78 
SULPHUJtOUS TAl’OUns. 
externally by the slow action of tbc vapours of sulphurous 
acid gas. These vapours rise in abundance; and what is 
rather remarkable, through crevices which seem to have no 
communication with the apertures that emit aqueous vapours. 
We may be convinced of the presence of the sulphurous 
acid, by examining tbc fine crystals of sidpliur, which are 
everywhere found in the crevices of the lava. This acid, 
combined with the water with which the soil is impregnated, 
is transformed into sulphuric acid by contact with the oxygen 
of the atmosphere. In general, tlie humidity in the crater 
of the peak is more to be feared than the heat ; and they 
who seat themselves for a while on the ground find their 
clothes corroded. The porphyritic lavas are affected by the 
action of the sulphuric acid: the alumine, magnesia, soda, 
and metallic oxides gradually disappear ; and often nothing 
remains hut the silex, which unites in mammillary plates, 
like opal. These siliceous concretions,* which M. Cordier 
first made known, are similar to those found in the isle of 
Ischia, in the extinguished volcanoes of Santa Fiora, and in 
the Solfatara of Puzzuoli. It is not easy to form an idea of 
tlie origin of these incrustations. The aqueous vapours, 
discharged through great spiracles, do not contain alkali in 
solution, like the waters of the Geyser, in Iceland. Perhaps 
the soda contained in the lavas of the peak acts an impor- 
tant part in tlie formation of these deposits of silex. There 
may exist in the crater small crevices, the vapours of which 
are not of the same nature as those on which travellers, 
whose attention has been directed simultaneously to a great 
number of objects, have made experiments. 
Seated ou the northern brink of the crater, I dug a hole 
of some inches in depth ; and the thermometer placed iu this 
hole roso rapidly to 42°. Hence we may conclude wluit must 
bo the heat in this solfatara at the depth of thirty or forty 
fathoms. The sulphur reduced into vapour is condensed 
into fine crystals, which however are not equal in size to 
those M. Holomieu brought from Sicily. They are semi- 
diaphanous octoliedrons, very brilliant on the surface, and of 
* Opalarliger kieselsinier. The siliceous yurh of the volcanoes of the 
Isle of France contains, according to Klaproth, 0'72 silex, and 0'21 
water; and :1ms comes near to opal, which Karsten considers as a hydrated 
silex. 
