THE VOYAGE OF THE BEAGLE 



71 



appeared, from the number of minute entangled air or per- 

 haps steam bubbles, like an assay fused before the blowpipe, 

 i The sand is entirely, or in greater part, siliceous; but some 

 points are of a black colour, and from their glossy surface 

 possess a metallic lustre. The thickness 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 Transac- 

 tions, 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 fur- 

 rows. Judging from the uncompressed fragments, the meas- 

 ure 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 11 succeeded in making tubes, in 

 most respects similar to these fulgurites, by passing very 

 strong shocks of galvanism 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 extraordinarily refractory as quartz ! 



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

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

 u Annals de Cflinlie et de Physique, torn, xxxvii. p. 319. 



