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THE POPULAR SCIENCE MONTHLY. 



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 



Fig. 1. Block of Diorite from the Pic-du-Midi, shcwlng on its Surface a Melted and 

 Vitrified Track produced by the Passage of Lightning. (From a specimen iu tue 

 Museum ; figure one fourth the natural size.) 



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 grani- 

 toid diorite, that is, a mixture of triclinic feldspar and hornblende am- 

 phibole. The melted portion does not constitute a veneer, as in the 

 examples previously mentioned ; it is a track exactly marking the 

 course of the electic spark and ramifying as it did. The vitrified por- 

 tion 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 out- 

 side, 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 mu- 



