LAVA. In addition to the observations of Sir William Hamilton, Bergmann, Formes, and Dalmieu, on the composition of different lavas, which have been given in the Encyclopædia, we cannot refuse ourselves the pleasure of noticing, in this place, those of Sir James Hall. From a number of well-devised experiments, Sir James thinks himself warranted to conclude, that lava and whinstone are intrinsically the same substance; and that their apparent differences arise wholly from the circumstances under which they have passed from a liquid to a solid state. The lavas, it is well known, have been cooled rapidly in the open air, and the whins (according to Dr Hutton's theory, which Sir James seems willing to adopt) slowly in the bowels of the earth.
Though we are far from adopting that theory in all its parts, to which we think insuperable objections may be made (see EARTH, Encycl. n° 120), we admit, that the experiments of Sir James Hall go far to establish the identity of lava and whinstone. These experiments were made upon seven different species of whinstone and six lavas, of which four were broken from the currents of Etna and Vesuvius by Sir James himself. Each of the original whinstones was reduced, by fusion and subsequent rapid cooling, to a state of perfect glass. This glass, being again placed in the furnace, was subjected to a second fusion. The heat, being then reduced to a temperature generally about 28° of Wedgewood, was maintained stationary for some hours; when the crucible was either immediately removed, or allowed to cool with the furnace. The consequence was, that in every case the substance had lost the character of glass, and by crystallization had assumed in all respects that of an original whinstone. It must be owned, that in most cases the new production did not exactly resemble the particular original from which it was formed.
ed, but some other original of the same class; owing to accidental varieties in the mode of refrigeration, and to chemical changes which unavoidably took place during the process. In the case, however, of the rock of Edinburgh castle, and of that of the basaltic columns of Staffa, the artificial substances bear a complete resemblance to their originals, both in colour and texture.
The lavas were now treated in the same way, and were each, by fusion and rapid cooling, reduced, as the whinstones had been, to glass. This glass, when fused again and cooled slowly, yielded the same kind of crystallized, stony, or earthy masses, completely resembling an original whin or lava.
Although the internal structure of lava was thus accounted for, yet Sir James was embarrassed with the state of its external surface; which, though cooled in contact with the open air, is seldom or never vitreous, holding an intermediate station between glass and stone; but this difficulty was removed by a circumstance which took place in the course of these experiments. It was found, that a small piece of glass of any of the lavas, or of several of the whins, being introduced into a muffle, the temperature of which was at any point between the 20th and the 22d degree of Wedgewood's scale, the glass became quite soft in the space of one minute; but, being allowed to remain till the end of a second minute, it was found to have become hard throughout in consequence of a rapid crystallization, to have lost its character of glass, and to have become by 12 or 14 degrees more infusible, being unaffected by any heat under 30, though the glass had been fusible at 18° or at 16°. This accounted for the scoria on the surface of lavas; for the substance even at the surface, being in contact with the flowing stream, and surrounded with heated air, could not cool with excessive rapidity: and the experiment shews, that should any part of the mass, in descending heat, employ more than one or two minutes in cooling from 22 to 20, it would infallibly lose its vitreous character.
Independently of any allusion to system or to general theory, Sir James Hall flatters himself that these experiments may be of some importance, by simplifying the history of volcanoes; and, above all, by superceding some very extraordinary, and, he conceives, unphilosophical opinions advanced with regard to volcanic heat, which has been stated as possessing very little intensity, and as acting by some occult and inconceivable influence, or with the help of some invisible agent, so as to produce liquidity without fusion. These suppositions, which have been maintained seriously by some of the most celebrated naturalists in Europe, have originated from the difficulty of accounting for the stony character of lavas when compared with that of glass, which they assume in consequence of fusion in our furnaces. But now he hopes we may be relieved from the necessity of such violent efforts of imagination, since the phenomena have been fully accounted for by the simple, though unnoticed, principle of refrigeration, and have been repeated again and again with ease and certainty in a small chamber furnace.