14 Exl>erim(nts on Whitiftone and Lava. 



found to be quite hard, though the temperature had remained ftationary. Tlie fubftancc, 

 thus hardened, had undergone a change throughout ; it had loft the vitreous charafter i 

 ■when broken, it exhibited a frafture like that of porcelain, with little luftre ; and its co- 

 lour was changed from bkck 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 cleareft manner, that, in any 

 temperature, frorn 2 » to 28 inclufive, the glafs of this whin pafles from a foft, or. liquid 

 ftate, to a folid one, in confequence of cryftallization; which is differently performed 

 at different points of this range. In the lower points, as at 23, it is rapid and imper- 

 fe£l ; in higher points, flower and more complete, every intermediate temperature afford- 

 ing an intermediate refult. I likewife found, that cryftallization 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 paffnig from 21 to 23, or from 23 to 21, the mafs will infallibly 

 cryftallize, and lofe its vitreous chara£ler. 



Thefe fa£ts 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 cryftallization, I had allowed 

 It to be paffed over rapidly by the defcending 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 unexpedted refult, which I ob- 

 tained in endeavouring to convert the glafs' of this ftone into cryftallite. The fire applied 

 to the crucible, containing fragments of the glafs, had been raifed very flowly, which I 

 know to have been the cafe by fome circumftances of the experiment. The glafs had 

 foftened by the firft application of heat, but had cryftallized again as the heat gradually 

 rofe; fo that the fubftance confolidatcd, while ftill.fo vifcid as to retain the original 

 fliape of the fragments ; at the fame time it acquired fuch infufibility as to refift the appli- 

 cation of higher degrees of heat daring the reft of the procefs. ' ' 



No. 2. Whin of the Rock of Edinburgh Caftle. 



This is a bafalt of a blacklfli blue colour. Its grain is fine, and its frafture uneven, par- 

 taking of the fplintery. It is in general homogeneous, although, in fome pieces, a very 

 few minute cryftals of hornbleiid are perceptible. It has fome luftre, from a number of 

 fmall fliining facettes ; has an earthy fmell when breathed on j and gives fire flightly with 

 fteel. 



The pure glafs which this whin yielded, by rapid cooling after a moderate heat, was 

 cryftaUized in three experiments, and produced maffes greatly refembling the original. In 

 one of thefe, formed on a large fcale In the glafs-houfe, the refemblance is fo ftrong, both as 

 to colour and texture, that it would be difficult, or perhaps impoffible, to diftinguifh them, 



but 



