ITALY. L E T T E R XI. 1 6$ 



on the exterior fide of the lava hill, which in- 

 clofes the Solfatara, ifluing there with fome noife 

 under ground in a valley at the foot of this hill, 

 which by fuiphureous acid is changed intowhiteclay, 

 and is called monte Secco. The murmuring noife of 

 the wacer may be explained by its boiling from the 

 heat of fubterraneous fire ; or by an ignition and 

 fermentation of humected martial fulphur pyrites ; 

 or by a fermentation of the fuiphureous acid with 

 calcareous fubftances, which may be fuppofed ex- 

 ifting under ground; fince it is highly probable, 

 that the calcareous Apennines, which inclofe Na- 

 ples on every fide, are inferior here to the fuper- 

 incumbent volcanic covering. The preience of 

 the fuiphureous acid in the fubterraneous caverns 

 of Solfatara is plainly evidenced, by the native 

 cryftalline fulphur flowers, fublimated on the argil- 

 laceous furface of its floor and fides, and by the 

 alum, vitriol, and felenite, which is here pro- 

 duced. But falmnc being likewife gained in 5>ol- 

 fa .ura, there fhould be in its earth the requifite 

 fubitantial acid of common fait and a volatile 

 alkali. 



The rocks and walls, which furround the plain 

 floor of Solfatara, are for the moft part ftratified and 

 white like lime-done. They might even, by their 

 appearance, be miftaken for lime-done ; but upon 

 nearer examination they appear to be argillaceous, 

 M 3 and 



