40 CATALOGUE OF PHILOSOPHICAL APPARATUS, &c. 



The Obsidian of Lipari. Pumice of ditto. 



Silica 74.05 73.70 



Alumina 12.97 I2 - 2 7 



Oxide of Iron 2.73 2.31 



Lime 0.12 0.65 



Magnesia 0.28 0.29 



Soda 4.21 4.52 



Potash 5.11 4.73 



Chlorine 0.31 0.31 



Water 0.22 1.22 



so that these bodies may be regarded as different conditions of the same mineral. Obsidian 

 indeed may be converted into a substance much resembling pumice, by the mere application 

 of a strong heat. Its vitreous character is not simply due to rapid cooling, for if so, ordinary 

 lavas would assume it, but is doubtless connected with the presence of a large proportion of 

 alkali ; why however the mineral should assume the spongy and fibrous form of pumice by 

 simply heating it, is still I believe undetermined. It will be observed, that it approaches trachyte 

 in chemical composition, for the Obsidian of Lipari is RO, Si O 3 + R 2 03, 3 Si O 3 , which is the 

 composition of Albite ; whereas Labradorite, which is the kind of felspar present in most lavas, 

 is RO, Si O 3 + R 2 O 3 , Si O 3 . It seems necessary therefore, that Silica should be present in suffi- 

 cient quantity to form a trisilicate with the Alumina, and also that alkaline bases should exist 

 in the mineral sufficient to combine with the remaining Silica, in order that a vitreous body 

 like Obsidian should result from the fusion. See Volcanos, ch. ii, and Abich Geog. Beob. 



Drawer 1 19. Black pumice, scoriform and compact lava, specular iron ore, 

 augite crystals, Stromboli. 



SICILY. 



Drawer 120. Ejected masses from Mount Etna, and specimens of the older 

 and more modern lavas. 



Amongst the latter are specimens of the currents of 1669, 1679, 



1842. 



Similar in composition to the modern lavas of Vesuvius, but entirely different from those 

 of the Lipari Islands. 



Drawer 121. Volcanic rocks from Mount Etna and from Graham's Island; 

 Palagonite from Palagonia, Val de Noto. 



Drawer 122. Palagonite from Aci Eeale, with Analcime and other imbedded 

 minerals. 



Palagonite is a name given by Waltershausen to the tuff of Sicily, Iceland, and elsewhere, 

 from the town of Palagonia where he first observed it. 



Its composition is (3 RO, 2 Si O 3 + 2 R 2 O 3 , Si O 3 ) + 9 HO. 



