the gypsiferous beds and dolomites of the Trias. At Birmenstorf the natural leaching of the gypsum rocks is imitated artificially; and either salt is dissolved according as it predominates in the particular stratum on which the process is performed; so that two kinds of medicinal waters are prepared. The purgative springs of Sedlitz, Seidschütz, and Püllna in Bohemia, derive their sulphate of magnesia from the Tertiary marl which they have traversed; and, this fact once recognized, similar waters have been produced for half a century by washing the rock—an experiment which was the starting-point of the artificial mineral-water industry. The origin of most of the sulphureted waters, or, to speak more exactly, of their sulphuret of calcium, is explained by the facility with which the sulphates can yield their oxygen to organic matters; a thing which occurs notably when carbonaceous matters, lignite, stone coal, or bitumen, are found associated with gypsum.
The origin of gaseous or acid springs, which constitute one of the most important families in a hygienic aspect, is connected with exhalations of carbonic acid, which are in turn one of the most remarkable phenomena of the globe's interior economy. The emanations of carbonic acid, as well as the springs to which they give character, are most commonly grouped near volcanoes, active or extinct, and ancient volcanic rocks, basalts, and trachytes. The granitic table-land of central France, in the chain of the Puys, as in the masses of Mont-Dore, the Cantal, and the Nivarais, exhales daily torrents of the gas, either dry or in solution, from more than five hundred springs. There are Royat, with its tumultuous ebullition. Saint-Allyre at Clermont, Saint-Nectaire, where all the oozings of the ground, even the road-ditches, boil with gas. Disengagements of carbonic acid are frequent in the mines of Pontgibaud, which are situated on the side of a crater and an ancient lava-flow, and where veins of silver-bearing lead-ore furnish conduits for the asphyxiating gas.
Countries where the volcanic rocks do not appear on the surface, but which are broken by deep dislocations, may also be the seat of exhalations of carbonic acid. The gaseous springs of Pongues, and some others of the Niévre, are situated upon simple faults. In northern Germany, on the left bank of the Weser, the country is riddled with fractures which give passage to abundant disengagements of carbonic acid, especially upon the plateau of Paderborn, and in the neighborhood of Pyrmont, Disburg, and Meinsberg. Acid springs occur under nearly the same conditions of stratification as the warm springs of which we are about to treat; and, like them, they reach the surface with the assistance of quartz or other mineral veins.
Sometimes the carbonic acid is abundant enough to make the waters in which it is incorporated spurt up violently; as at Montrond, Loire, where the intermittent eruptions from a depth of five hundred metres attain a great height; and at Nauheim, in Vétéravie, where a