As the waters by which the springs are fed have often vast subterranean journeys to perform, their temperature is naturally independent of that of the seasons or of the changes of the atmosphere. Thus, cold springs occur in a tropical climate, when their subterranean channels descend from high mountains, and boiling sources gush forth in the Arctic regions when forced upwards from a considerable depth.
While the waters filter through the earth, they also naturally dissolve a variety of substances, and hence all springs are more or less impregnated with extraneous particles. But many of them, particularly such as are of a higher temperature, contain either a larger quantity or so peculiar a combination of mineral substances as to acquire medicinal virtues of the highest order.
The geological phenomena which favour the production of thermal springs are extremely interesting, and point to a deep-seated origin. By far the greater number of these fountains arise near the scene of some great subterranean disturbance, either connected with volcanic action, or with the elevation of a chain of mountains, or lastly by cliffs and fissures caused by disruption. Thus the thermal springs of Matlock and Bath accompany great natural fissures in the mountain limestone, and the hot springs of Wiesbaden and Ems, of Carlsbad and Toeplitz, all lie contiguous to remarkable dislocations, or to great lines of elevation, or to the neighbourhood of a volcanic focus.
One of the most remarkable phenomena of thermal springs is the constant invariableness of their temperature and their mineral impregnation. During the last fifty or sixty years, ever since accurate thermometrical observations and chemical analyses have been made, the most celebrated mineral sources of Germany have been found to contain the same proportion of mineral substances. This is truly astonishing when we consider that the latter are merely dissolved by the waters while passing through the bowels of the earth, and that a considerable number of them are frequently found together in the same source.
Another remarkable fact is, that, even in countries exposed to violent and frequent earthquakes, so many subterranean watercourses have remained unaltered for 2,000 years at least. The sources of Greece still flow apparently as in the times of Hellenic antiquity. The spring of Erasinos, two leagues south of Argos, on the declivity of the Chaonian mountains, is mentioned by Herodotus. At Delphi the Cassotis (now Wells of Saint Nicholas) still flow under the ruins of the temple of Apollo, and the hot baths of Aidepsos still exist in which Sylla bathed during the Mithridatic war.
Many springs exhibit the singular phenomenon of an intermittence which is independent of the quantity of rain falling in the district, or of the flux and reflux of the tide in a neighbouring river. In many cases the simple and well-known hydrostatical law exemplified in the common siphon[[3]] affords a very ready and sufficient explanation of the phenomenon.
In the annexed diagram the vessel a communicates, by a tube c, with the siphon tube b, and it is manifest that when the water in a rises above the level of the top of b, it will begin to flow over and escape, as at d. But as soon as this is the case the tube b begins to act as a siphon, and draws off all the water in a, so that if a constant supply is poured into a, but at a rate slower than the rate of the discharge at d, there will be an intermittent discharge, the interval depending on the relation of the rate of filling to that of emptying.
SECTION OF AN INTERMITTENT SPRING.