TUBES FORMED BY LIGHTNING, 77 



of minute entangled air or perhaps steam bubbles, 

 like an assay fused before the blowpipe. The sand 

 is entirely, or in gi-eater 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 Geo- 

 logical Transactions, the tubes are generally com- 

 pressed, and have deep longitudinal fuiTows, so as 

 closely to resemble a shrivelled vegetable stalk, or 

 the bark of the elm or cork tree. Their circum- 

 ference is about two inches, but in some fragments, 

 which are cylindrical and without any furrows, it 

 is as much as foui inches. The com2:)ression 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 fi-om the uncompressed fragments, the 

 measure or bore of the lightning (if such a teiTn 

 may be used) must have been about one inch and 

 a quarter. At Paris, M. Hachette and M. Beu- 

 dant* 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 fusibili- 

 ty, 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 di- 

 ameter of -OlO 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 

 ♦ Annales de Chimie et de Physique, torn, xxxvii., p. 319. 

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