179 



contained them, but a part also of my miner 

 alogical journal. The heat of the vapors, which 

 issue from the crevices of the Caldera, is not suf- 

 ficiently great, to combine the sulphur, while in 

 a state of minute division, with the oxygen of 

 the atmospheric air; and after the experiment 

 which I have just cited on the temperature of 

 the soil, we may presume, that the sulphurous 

 acid is formed at a certain depth *, in cavities to 

 which the external air has free access. 



The vapors of heated water, which act on the 

 fragments of lava scattered about on the Cal- 

 dera, reduce certain parts of it to a state of paste. 

 On examining, after I had reached America, 

 those earthy and friable masses, I found crystals 

 of sulphat of alumin. Messrs. Davy and Gay- 

 Lussac ^ have already made the ingenious re- 

 mark, that two bodies highly inflammable, the 

 metals of soda and potash, have probably an im- 

 portant part in the action of a volcano ; now the 



* An observer, in general, very exact, Mr. Breislack, asserts 

 (Geologia, t. ii, p. 232), that the muriatic acid always pre- 

 dominates in the vapours of Vesuvius. This assertion is con- 

 trary to what Mr. Gay-Lussac and myself observed, before 

 the great eruption of 1805, and while the lava was issuing 

 from the crater. The smell of the sulphurous acid, so easy 

 to distinguish, was perceptible at a great distance $ and when 

 the volcano threw out scoriae, the smell was mingled with 

 that of petroleum. 



t Davy, on the Decomposition of fixed Alkalies, Phil. 

 Trans. 1808, P. 1, p. 44. 



N 2 



