Page:Popular Science Monthly Volume 32.djvu/835

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O understand the chemical composition of subterranean waters, we must inquire anew of the geological constitution of the country, and it will usually answer with precision and certainty.

Water does not have to remain long in the soil to dissolve and remove various substances from the rocks. Chemical analysis has already shown that such substances exist in the Water-sheets of alluviums in more notable proportions than in the neighboring rivers. The difference is sufficient to explain why in Hungary, Egypt, India, and China, river-water is preferred to well-water for culinary uses. In the subsoil of inhabited places, water is not charged with mineral substances only; but the liquids of manure-heaps and other elements of corruption are transmitted to it by sewers, factories, and cemeteries. The unhealthy effect of the impurities thus conveyed to the wells has been frequently recognized; and it would be surprising if water exposed for centuries to such infiltrations did not cease to be potable.

The most common bodies found to be contained in subterranean waters are oxygen, nitrogen, and carbonic-acid gases, chlorides, carbonates, silicates, and sulphates of lime, magnesia, and soda, and organic substances, in the presence of excessive proportions of which water may cease to be drinkable, or even fit for domestic uses.

Water sometimes acquires also useful properties during its subterranean course. Springs which are employed as therapeutic agents are endowed with mineral qualities. The name of mineral is sometimes extended to other springs the high temperature of which makes them susceptible of similar uses, even when the amount of foreign matter they contain is inferior to what is included in many potable waters.

Chloride of sodium, or sea-salt, is sometimes present in so feeble proportions as not to be perceptible to the taste. It is derived from very widely distributed rocks, which contain traces of it. In other springs it exists in much stronger proportions. Such springs derive their salinity from beds of rock-salt, which it has been found profitable to mine or bore for directly.

Gypsum is dissolved in water under similar conditions. It is present in large masses and in a very fine state of division in the Parisian Tertiary beds, and, being freely soluble, gives hardness to many springs. It is often associated with other substances, which give therapeutic qualities to water, as in the cold springs of Contrexeville and Vittel in the Vosges, and the hot springs at Ago vie and Schinznach in Baden. The mineralization in these cases is due to the presence of the soluble sulphates of lime and magnesia, which are furnished by