6o EXPERIMENTS on WHINSTONE and LAVA. 



refumes its ftony character. But, not content with analogy- 

 alone, I refolved^to afcertain the truth of thefe conclufions in a 

 direct manner, and performed the following experiments with 

 fpecimens of fix different lavas, four of which, to my certain 

 knowledge, had made part of external volcanic currents. In the 

 prefent ftate of geology, too much pains cannot be beftowed in 

 afcertaining that the fpecimens collected are really lavas, fince 

 this circumflance has been frequently overlooked, as I mall en- 

 deavour to fhew, when I fpeak of the differences between them.:, 

 and whinftone. 



No. i. Lava of Catania. 



This is the celebrated lava, which, in 1669, laid wafte great 

 part of the town of Catania. The interior part of the cur- 

 rent, (accurately defcribed by M. Dolomietj, IJles Ponces, 

 p. 256.*), from which the fubjeel: of our experiment was taken, 

 confifts of a light grey bafis, interfperfed with cryftals of fel- 

 fpar and of fchorl, (augit). It bears a general refemblance to 

 the rock of the bafaltic columns on Arthur's Seat, and exhibited 

 the fame phenomena in our experiments. After flrong heat, the 

 whole was reduced, by rapid cooling, to pure black glafs ; but 

 when the heat applied was moderate, the felfpars remained 

 unchanged. Being maintained, after a fecond fufion, in a 

 temperature of 28, both thefe glafTes yielded ftony arid cryftal- 

 lized fubftances, fomcwhat lefs fufible than the original ; and 



when 



* " Elle eft formee d'une pate de roche de corne grife, a grains fins, melee 

 e( d ; ecailles, et de criftaux de feld-fpath de meme couleur; elle contient un tres 

 ** grand nombre de criftaux de fchorl noir, et de grains de cryfolites jaunes, les uns 

 " et les autres quelquefois chatoyans, de differentes ccruleurs dans leurs fradures. — 



i( — Cette lave a une caffure fechie, et un grain rude, furtout dans 



" le centre des courans ; c'eft la oil elle a toujours conferve une couleur plus claire, 

 Ji qui doit etre cellc de fa bafe ; fur les bords es les furfaces elle s'eft fort noircie ; 

 " elle y a acquis une affez. forte aftion fur Vaiguille aimantee que celle du centre n'a 

 u prefque point," 



