ITALY. LETT E R XI. l6j 



rally thefe changed volcanic materials are of a 

 white colour j but there are too, red, cinericeous, 

 blueifh, and black-coloured ones, as for example, 

 near the before-mentioned Pifciarelle. 



This undoubted change of volcanic vitreous 

 materials into an argillaceous nature by the ful- 

 phureous acid, which did, during fo many years, 

 flowly penetrate, difiblve, and change them, is cer- 

 tainly one of the mod fingular, and moll fignifi- 

 cant phenomena in Nature. 



It certainly is worth white to expofe Vefuvian 

 lavas and other (tones to the fleams of Solfatara, 

 and to obferve their fucceffive alterations. Pro- 

 fefibr Valro at Naples has promifed me to make 

 thefe experiments. 



It is a known fact, that clays, by burning in 

 fire, are deprived of their vifcid coherent quality, 

 which cannot be reflored to them, neither by the 

 fined pulverization, nor by the moft curious hu- 

 mectation. But the fulphureous Solfatara fleams 

 reftore it, as may be eafily obferved on the broken 

 pots wherein they gather the falmiac. Though 

 Tery well baked and burnt at Naples, they are 

 mollified again by the acid fleams into a vifcid 

 clay, which keeps the former fire-burnt colour. 



The clays and ftones at the Solfatara are, by 



the fuper-abnndance of the fulphureous acid, 



brought to diftill into aluminous lumps, to which 



M 4 very 



