IgQ ON THE CRYSTALS IN LAVAS. 



in the base of Etna in 165q, which raised a cone 4300. 



paces in circumference at its base, whence issued the enor- 



nioas lava that we see in existence, and the bulk of which 



astonishes us, exhibits a singularly striking example. The 



thos- wthout summit of this crater is covered with these schoerls mixed 



the crater with small scoriae; and this remarkable circumstance at- 



encrusted with • i « 



lava; tends them, the schoerls on the outside of the crater have 



those within, all without exception retained on their surface a crust of 

 the lava that included them, while those within exhibit their 

 native polish. 



This accounted I will here explain the cause of this difference, and I be- 



for by the ac- |-g^,g j ^Yie first who have attended to it. The sulphu- 

 tion of the vol- ^ 



cjinicfumeson rous acid fumes of the volcano penetrate and decompose the 

 e crust. surface of the lavas and scoriae, that are exposed to it; and 



the schoerls, which these fumes do not attack, then appear 

 m relief, and entire throughout, being perfectly cleaned 

 from the lava that surrounded them ; as rock crystals, when 

 they are covered, as they are sometimes, with a calcareous 

 tufa, are freed from it' by nitric acid, and appear with all 

 their briUiancy. This operation proves, that there is no 

 chemical affinity between the lava and the pyroxene schoerl 

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

 other is not. The effect of this is sometimes a pleasing 

 sight, as it exhibits schoerls of all sizes, even microscopic, 

 fixed on the lava, the surface of which has been decompo- 

 sed, shining with their native polish and their singles per- 

 fectly sharp. 

 Sometimes the It sometimes happens, that the schoerls themselves are 

 pyroxenes attacked, and their colour altered to such a degree, as to 

 tacked. " appear like small crj^stals of sulphur, or still whiter. This 



effect is produced, no doubt, when the fumes contain a mix- 

 ture of acids, that act on schoerl when combined, which 

 they could not do separately: a chemical operation of 

 which we have a well-known instance in the aqua regia, 

 composed of the nitric and muriatic acids. 

 In the volcanic To these facts, which evidently prove, that these crj'sta!- 



a-.hes at Pom- |j.2y(j substances were anterior and foreign to the lava in- 

 peii a leucite " 



united to a eluding them, I shall add as a superabundant proof a singu- 

 schoerl. larity, found among the ashes that covered Pompeii, and 



now in my collection of volcanic substances. This is a so- 



littiry 



