EXPERIMENTS on WHINSTONE and LAVA. 51 



refults already mentioned. " I had placed in the muffle a long 

 and flender fragment of this glafs, with its extremities refting 

 on two fupports of clay, and its middle unfupported. Having 

 then increafed the temperature by flow degrees, I expedled to dif- 

 cover the loweft point of emoUefcence, by pbferving when the 

 fragment funk by its own weight. The muffle having attained 

 a moderate heat, I obferved the glafs to lofe its fliape a little. 

 Wifhing to fee it completely melted, the fame heat was con- 

 tinued, but no further change took place. The heat was then 

 raifed feveral degrees, but without effedl. At laft, being urged 

 ftill further, the glafs funk down completely between its fup- 

 ports. The pyrometer being then withdrawn, denoted a tem- 

 perature above 30. 



It occurred to me, that, on this occafion, the glafs, by the 

 firft application of heat, had foftened, and then had cryftallized, 

 fo as to becoiue hard again ; that, in cryftallizing, it had ac- 

 quired fuch infufibility as to yield to no heat under 30. I im- 

 mediately confirmed this conjedure by the following experi- 

 ment. 



A PIECE of the fame glafs, placed in a cup of clay, was in- 

 troduced into the muffle, heated to 21. In one minute it be- 

 came quite foft, fo as to yield readily to the preffiare of an iron 

 rod. After a fecond minute had elapfed, the fragment, being 

 touched by the rod, was found to be quite hard, though the 

 temperature had remained ftationary. The fubftance, thus 

 hardened, had undergone a change throughout ; it had loll 

 the vitreous charadler ; when broken, it exhibited a fradture 

 like that of porcelain, with little luftre ; and its colour was 

 changed from black to dark brown. Being expofed to heat, it 

 was found to be fufible only at 31 ; that is, it was lefs fufible 

 than the glafs by 13 or 14 degrees. 



Numerous and varied experiments have fince proved, in the 

 clearefl manner, that, in any temperature, from 21 to 28 inclu- 



G 2 five 



