EKpiriment on W7}injlcni and Lava. ^^ 



The truth of this laft opinion is demonftrated by Xome fa£ls which I accidentally 

 obferved, long before my prefent views had occurred, when, in fpring 1785, I had an op- 

 portunity of examining a ftream of lava, which flowed from Vefuvius. The eruption was 

 comparatively fo gentle, that I was able, though not without inconvenience, to approach 

 and examine the fiery ftream on three different days. It was in general concealed by a 

 thin white fmoke, which the wind blew afide occafionally, fo that I could diftinflly fee the 

 lava as it burft from the fide of the hill. It was then of a bright white heat, and flowed 

 with the agility and rapidity of water, in all refpedts refembling melted iron running 

 from the furnace. The liquid, at its firft emergence, manifefted a ftrong effervefcence, 

 which fubfiding as the heat abated, fhewed itfelf at laft only in the burfting of fome very 

 large bubbles, accompanied with a white fmoke. Where I approached the ftream, it was 

 ftill of a ftrong red heat, and had the confiftence of honey. I thruft a ftick into it with 

 cafe, to the end of which fome of the lava adhering, by its vifcidity, allowed itfelf to ba 

 drawn out into threads, and was found, when cold, to have a fhining furface, and a 

 vitreous fracture. 



Being thus convinced that I had met with a lava of glafs, I prepared fome moulds of 

 ftucco, in which I meant to take cafts with that rare fubftance ; and with this view re- 

 turned to the mountain. I found the ftream was not fo liquid as at firft, but I was able, 

 by means of a ladle fixed on the end of a pole, to lift the fpecimen now before lis in a ftate 

 like dough. I then prefled it with a feal, by which means, though too coarfe to receive 

 an accurate imprefllon, it took the fliape it now bears, which is that of the ladle. It is 

 very porous, one-third of it nearly being occupied by air holes. It contains a great 

 number of fmall white cryftals of Vefuvian garnet, embedded in a black fubftance, which 

 completely refertibles the glafs obtained in our experiments from lava by rapidcooling after 

 fufion. Befides all their other properties, it pofleffes the fufibility of the glafles, fince it 

 foftens completely at 18, that is, 14 or 15 degrees below the foftening point of any of the 

 ftony lavas. Being expofed to the procefs of regulated cooling, it gave the fame refult as 

 all the other lava glafles. In the lower points it yielded a liver cryftallite infufible under 

 30, and in the higher a ftony fubftance like a common lava or whin, and fufible 

 only at 35. 



What has been faid is applicable to the interior parts of lavas ; but I was at a lofs to 

 underftand the ftate of their external furface, which, cooling much more rapidly, might 

 be expedled to poflefs a vitreous charader ; yet glafs is not found on the furface of lavas, 

 except in a very few cafes, and has occurred only in a fingle fpot on JEtna. This difli- 

 culty was removed, however^, by the following confideration : though the furface of a lava 

 cools with far more rapidity than the reft of the mafs, yet, owing to the contad of the 

 fiery ftream, that rapidity can never be very great; and we muft fuppofe that the tem- 

 perature of the furface employs more than a minute or two in defcending from 23 to 21. 

 Where this happens, we have Jhown that the fubftance confolidates into the liver cryftallite, 

 which completely refembles the fcoria of a lava. A fmall fragment of the mafs, which I- 



I 2 took 



