Attention to some common phenomena may suffice to show, that for this difficulty there is no foundation; and that, on the contrary, the general laws of nature seem to give on the whole a tendency to material bodies, when they are in a moist or softened state, promptly to acquire that degree of hardness, which under the existing circumstances they are capable of attaining. We know, for example, how soon a wet lump of clay becomes very hard by the ordinary process of exsiccation under exposure to the atmosphere. Under a solstitial sun, and in a season of drought, the surface of the earth is rendered, even in our temperate clime, impenetrable to the instruments of agriculture and we are thus enabled to enter into the full import of the curse denounced against the Jewish nation in the event of their rebellionagainst God; "Thy heaven that is over thy head shall be brass,” scorching as a brazen vault; “and the earth that is under thee shall be iron*." The return of metals to a fixed state by cooling is extremely rapid. So likewise is the crystallisation of metallic and other neutral salts. The tenden

* Deut. xxviii. 23.

cy of gypseous substances to almost instantaneous induration is exemplified in the familiar instance of the consolidation of fluid plaster. A kindred tendency is manifested in lime by the formation of mortar. In crude calcareous earth, when diffused in the shape of invisible atoms in water, it is rendered equally apparent by all the varieties of stalactites and sparry concretions pendent from the domes or accumulating on the walls of caverns: and by the speedy and dense incrustation of every object which lies in the course of the stream from a petrifying spring*. That the same propensity exists in siliceous earth, is proved by the high mounds of flint raised on every side by deposition from the boiling fountains of Geyser in Icelandt. Nay, it is well understood by persons accustomed to witness chemical experiments, that the mixture of two bodies, each of which is previously converted into a perfectly transparent and aeriform state, may instantane

* As at Matlock, and in other limestone districts. † See Letters on Iceland by Von Troil, second edition, p. 342, &c. ; and the description of the boiling fountains of Geyser in the recent publication by Sir James Mackenzie.

ously produce, by their union, a visible and solid substance. Thus if muriatic acid, and the salt commonly known by the denomination of volatile alcali, (in the systematic language of chemistry termed ammonia,) be separately transformed, as may be effected by a very easy process, into transparent gases, and then mixed together, a cloud of white powder is instantly produced by their incorporation; the gases are completely absorbed; and the powder proves, on examination, to be the neutral salt which results from the chemical union of the two ingredients in their ordinary states. I may add, as illustrative of the subject, an example which, many years ago fell very frequently under my notice at Scarborough, of the rapid formation of a hard ponderous substance by the union of its component parts in an aeriform condition. The cliffs on the shore between the town and the Spa consist of red clay, and contain pyrites. The spring-tides, driven against the base of the cliffs, had washed away portions of the clay; fragments of which, bleached, rounded into the shape of pebbles, but remaining soft to the touch, were daily to be seen lying mixed with the gravel near the mark of high water. Of

these clay pebbles, if I may use the expression, many had soon acquired a sort of metallic hue, sufficient to enable the eye, after some practice, to single them out at a little distance amidst the gravel. On examination they were found to be wholly covered with a coating of a new and hard substance of a uniform structure, in some cases barely thicker than a mere film, in others as thick as a shilling, in others as an eighth or a quarter of an inch, or even much more. In those of recent formation the white nucleus of clay was yet moist and soft; in the others, dry, but sometimes detached from the crust, and, when shaken, rattling within it like the contracted kernel of a nut within the shell, or the chrysalis of a silk-worm within its coccoon; in those of older date, perfectly hard, and incorporated with the crust, but always distinctly marked by its hue. Some of the specimens collected by me are still in my possession, and are now before me and the pallid colour of the nucleus is as strongly contrasted as it was at first with the dark ferruginous brown of the crust. The crust appears evidently to be ironstone. Its metallic contents seem to have been furnished from the cliff, as likewise

E