APPENDIX— HALL'S EXPERIMENTS. ^ 141 



raised very slowly, which I know to have been the case 

 by some circumstances of the experiment. The glass had 

 softened by the first application of heat, but had crystal- 

 lized again as the heat gradually rose ; so that the sub- 

 stance consolidated, while still so viscid as to retain the 

 original shape of the fragments ; at the same time it acquired 

 such infusibility as to resist the application of higher de- 

 grees of heat during the rest of the process. 1 '' (Trans, of 

 Royal Soc. of Edin. vol. v. p. 5.) Besides the greenstone of 

 Bell's Mills, Sir James Hall fused and recrystallized several 

 other of the trap-rocks around Edinburgh. We shall note 

 his observations in regard to these. 



Whin (Basaltic Clinkstone) of the rock of Edinburgh Castle. 



" The pure glass which this whin yielded, by rapid cool- 

 ing after a moderate heat, was crystallized in three experi- 

 ments, and produced masses greatly resembling the original. 

 In one of these, formed on a large scale in the glass-house, 

 the resemblance is so strong, both as to colour and texture, 

 that it would be difficult, or perhaps impossible, to distin- 

 guish them, but for a few minute air-bubbles visible in the 

 artificial crystallite. The glass is less fusible than that re- 

 sulting from the greenstone of Bell's Mills, and seems not 

 to possess the property of producing the liver crystallite. 11 

 —p. 53. 



Whin of the basaltic columns on Arthur's Seat, near Edinburgh. 

 (Basalt of the summit.) 



" In the temperature of 100° or upwards, the whole was 

 changed to pure black glass ; but in a more moderate heat 

 (about 60°), the felspar remained unchanged, while the horn- 

 blende disappeared and formed a glass along with the ba- 

 sis of the stone. Both kinds of glass yielded highly cha- 



