5* EXPERIMENT'S on WHINSTONE and LAVA. 



five, the glafs of this whin pafTes from a foft, or liquid date, 

 to a folid one, in confequence of cryftallization ; which is diffe- 

 rently performed at different points of this range. In the lower 

 points, as at 23, it is rapid and imperfect ; in higher points, 

 flower and more complete, every intermediate temperature 

 affording an intermediate refult. I likewife found, that cryftal- 

 lization takes place, not only when the heat is ftationary, but 

 likewife when rifing or finking, provided its progrefs through 

 the range juft mentioned is not too rapid. Thus, if the heat of 

 the fubftance, after fufion, exceeds one minute in paffing from 

 21 to 23, or from 23 to 21, the mafs will infallibly cryftallize, 

 and lofe its vitreous character. 



These facts enabled me to account for the production of the 

 fubftance refembling the liver of an animal, which I obtained 

 in my firft attempts to cryftallize the melted ftone. Not being 

 then aware of the temperature proper for complete cryftalliza- 

 tion, 1 had allowed it to be paffed over rapidly by the defend- 

 ing heat, and I had begun the flow cooling in thofe lower 

 points, at which the formation of this intermediate fubftance 

 takes place. 



By the fame means I was enabled to explain the other unex- 

 pected refult, which I obtained in endeavouring to convert the 

 glafs of this ftone into cryftallite. The fire applied to the cru- 

 cible, containing fragments of the glafs, had been raifed very 

 flowly, which I know to have been the cafe by fome circumftan- 

 ces of the experiment. The glafs had foftened by the firft appli- 

 cation of heat, but had cryftallized again as the heat gradually 

 rofe ; fo that the fubftance confolidated, while ftill fo vifcid as 

 to retain the original fhape of the fragments ; at the fame time 

 it acquired fuch infufibility as to refift the application of high- 

 er degrees of heat during the reft of the procefs. 



No. 2. 



