348 Mr Porbes's Physical Notkes of the Bay of Naples. 



the memorable experiments of Black demonstrated the exist- 

 ence of 16 per cent, of that alkali in the siliceous compound. 

 The remarkable effect of this substance in promoting the solu- 

 tion of silica in a state of fine division of parts, and especially 

 at high temperatures, sufficiently explains why these depositions 

 should be entirely confined to countries now or formerly under 

 the action of volcanos. 



Other parts of Ischia present specimens of the same sub- 

 stance. In the extinct crater of Canali, Breislak found a mass 

 incrusted with a stalactitic siliceous crust to the thickness of 

 three lines. * Besides, at Vesuvius it occurs in the rock of the 

 Solfatara, and in the tufa at its base ; in the latter, Dr Thomp- 

 son was able to discover some of the triangular facettes of the 

 six-sided pyramid. It is also met with at Astroni. The on- 

 ly example, as far as I know, of the detection of silica as a 

 constituent in the water of a spring in the Bay of Naples, is 

 in the Gurgitello above described. Tenore-|- claims the 

 honour of this discovery, made by him in 1801, and since 

 disputed. It was not, however, pubhshed till 1816. Some 

 of the mineral contents of the spring are said to exist in the 

 state of hisilicates. 



One other fact connected with the island of Ischia, and that 

 a very interesting one, we have alone to notice. Near Lacco, 

 on the north side of the island, is a grotto or cavern, formed 

 of loose blocks of lava, from which issues constantly a cold 

 wind, which seems first to have been observed by Saussure. 

 He found the temperature of the stream of air in March 177B 

 to be 45.°5 Fahr., the external air being 63. °5. X He adds^ 

 that he was told that it was greatly colder in summer ; but it 

 is hardly necessary to observe, that this mistake is in similar 

 circumstances constantly repeated by those who judge only by 

 their feelings. And as we might suppose, it appears to be 

 much the reverse, for Breislak observed the temperature of 

 the cavern to be 13R. =:61J°. F. when that of the air was 

 21 II. = 795°. F. § The difference of temperature in both 



• Vol. ii. p. 215. 



+ Esitai sur la Geograpihie thysique du Royaume de Naples, p. 28, 

 X Saussure, Voyages dans les A/ pes, § 1414. 

 . § Campanie, ii. 214. 



