EXPERIMENTS on WHINSTONE and LAVA. 49 



to mpft perfect black glafs. A crucible, filled with fragments 

 of this glafs, being then expofed to a heat, which, from previous 

 trials, was judged to be more than fumcient to reduce its 

 contents to fufion, the fire was very gradually lowered till 

 all was cold. I thus expected to obtain a refult fimilar to 

 that laft mentioned, but found, to my great furprife, that 

 the fragments had never been in complete fufion, fince they 

 dill, in a great meafure, retained their original lhape. This 

 extraordinary fact, which afterwards led to the difcovery of fome 

 curious properties of whinftone, will be fully accounted for in a 

 fubfequent part of the paper. 



Another portion of the fame glafs being perfectly melted 

 by a very ftrong heat, the temperature was reduced to about 28 

 of Wedgwood, and was maintained at that pitch during fix 

 hours. The refult was a perfectly folid mafs, cryflallized to a 

 certain depth from the outfide, though flill vitreous in the heart. 

 In another experiment, performed like the laft in all refpects, 

 except that the heat was maintained at 28 during twelve hours, 

 I obtained a mafs entirely cryftalline and ftony throughout, 

 with facettes appearing in the folid parts, and fmall cryftals 

 fhooting into fome of the cavities. 



Soon after I had communicated thefe refults to Dr Hope, 

 he performed, with complete fuccefs, an experiment fimilar to 

 the firft, in which I had obtained a cryftallized fubftance, by 

 the gradual cooling of the melted ftone. The fame was likewife, 

 foon afterwards, performed by Mr Boswell of Auchinleck. 



My experiments, already defcribed, were confined to one 

 fpecies of whinftone ; but have fince been extended to fix other 

 varieties. They were all firft reduced to glafs by the application 

 of a ftrong heat, and fubfequent rapid cooling. After a fecond 

 fufion they were cryftallized, by being kept long in a ftation- 

 ary temperature, between 28 and 30. This laft operation was 

 beft performed in a long and narrow muffle, wholly furround- 



Part I. G ed 



