which it circulated, by vestiges of different kinds which permit us to reconstitute the various circumstances of its course.
The external features of an organized being make its constitution known only in an incomplete manner. An adequate anatomical study must penetrate to its interior organs and tissues. Thus, existing thermal springs, even if we take care to scrutinize in the most careful manner the constitution of the country and the conditions where they issue, do not suffice to reveal their economy with precision. Their constantly flowing columns of water, even when they are not accompanied with irrespirable gases, prevent our reaching their channels of ascent. In the very exceptional cases in which it is possible to penetrate below their orifices of emergence, as at Bourbonne and Plombières, the curious facts which we observe cause us to regret that we can not descend lower. Nature seems to have desired to withdraw from our sight the actual workings of subterranean waters, especially when they are engendering minerals. Water is not more rare than heat in the masses of the interior of the globe. Even when it does not circulate in natural channels, it is at least present, held imbibed in the most compact rocks. In clays, although combined, it is not less susceptible of acting chemically than in the free condition. Thus, what we have obtained only with many difficulties in our experiments, the action of superheated water, is found vigorously exemplified everywhere in the interior of the rocks, where the effective resistance to enormous pressures permits the realization of more complete results than are possible with the fragile apparatus of our laboratories.
The circumstance that heat stored in masses of so little conducting power as stony substances is preserved for a very long time, is eminently favorable to chemical combinations and to crystallization. Nature possesses another superior advantage over man in having extremely long lapses of time at her disposal. The importance of this advantage, in the application in which we are now regarding it, appears plainly from what has occurred in the Roman masonry of Plombières. Besides this, reactions which go on slowly do not require so high a temperature as those that are of shorter duration.
The study of waters in their course and effects in ancient epochs thus seems to complete the history and broaden the view of their subterranean works. Here, then, a real exchange of light takes place. The past illuminates the present as much as the present illuminates the past. There is nothing, moreover, to prove that phenomena of this character do not continue down to our own days. We have a right to believe that similar actions are still going on, but in interior regions beyond the reach of our powers of observation. Superheated water, which betrays its ex-
