1832-3.] TUBES FORMED BY LIGHTNIXG. 57 



of the wall of the tube varies from a thirtieth to a twentieth of an 

 inch, and occasionally even equals a tenth. On the outside the 

 grains of sand are rounded, and have a slightly glazed appearance : 

 I could not distinguish any signs of crystallization. In a similar 

 manner to that described in the Geological Transactions, the tubes 

 are generally compressed, and have deep longitudinal furrows, so 

 as closely to resemble a shrivelled vegetable stalk, or the bark of 

 the elm or cork tree. Their circumference is about two inches, 

 but in some fragments, which are cylindrical and without any 

 furrows, it is as much as four inches. The compression from the 

 surrounding loose sand, acting while the tube was still softened 

 from the effects of the intense heat, has evidently caused the 

 creases or furrows. Judging from the uncompressed fragments, 

 the measure or bore of the lightning (if such a term may be used) 

 must have been about one inch and a quarter. At Paris, M. 

 Hachette and M. Beudant * succeeded in making tubes, in most 

 respects similar to these fulgurites, by passing very strong shocks 

 of galvinism through finely-powdered glass : when salt was added, 

 so as to increase its fusibility, the tubes were larger in every 

 dimension. They failed both with powdered felspar and quartz. 

 One tube, formed with pounded glass, was very nearly an inch 

 long, namely, '982, and had an internal diameter of -019 of an inch. 

 When we hear that the strongest battery in Paris was used, and 

 that its power on a substance of such easy fusibility as glass was 

 to form tubes so diminutive, we must feel greatly astonished at 

 the force of a shock of lightning, which, striking the sand in 

 several places, has formed cylinders, in one instance of at least 

 thirty feet long, and having an internal bore, where not compressed, 

 of full an inch and a half; and this in a material so extraordi- 

 narily refractory as quartz ! 



The tubes, as I have already remarked, enter the sand nearly 

 in a vertical direction. One, however, which was less regular 

 than the others, deviated from a right line, at the most consider- 

 able bend, to the amount of thirty-three degrees. From this same 

 tube, two small branches, about a foot apart, were sent off; one 

 pointed downwards, and the other upwards. This latter case is 

 remarkable, as the electric fluid must have turned back at the 

 acute angle of 26, to the line of its main course. Besides the four 

 tubes which I found vertical, and traced beneath the surface, thero 

 * Annals de Chinrie et de Physique, torn, xxxvii. p. 319. 



