The mechanical impurities of water, or the solid particles rendering it muddy or milky, may in most cases be removed by mechanical means. The two processes for this purpose are subsidence and filtration. The effects of subsidence are strikingly seen in the case of rivers that pass through lakes. See GENEVA, LAKE OF. The subsidence of solid particles depends on their own weight, as compared with the weight of an equal bulk of water. To favor the process, the most, perfect stillness should he allowed. It is expe dient to have partitions placed in the subsiding reservoirs at short intervals, more effec tually to prevent the agitation of the water. The water should be run off from the top, and not from the bottom. By making the bottom of the subsiding reservoir form a declivity from opposites sides, and providing means to let off the water occasionally from its lowest depth, it is possible to get quit of the subsided mud. It is always found of advantage in clearing water from solid particles, whether by subsidence or by filtration, to mix together streams of different qualities.
In constructing an artificial filter on a large scale, a basin is formed, having the floor nearly level, but slightly inclining toward a center line, and made water tight by pud dling the bottom and sides with Glay. On the floor is laid a series of layers of gravel, coarse at first, and getting gradually finer upward; next, a layer of slate-chips or sea shells, then one of coarse sand,-on which is placed the actual filtering layer of fine sand. The depth of this layer is from twelve to thirty inches, that of the entire mass from four to six feet. The water being admitted gently on the top of the sand, sinks down and is conducted by a series of channels, generally of tile-pipes, into the main drain. A filter in a clean state will pass from twelve to eighteen vertical feet of water in twenty-four hours. The solid matter intercepted does not penetrate more than than three-fourths of an inch into the sand, so that, by removing a very thin film from the surface, the filter is again clean. What is scraped off the top, is capable of being washed and put again to use.." This process of filtration," says prof. Clark, is "efficacious in removing mechani cal impurities to an extent that could scarcely be believed without seeing the process." The cleansing power of sand can hardly be accounted for on the theory of mere mechanical interception. Though there is no chemical action, strictly speaking there is no doubt that the attraction of adhesion is at work—a power that plays a greater part in natural processes than has generally been assigned to it. Some substances manifest this adhesive attraction more strongly than sand, and have therefore still greater efficacy as filters; though practically, and on the large scale, sand is the most eligible. Powdered charcoal has long been known as a powerful filtering medium, attracting and detaining especially organic matter. Animal charcoal, or that derived from burning bones, is still more efficacious than wood charcoal. A filter of animal charcoal will render Lon don porter almost colorless.
According to recent researches, it would seem that loam and clay have similar prop erties, and may be made available as fillers. Professor Way states that "they have
powers of chemical action for the removal of organic and inorganic matters from water to an extent never before suspected." The filthiest liquids, Such as putrid urine and sewer-water, when passed through clay, dropped from the filter colorless and inoffensive. The clay used was that known as pipe-clay.
For filters for domestic use, see FILTER.
Sylening of Water rendered Hard by Process.—This is one of the most beautiful applications of science to the arts of life that could perhaps be named. We extract the inventor's own account of it (retaining the old nomenclature) as read at the meeting of the society of arts: "In order to explain how the inventor operates, it will be necessary to glance at the chemical composition and some of the chemical properties of chalk; for while makes up the great bulk of the matter to be separated, chalk also contains the ingredient that brings about the separation. The invention is a chemical one for expelling chalk by chalk. Chalk, then, consists, for every 1 lb. of 16 oz., of lime, 9 oz.; carbonic acid, 7 oz.
"The 9 oz. of lime may be obtained apart, by burning the chalk, as in a lime-kiln. The 9 oz. of burnt lime may be dissolved into any quantity of water not lest than 40 gallons. The solution 'would be called lime-water. During the burning of the chalk to convert it into lime, the 7 oz. of carbonic acid are driven off. This acid, when uncom blued, is naturally volatile and mild; it is the same substance that forms what has been called soda-water, when dissolved in water under pressure.
"Now, so very sparingly soluble in water is chalk by itself, that probably upward of 5,000 gallons would be necessary to dissolve 1 lb. of 16 oz.; but by combining 1 lb. of chalk in water with 7 oz. addititional of carbonic acid—that is to say, with as much more carbonic acid as the chalk itself contains—the chalk becomes readily soluble in water, and when so dissolved, is called bicarbonate of lime. If the quantity of water containing the 1 lb. of chalk with 7 oz. additional of carbonic acid, were 400 gallons, the solution would be water of the same hardness as well-water from the chalk-strata, and not sensibly different in other respects.
"Thus it appears that 1 lb. of chalk, scarcely soluble at all in water, may be ren dered soluble in it by either of two distinct chemical changes—sOluble by being deprived entirely of its carbonic acid, when it forms linie-water, and soluble by combining with a second dose of carbonic acid, making up bicarbonate of lime.
"Now, if a solution of the 9 oz. of burnt lime, forming lime-water, and another solu tion of the 1 lb. of chalk and the 7 oz. of carbonic acid, forming bicarbonate of lime, be mixed together, they will so act upon each other as to restore the 2 lbs. of chalk, which will, after the mixture, subside, leaving a bright water above. This water will be free from bicarbonate of lime, free from burnt lime, and free from chalk, except a very little, which we keep out of account at present for the sake of simplicity in this explanation. The following table will show what occurs when this mutual action takes place: