138 Mr Forbes's Phij.sical Notices of the Bmj of Naples. 



and if the acid be, as we have already supposed, disengaged 

 by a chemical affinity exerted at a high temperature with the 

 clay or sand present, the base will afterwards be in a condition 

 to unite at the surface of the plain with the sulphurous and 

 sulphuric acid vapours. 



The only other sulphate we have to notice is that of Iron, 

 which occurs generally in greenish acicular crystals under four 

 lines in length, along with the native alum in the vicinity of 

 the " fumerole." The sulphuret of this metal is here abun- 

 dant, which is the more remarkable, as, though long considered 

 the most approved prime mover in the theories of volcanic ac- 

 tion, Dolomieu notices but one specimen in Mount Etna, and 

 Gioeni none in Vesuvius, notwithstanding the erroneous state- 

 ment of Sir William Hamilton ; nor could Spallanzani detect 

 it in Stromboli and Volcano. In the Solfatara it occurs in two 

 forms, not only incorporated in the rocks of the crater, or lin- 

 ing drusy cavities, but in a state of sublimation from the active 

 emissaries. Since sulphur only accumulates for a long period, 

 and becomes an important feature in volcanos when half ex- 

 tinct, perhaps we should not be surprised to find its compound 

 with absolute iron ore, which abounds in almost all lavas, in a 

 case like the present. The octohedral magnetic iron ore is 

 here very abundant, both in the solid rocks of the mountain 

 and in the ferruginous sand where the sea washes its base. 



These and some minor products of the Solfatara give the 

 spots which they incrust a very peculiar, and often beautiful 

 appearance. The shades of the sulphur softening from the 

 deep-orange formed by the combination of arsenic, through all 

 tints to the palest straw-colour, from the mixture of the various 

 salts which have been enumerated, have an attractive appear- 

 ance ; nor less so the more unusual colour of green vitriol, si- 

 milarly diversified ; and both the yellow and green merging 

 at last as we retreat from the immediate action of the " fume- 

 role" into the monotonous white which characterizes the whole 

 plain, rendered here and there more conspicuous by the silice- 

 ous sinter which was first discovered by Dr Thompson, and in 

 some places forms a white crust of two or three lines in thick- 

 ness. 



Such being an account of the Solfatara and its productions, 



