EXPERIMENT'S on WHINSTONE and LAVA. 65 



points it yielded a liver cryftallite infufible under 30, and in the 

 higher a ftony fubftance like a common lava or whin, and fu- 

 fible only at ^S- 



What has been faid is applicable to the interior parts of la- 

 vas ; but I vsras at a lofs to underfland the ftate of their exter- 

 nal furface, which, cooling much more rapidly, might be ex- 

 pedled to poflefs a vitreous charadler ; yet glafs is not found on 

 the furface of lavas, except in a very few cafes, and has occur- 

 red only in a fingle fpot on iEtna. This difficulty was remo- 

 ved, however, by the following confideration : Though the fur- 

 face of a lava cools with far more rapidity than the refl of the 

 mafs, yet, owing to the contadl of the fiery ftream, that rapi- 

 dity can never be very great ; and we muft fuppofe that the 

 temperature of the furface employs more than a minute or two 

 in defcending from 23 to 21. Where this happens, we have 

 fliown that the fubftance confolidates into the liver cryftallite, 

 which completely refembles the fcoria of a lava. A fmall frag- 

 ment of the mafs, which I took from the running ftream, being 

 placed in the temperature of 22, loft its vitreous chara(n:er in 

 two minutes, as already ftated ; and had the mafs itfelf been al- 

 lowed to remain but a very little longer in the ftream, it would 

 certainly have acquired, as well as the reft of the furface, the 

 dull charadler of fcoria. 



The fame property accounts for the cruft which is formed on 

 the furface of flowing lavas, and which conftitutes fo remark- 

 able a feature in their hiftory. Were lava to congeal after the 

 manner of pitch or wax, by an uniform and gradual increafe 

 of vifcidity throughout, no cruft would be formed, or if, by 

 the a(5Hon of cold air, the upper furface were to harden a little, 

 it might be foftened again by an influx of frefli matter a 

 very little hotter than itfelf. In lavas, however, as we have 

 proved, when the furface cools down to 21, it rapidly congeals 

 to a hard fubftance, capable of refifting any heat under 30. The 



Part I. I cruft 



