No. VII. — Islands of Procida and Ischia. 347 



ternal heat. Down to the very shore its influence extends, and 

 the sands of Vico, at two feet below the surface, have a tem- 

 perature of 110° F. * 



The " Fumarole" or emissaries of aqueous vapour, of a high 

 temperature and elastic condition, are also numerous and re- 

 markable. They rise through the fissures of lava beds, and, 

 what is most interesting, they produce siliceous incrustations 

 of a nature similar to those of the Geyser in Iceland. This 

 phenomenon was first observed by Dr Thompson of Naples, 

 in 1795 at Monticeto, where the vapour had a temperature of 

 75.5° K. = 20^° Fahr. To this substance he has given the 

 name of Fiorite, from Santa Fiora in Tuscany, -|- where it was 

 discovered by Professor Santi of Pisa. In 1794, Thompson 

 established its occurrence in the lavas of Vesuvius. The fti- 

 marole of Monticeto in Ischia, deposit sulphates of lime, alu- 

 mina, and magnesia, and as these appear to result from the 

 action of a small quantity of sulphuric acid evolved upon the 

 components of the lava, the silica which that rock contained 

 seems likewise to have become the subject of real chemical ac- 

 tion, and is not merely deposited it dull crusts upon the tufas 

 thus formed, but in vermicular and botryoidal forms, with vi- 

 treous fracture and considerable hardness and transparency. 

 This mineral, which differs slightly from the siliceous sinter of 

 Iceland in having frequently a pearly lustre, is chiefly found 

 in Italy, — the Bay of Naples, Tuscany, and the Vicentine, being 

 its principal localities. In its origin and important characters, 

 it is closely assimulated to the productions of the Geysers and 

 of the volcanic districts of Teneriffe and Lanzerote, as observed 

 by Humboldt and Von Buch ; and the singular phenomenon 

 of its solution in water must be explained in all by the same 

 cause. We know that silica is minutely soluble in water in its 

 ordinary condition, and the experiments of Berzelius lead us 

 to believe that in its newly formed state it is very consider- 

 ably so ; but the main agent is undoubtedly the presence of 

 an alkali, which here we have seen is not awanting, for the 

 clays of Ischia and likewise its mineral springs contain it in 

 considerable abundance. It is here soda as in Iceland, where 



* Daubeny. 



t Not in Ischia as Philips says : Mineralogy, Art. Fiorite. 



