for renewing theseveral materials. Experiment has shown that, when the upward current is applied undera pressure of 26 feet head, it tloes not remove one-tenth part of what has been deposited : itseems necessary to produce great agitation between all the particles before the impuritieswill detach themselves from the grains of sand.

Impurities usually found in Wafer, are the mineral or saline, the vegetable, the animal, andmechanical. The saline are earthy salts, salts of lime, and salts of magnesia ; common saltis also usually present, and sometimes bicarbonates of soda and potash. The most importantof the earthy salts is the bicarbonate of lime. The whole saline matter consists of two por-tions the neutral and the alkaline. The latter are entirely bicarbonates, as those of limeand magnesia, to which sometimes are added those of potash and soda. The neutral por-tion consists of the neutral salts, of earths and alkalies, such as gypsum and common salt.Salts of iron are sometimes found, giving an inky taste to the water, and a yellowish tint tolinen washed in it.

The earthy salts and iron salts are the principal cause of hardness , which effect is oftenalso produced by an excess of carbonic acid, at least when it is in a greater quantity thanis sufficient to form the bicarbonates present.

Filtration luts no effect upon the hardness of water, but it is softened by exposure to theair, and sometimes by the process of boiling, when the earthy bicarbonates are decomposed.The New River water contains, according to Mr. Clark’s evidence, given before the Com-missioners of Inquiry into the State of Large Towns, 13 grains or more of carbonate of limein every gallon. In the middle of May, water taken from the Thames at Mortlake con-tained 14 grains, that from the East London Waterworks upwards of 16 grains, that ofthe Vauxhall Company, on the Surrey side of the river, 13 grains.

Thomas Clarke's, M-D., Process for Purifying Water. — To understand its nature, it isnecessarv to advert to a few chemical properties of the familiar substance chalk, whichat once forms the bulk of the impurity that the process will separate from water, andis the material whence the ingredient for effecting the separation is obtained. In waterchalk is almost or altogether insoluble ; but it may be rendered soluble by either of twoprocesses of a very opposite kind. When burned, as in a kiln, it loses weight: if dry andpure, only 9 ounces will remain out of 16; these 9 ounces will be soluble in water, butthey will require not less than 40 gallons of water for entire solution. Burnt chalk orquicklime, when held in solution, forms lime-water, which is perfectly clear and colourless.The 7 ounces lost when the chalk is converted into lime is carbonic acid gas. The othermode of rendering chalk soluble in water is nearly the reverse. In the former mode, apound of pure chalk becomes dissolved in consequence of losing 7 ounces of carbonic acid.To dissolve in the second mode, not only must the pound of chalk not lose the 7 ouncesof carbonic acid that it contains, but it must combine with 7 additional ounces of thatacid.

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 7ounces of carbonic acid would form a solution not sensibly different in ordinary use from thefiltered water of the Thames in the average state of that river. Chalk, or carbonate of lime,becomes a bicarbonate when it is dissolved in water by carbonic acid. Any lime-water maybe mixed with another, and any solution of bicarbonate of lime with another, w ithout anychange being produced ; the clearness of the mixed solutions would be undisturbed ; not so,however, if lime-water be mixed with a solution of bicarbonate of lime. Very soon a hazi-ness appears, this deepens into a whiteness, and the mixture soon acquires the appearance ofa well-mixed whitewash. When the white matter ceases to be produced, it subsides, andin process of time leaves the water above perfectly clear; the subsided matter being nothingbut chalk. What occurs in this operation will be understood, if we suppose that 16 ouncesof chalk, after being converted by burning into 9 ounces of quicklime, is dissolved, so as toform 40 gallons of lime-water ; that another 16 ounces is dissolved by 7 ounces of extra car-bonic acid, so as to form 560 gallons of a solution of bicarbonate of lime, and that the twosolutions are mixed, making up together 600 gallons. The 9 ounces of quicklime from the16 of chalk unite with the 7 extra ounces of carbonic acid that hold the other 16 ounces ofchalk in solution. These 9 ounces of quicklime and 7 ounces of carbonic acid form 16ounces of chalk, which being insoluble in water become visible at the same time that theother pound of chalk, being deprived of the extra 7 ounces of carbonic acid that kept it insolution, reappears. Both pounds of chalk are found at the bottom after subsidence. The609 gallons of water will remain above clear and colourless, without holding in solution anysensible quantity either of quicklime or of bicarbonate of lime.

The weight of chalk separated from the whole waters with which the several companiesin London supply the public annually is estimated by Dr. Clarke to be equal to 9000 tons,and that to purify it after the above method would cost only 10 1. per day.