Popular Science Monthly/Volume 20/March 1882/Effects of Lightning on Rocks and Soil
THE effects of lightning on the tops of mountains are often very intense. Among them have been cited such works as the transportation of large masses to a considerable distance. They also frequently include the development of high degrees of heat. The clearest possible proof of this fact is afforded by the superficial fusion of rocks, even of such refractory ones as granite and other crystalline stones. Laussure long ago described vitrifications of this kind on the summit of Mont Blanc; Ramond has found them on the Pic-du-Midi, and Humboldt in Mexico; and the characters they exhibit are everywhere uniform.
The vitrifications are generally only a few tenths of a millimetre thick, but they sometimes extend over surfaces of nearly a square metre, on which the rock appears to be veneered with a kind of gray or yellowish enamel, in which bubbles or swellings several millimetres in diameter may be distinguished. The specimen represented in Fig. 1, which is exhibited in the geological department of the museum at
Paris, was found on the top of the Pic-du-Midi by MM. Baylac and Albert Tissandier. It is of special interest. The rock is of a granitoid diorite, that is, a mixture of triclinic feldspar and hornblende amphibole. The melted portion does not constitute a veneer, as in the examples previously mentioned; it is a track exactly marking the course of the electric spark and ramifying as it did. The vitrified portion extends along the natural external surface of the rock, and then plunges into a fissure, within which it disappears. In this respect the fulguration is extremely like another accident which is known under the name of fulgurites or fulminated tubes, splendid specimens of which may be seen at the museum. As the two cuts in Fig. 2 show, there are irregular tubes, the substance of which, a kind of natural glass, is the product of the solution of siliceous sands that have been struck by lightning. The tube is smooth within, but rugose on the outside, on account of the agglutination of imperfectly melted particles of sand. Fulgurites are generally ramified at their lower end. Their interior diameter varies from a millimetre to five centimetres, or two inches; and their length, which is variable, may reach ten metres, or more than thirty feet.
These curious accidents do not seem to have been remarked before 1711, when Hermann observed them in Silesia; since then all the museums have secured specimens of them for their collections. It is hard to get large ones, and they command a high price. At first, their true nature was misapprehended. They were regarded successively as incrustations formed around roots that had disappeared; as the cells constructed by worms of extinct species; and as a kind of stalactites. Hentzen seems to have been the first one who attributed them to lightning;
and his opinion has been shown to be correct by Blumenbach and by Tiegler. More recently, Nature has been caught in the act; that is, fulgurites have been found in sand which was still hot, at the spot where the lightning had been seen to strike. Besides, several experimenters, as Beudant, Huchette, and Savart, have obtained tubes analogous to fulgurites by discharging the great electrical battery of the Conservatoire des Arts et Metiers through beds of pounded glass, or of sand mixed with sea-salt to make it more fusible. Fulminated tubes are found principally in places where beds of sand lie upon a soil which contains water, and is consequently a conductor of electricity; for example, at certain points in Silesia, in Eastern Prussia, Poland, Cumberland, and Brazil.
- Translated from "La Nature."