gS , Experimtnts on Whinjlom and Lava. 



II. 



Exptriments on Whinjlone and Lava. By Sir James Hall, Bart. F.R.S. & F.A.S. 



Edin *. 



(Concluded from page \%.) 



HE fuppoGtIons which thefe gentlemen have thus advanced, and have ferioufly main- 

 tained in various parts of their works, have arlfen in both from the belief, that, in our 

 fires, nothing but glafs can be produced from a lava after complete fufion. This being 

 taken for granted, it would certainly be very difficult to explain the phenomena of aftual 

 eruptions, by means of the known agents of nature. Recourfe has therefore been had, 

 by one of thefe gentlemen, to a hypothetical modification of thefe agents ; and by the 

 other to the influence of fubftances, which have left behind them no trace of their 

 exiftence f , and which, had they been prefent, could not have produced the efFefils 

 afcribed to them. 



According to both fuppofitions, the heat of volcanos is conceived to be of very little 

 intenfity ; but the few obfervations I had occafion to make; which are confirmed by in- 

 numerable fadts related by travellers, convince me that it muft far exceed what is requifite 

 for the moft perfect fufion of the lavas, and of all the fubftances contained in them X ; and 

 the experiments already defcribed fuperfedc the neceffity of fuppofing any thing different 

 from the common courfe of nature ; for they afford, analogically, an eafy folution of the 

 difficulty, by fhowing that glafs is not the only refult of fufion, and that whin, a fubftance 

 like lava, when cooled flowly after fufion, refumes its ftony chara£ler. But, not content 

 with analogy alone, I refolved to afc'ertain the truth of thefe conclufions in a direft man- 

 ner, and performed the following experiments with fpecimens of fix different lavas, four 

 of which, to my certain knowledge, had made part of external volcanic currents. In the 

 prefent ftate of geology, too much pains cannot be beftowed in afcertaining that the 

 fpecimens colleded are really lavas, Cnce this circumftance has been frequently over- 

 looked, as I fliall endeavour to fliew, when I fpeak of the differences between them and 

 whinftone. 



* It ought to have been noticed in thejaft Number that this paper is infertcd in the Edinburgh Tranf- 

 aflions,' though, by fome culpable negleft, the copies of that work have not yet reached London. N. 



f None of the lavas 1 have feen contained the fmalleft veftige of petroleum ; nor did I meet with any 

 fulphur but what was evidently produced by the condenfation of vapours, rifing through crevices, long after 

 (he eruptions had ceafed. 



J I conceive, therefore, that the formation of the infulated fubftances contained in lavas, as well as the 

 other peculiarities of internal ftrufture, poffefled by lavas in common with granite and bafalteS, muft be 

 afcribed in all of them to cryftallization during flow cooling after fuCon, as I had ftated formerly in Spring 

 i79°> (Tranf. Ed'm. 'vol. HI.) The year following. Dr. Beddoes prefented to the Royal Society of Lon- 

 don a paper, in which he alfo explains the character of granite and bafaltes by cryftallization, in confequence 

 of flow cooling. 



No. 



