When the rain-water strikes the earth's surface it at once commences to take up the soluble bodies to be found therein, and gradually becomes more and more impure. As it percolates through the different strata of which the earth's crust is composed, it dissolves out special objects, the nature of which will obviously depend upon the nature of the compounds to be found in those strata. Usually, however, in addition to oxygen and nitrogen, a large proportion of free carbonic acid is present, which adds considerably to its palatable taste and pleasing appearance. These desirable acquisitions are destroyed, or at any rate materially modified, on boiling the water, owing to the expulsion of the gas. The presence of the carbonic anhydride in the water is probably due to the decomposition of the organic matter in the ground through which it permeates, and to its evolution from certain subterranean, sources, such, as caverns, mines, etc., and where the water is frequently-charged under considerable pressure. The dissolved solids chiefly found in spring-water are calcium, magnesium, sodium, potassium, iron and manganese carbonates, sulphates, chlorides, sulphides and silicates. In some springs these substances are present in very trifling quantities, but in others to such a great extent as to unfit them for ordinary potable purposes. When such is the case they are spoken of as "natural mineral waters". These are divided, according to the constituents that are characteristic of them, into sulphurous, saline, carbonated, silicious, etc. See Chapter on Mineral Waters. Not only do different springs exhibit wide differences in the amount of their chemical constituents, but also their temperatures vary considerably; thus, whilst some are but little removed from the freezing point of water, others are as hot as 97° C. - three degrees below its boiling point.

In sinking wells one must expect to dig until it reaches the plane of saturation, that is, the surface of the stock of water underlying the whole neighborhood, just as rain falling on a large vat or other vessel filled with sand or gravel would sink, by soakage, until it reached the plane of saturation, that is, the level of water that had previously fallen on the surface and collected in the bottom of the vessel. The plane of saturation is not itself level in the sense in which the water of an ordinary lake is level, for the water is held in the whole mass of the hill as in a sponge, by capillary attraction or adhesion. The plane is in fact an inclined plane as regards any few yards of its surface, and, as regards its whole mass, a sort of low cone, or hill within the hill, its sides on a much less sharp incline than the incline of the hill, whatever that may be. Then, too, the surface of the cone of water will scarcely be a perfectly regular surface, inasmuch as the material forming the hill will probably vary in porosity, and water is sucked up into narrow pores to a greater height than into wide pores; hence the height of water in contiguous wells may not be absolutely regular. The stream of water yielding a spring will pass more readily through loose than through close ground; hence in sinking wells into spring-laden strata one may have to go much deeper for water in some places than in others.

Well-water, whether drawn from the underground stock of water common to the district, or from a true spring supplied from a distance, will, as already explained, be aerated by reason of the presence of the oxygen and nitrogen gases naturally dissolved from the air, and the carbonic acid gas partly dissolved -from the air, but more especially produced within the water itself by the true burning of dissolved vegetable or other carbonaceous matter by the contained and always renewed oxygen. The water will also contain the usual small amounts of various saline and calcareous substances dissolved from the soil through which the water has percolated.

Unfortunately, well-water is also liable to contain a certain proportion, sometimes more, sometimes less, of the incompletely purified drainage waters of the surrounding country. The use of the well-water in the City of New York has been prohibited by the Board of Health. Situated in the vicinity of a place of interment, a well may contain the decaying animal matter of the dead; situated near a dwelling having old-fashioned sanitary arrangements, or one having modern but faulty pipe-sewerage systems, it may contain the decaying animal matter of the living. Even highly manured meadows or gardens may contribute impurities to water unless rain falling on the area has to percolate through some feet of porous air-laden subsoil before reaching the well. Shallow wells are most likely thus to be badly fouled. First, because their nearness to the source of contamination favors the minimum of dilution of the contaminating matter by the rainfall of the immediate vicinity. Secondly, because the oxidation, or true burning out of animal and vegetable matter in the water by the air in that water, depends on the extent of exposure of the water to the air in the pores of the soil through which the water percolates, and that exposure is clearly less if the reservoir or well, or rather stock of. water therein, is only a few feet than if it is many feet below the surface. Indeed, the only ordinary source of contamination of deep well-water by surface impurities is the running of impure surface water down the sides of the well. The exposure of such impure water to the air whilst it trickles down the well will certainly be quite insufficient to burn out the impurities; whereas the thorough admixture of the impure water with the air, that is with the concentrated oxygen of the air, in the pores of the soil, during the percolation of the water through the soil to the level of the water in the deep well, will sometimes suffice to burn out the impurities and convert the water into pure water - convert it by the method always adopted by nature - the method which transforms harmful carbonaceous matter into the useful carbonic acid gas, and harmful nitrogenous matter into useful nitre - the method by which nature enables us to use over and over and over again the constant stock of the water of the world.

The best means of preventing the pollution of deep well-water by impure surface water is to line the sides of the well with something im. pervious to water, extending the lining a foot above the ground, and to such a distance down as may be deemed desirable. If the sides be iron tubes the joints will of course be flanged and be properly bolted together. If the sides be formed of brickwork the bricks should be set in cemenl and the front face be "floated," that is, plastered over with the cement, If the well is already constructed and the bricks have been set with mor, tar, or, as more usual, without mortar, the inner face should be covered with at least an inch of good cement well prepared and well applied.

Deep well-waters are among the best varieties of water for carbonating purposes. Not only are they, usually, free from contamination, but art. not excessively cold in winter and are deliciously cool in summer - two requisites of great advantage in improving the gaseous nature of the drink.