APPENDIX.-HALL'S EXPERIMENTS. 130 



very gradually lowered till all was cold." — P. 49- On the 

 crucible being examined, Sir James was in no small degree 

 surprised to find that " the fragments had never been in 

 complete fusion, since they still, in a great measure, retained 

 their original shape." 



In another experiment the trap-glass was completely 

 fused, the temperature of the fire was reduced to about 28° 

 of Wedgwood's pyrometer, and allowed to remain so for 

 six hours. " The result was a perfectly solid mass, crystal- 

 lized to a certain depth from the outside, though still 

 vitreous in the heart ; 11 a similar mass of glass, however, 

 being experimented on in the same manner, but being sub- 

 jected to the heat for twelve hours, was on its consolidation 

 " entirely crystalline and stony throughout."" 



On investigating the fusibility of the glass obtained by 

 melting the greenstone of Bell's Mills, from experiment it 

 occurred to Sir James Hall, that the fact of the trap-glass 

 not thoroughly fusing on a former occasion, was to be ex- 

 plained by supposing that, on the first application of heat, 

 it had partially softened, and then had crystallized so as to 

 become again rigid, and that in the crystallizing process, " it 

 had acquired such infusibility as to yield to no heat under 

 30°." 



We shall quote from Sir James HalFs paper the experi- 

 ment by which he confirmed this conjecture, " A piece of the 

 same glass (the trap-glass), placed in a cup of clay, was in- 

 troduced into a muffle, heated to 21°. In one minute it 

 became quite soft, so as to yield readily to the pressure of 

 an iron rod. After a second minute had elapsed, the frag- 

 ment, being touched by the rod, was found to be quite hard, 

 though the temperature had remained stationary. The 

 substance, thus hardened, had undergone a change through- 

 out ; it had lost the vitreous character ; when broken, it 



