260 Mr Forbes's Physical Notices of the Bay of Naples. 



ages, since not merely have Pliny and Seneca recounted the 

 general phenomena, but Tiberius actually killed two slaves by 

 the vapour, — an example which, if we may believe report, has 

 been repeated in more modern times *. The explanation offer- 

 ed by Spallanzani seems satisfactory, that since the basis of 

 this whole volcanic region is undoubtedly the Apennine lime- 

 stone, and as we have abundant proof of the present action of 

 heat in the immediate vicinity manifested by hot springs and 

 sulphureous exhalations, the inference is obvious, that the car- 

 bonic acid is disengaged from the limestone, and rises through 

 the cracks of the strata ; and if we are inclined to admit 

 that the descriptions of the ancients are too lofty for the pre- 

 sent condition of the vapour, we may easily see how the quan- 

 tity emitted may be gradually on the decline. 



This opinion regarding the origin of the foul air, or Mofetta^ 

 as it is called in Italy, is strengthened by the consideration 

 that the Grotto del Cane, though themost remarkable example 

 in this neighbourhood, it is by no means a solitary one. Ha- 

 milton -f* gives us several examples, particularly of mofette ap- 

 pearing in spots where they had not been before known. In 

 the excavations of Pompeii they are very abundant. I recol- 

 lect one underground drain near the temple of Isis being 

 pointed out to me as aboundirfg with them. Similar exhala- 

 tions occur at Naples and at Mount Vesuvius, the latter con- 

 taining some sulphuric acid, and most baneful to vegetable as 

 well as animal life. Those who interest themselves in the in- 

 fluence of gases upon the vegetable physiology would do well 

 to notice the relations of Breislak regarding the Vesuvian mo- 

 fette, which, though they are perhaps of a nature to excite 

 incredulity, seem to be warranted by the observations and ex- 

 periments of that able naturalist. He remarks, that " it is a 

 very extraordinary phenomenon that this mephitic vapour, 

 which destroys all vegetation, and kills in a few days trees and 

 •shrubs from the root, has no bad effect either upon olive or 

 pear trees. It is a fact confirmed by all the cultivators of the 

 district, and which I have sometimes verified by seeing these 



• See Jorio, Pozzuoli e Contorui, 183. 

 t Campt Fhlegrcniy fol. Naples, i. 88. 



