crystallized Bodies contained in Lava. 303 



tidns of cooled glass and the crystals of the beds of our 

 moiinlains, all of consiant and regular form each in its spe- 

 cies. 



The pyrc'xcnated schorls are also often isolated, and some- 

 times in innumerable multitudes. The crater which opened at 

 the base of Mount iEtna in I669, which elevated a cone of 

 4300 paces in circumference at its base, from which pro- 

 ceeded the enormous lava which we now see, and the mass of 

 which astonishes us, shows a peculiarly striking example of 

 this. The summit of this crater is covered with these schorls 

 mixed with small scoriae, with this remarkable circumstance, 

 that the schorls at the exterior of the crater have all, without 

 exception, retained at their surface a crust of lava which en- 

 velops them,, and that those of the interior show their na- 

 tural polish. 



I shall here explain the cause of this difference, to which, 

 I think, I am the first observer who has directed his atten- 

 tion. The sulphurous acid fumes of the volcano penetrate 

 and decompose the surface of the lava? and scoriae which 

 are there exposed, and the schorls which they do not attack 

 appear at that time, in relievo, in their whole integrity per- 

 fectly cleaned from the lava which environs them, as rock 

 crystals, covered sometimes with a calcareous stone, are 

 sometimes cleaned of it by the nitrous acid, and appear in all 

 their lustre. Here is an operation proving that there is no 

 chemical affinity between lava and the pyroxenated schorl 

 it contains, since the one is attacked and dissolved, and the 

 other is not. This eft'ect sometimes presents a very curious 

 spectacle: it is pleasing to see these schoils of all sizes, 

 even microscopical, fixed upon lava, the surface of which 

 had been decomposed, brilliant ii\ their polish and very 

 sharp at their angles. 



It sometimes happens that the schorls themselves are at- 

 tacked and iheir colour allerc-dj so that their points appear 

 like small crystals of sulphur, or of a whiter tint ; this ef- 

 fect is produced, without doubt, when the fumes cohtain 

 a mixture of acids that act upon the schorl being united, 

 which they cannot do separately ; a chcrnlcal operation, of 



Vol. 27. No. 108. Maij 1807. \J which 



