496 WATER characters modified by the presence of foreign matters. That which falls from the clouds as rain or snow water holds in solution, besides the gases nitrogen, oxygen, and carbonic acid dis- solved from the atmosphere, small portions of ammonia and nitrous compounds, and a minute but variable amount of mineral matters which were previously suspended in the air. After falling on the earth these same waters become further impregnated with foreign ingredients. From decaying vegetation they take up two classes of substances : first, the organic prod- ucts of decomposition, the so-called soluble organic matters, which give to the waters of marshy districts their brownish color ; and sec- ond, the mineral matters which form an essen- tial part of all vegetation and constitute its ash, but are for the most part liberated in sol- uble forms during its slow decay. These con- sist chiefly of salts of potash, lime, and magne- sia, with phosphates and silica. At the same time the free oxygen of the atmospheric wa- ters is absorbed by the organic matter and replaced by carbonic acid derived therefrom. In rivers and waters exposed to the further action of the air, oxygen is again absorbed, and a slow oxidation of the dissolved organic mat- ters is effected. When the atmospheric waters sink into the soil, either directly or after being thus impregnated with the products of organic decay, they undergo still further changes, de- pendent upon the nature of the strata through which they pass. Ordinary soils contain no matters soluble in pure water, yet they are not without action on the infiltrating waters whose composition has just been described, especially if the soils are more or less clayey. In this case the silica and phosphates, together with much of the organic matter, are retained, while the potash salts are exchanged for those of soda and of lime. Carbonates of lime and magnesia, when present in the soil, are more- over taken into solution by the carbonic acid present. Hence the ordinary waters of wells and springs, supplied by this filtration, differ very much in their composition from the su- perficial waters. These reactions, in virtue of which the foreign matters derived by the superficial waters from the decay of plants are absorbed by the soil, are important alike for the nutrition of subsequent generations of plants and for the purification of the waters, which are thns rendered potable. Besides these reactions, which depend upon the min- eral matters previously dissolved by the atmos- pheric waters, there are others not less impor- tant due to the direct action of the water and its dissolved gases on the solid rocks, in vir- tue of which the silicated minerals of these are decomposed with the liberation in a soluble form of certain of their elements. In this way large quantities of alkalies, lime, and magnesia are set free and are dissolved in the form of carbonates, together with a considerable pro- portion of silica, which is liberated in a solu- ble condition. This process of decay has been going on from remote ages, and has effected the decomposition and disintegration of vast portions of the crystalline rocks, which have thereby been reduced to the condition of clays, while immense amounts of soluble matter have been added to the waters of the earth. The rivers which drain regions of crystalline rocks are, as a result of this process, remarkable for containing in solution carbonates of soda and potash, together with a large relative propor- tion of silica, as may be seen in the waters of the Ottawa, the Loire, and the Garonne. Wa- ters of a like origin are deprived by filtration through the soil of their silica and potash. A process similar in its results probably takes place at great depths under the influence of carbonic acid from subterranean sources, from which result waters more highly impregnated with alkaline salts, constituting some of the best known mineral springs. In addition to these various impregnating matters should be noticed those derived from the oxidation of metallic sulphurets, chiefly pyrites, giving rise to sulphates of iron and alumina, and indirectly to sulphates of lime and magnesia. Another and an important supply of foreign matters is derived from the soluble salts, such as chlo- rides and sulphates, which arc enclosed in many stratified rocks, while the carbonates of lime and magnesia of these rocks are themselves dissolved by carbonic acid, and with the iron oxide taken into solution through the interven- tion of the organic matters contribute to the complex constitution of the water of springs, rivers, and lakes. The quality called hardness in such waters depends upon the power they possess of decomposing soap, and is due to the salts of lime and magnesia, whether present as carbonates or in the form of sulphates and chlorides. Boiling, by expelling the excess of carbonic acid, precipitates the carbonates in an insoluble condition, and thus gives rise to incrustations. The sulphate of lime, from its sparing solubility at elevated temperatures, is deposited in like manner in high-pressure boil- ers. The ocean is the great receptacle of all soluble matters from the land, and from its waters have been deposited the greater part of the stratified rocks of the earth's crust. The waters of the ocean vary somewhat in composition, but contain on an average about 84 parts of solid matter in 1,000, though in the Mediterranean, where evaporation is rapid, this rises to 36 or even 40 parts, and in other regions falls very much lower, owing to the admixture of fresh water from the land. The saline matters of the sea may be regarded as consisting of from 78 to 80 per cent, of chloride of sodium or sea salt, with 2 per cent, of chlo- ride of potassium, 7 or 8 per cent, of chloride of magnesium, about 7 per cent, of sulphate of magnesia, and from 8 to 4 per cent, of sulphate of lime. A different arrangement of these elements in the water is conceivable, and is even probable ; but when exposed to sponta- neous evaporation, as in the manufacture of
