Such remarks as I have made on the eruptions of Mount Vesuvius, during my residence at Naples, have been transmitted to the Royal Society, who have done them more honour than they deserved. Many more might be made upon this active Volcano, by a person who had leisure, a previous knowledge of the natural history of the earth, a knowledge of chemistry, and was practised in physical experiments, particularly those of electricity[44]. I am convinced, that the smoak of Volcanos contains always a portion of electrical matter; which is manifest at the time of great eruptions, as is mentioned in my account of the great eruption of Vesuvius in 1767. The peasants in the neighbourhood of my villa, situated at the foot of Vesuvius, have assured me, that, during the eruption last mentioned, they were more alarmed by the lightning and balls of fire that fell about them with a crackling noise, than by the lava and the usual attendants of an eruption. I find in all the accounts of great eruptions mention made of this sort of lightning, which is distinguished here by the name of _Ferilli_. Bracini, in his account of the great one of Vesuvius in 1631, says, that the column of smoak, which issued from its crater, went over near an hundred miles of country, and that several men and beasts were struck dead by lightning, issuing from this smoak in its course.
The nature of the noxious vapours, called here _mofete_, that are usually set in motion by an eruption of the Volcano, and are then manifest in the wells and subterraneous parts of its neighbourhood, seem likewise to be little understood. From some experiments very lately made, by the ingenious Dr. Nooth, on the _mofete_ of the Grotto del Cane, it appears that all its known qualities and effects correspond with those attributed to fixed air. Just before the eruption of 1767, a vapour of this kind broke into the King's chapel at Portici, by which a servant, opening the door of it, was struck down. About the same time, as his Sicilian Majesty was shooting in a paddock near the palace, a dog dropped down, as was supposed, in a fit; a boy going to take him up dropped likewise; a person present, suspecting the accident to have proceeded from a _mofete_, immediately dragged them both from the spot where they lay, in doing which, he was himself sensible of the vapour; the boy and the dog soon recovered. His Sicilian Majesty did me the honour of informing me himself of this accident soon after it had happened. I have met with these _mofetes_ often, when I have been making my observations on the borders of Mount Vesuvius, particularly in caverns, and once on the Solfaterra. The vapour affects the nostrils, throat, and stomach, just as the spirit of hartshorn, or any strong volatile salts; and would soon prove fatal, if you did not immediately remove from it. Under the ancient city of Pompeii, the _mofetes_ are very frequent and powerful, so that the excavations that are carrying on there are often interrupted by them; at all times _mofetes_ are to be met with under ancient lavas of Vesuvius, particularly those of the great eruption of 1631. In Serao's account of the eruption of 1737, and in the chapter upon _mofetes_, he has recorded several curious experiments relative to this phænomenon. The Canonico Recupero, who, as I mentioned to you in a former letter, is watching the operations of Mount Etna, has just informed me, that a very powerful _mofete_ has lately manifested itself in the neighbourhood of Etna; and that he found, near the spot from whence it rises, animals, birds, and insects, dead, and the stronger sort of shrubs blasted, whilst the grass and the tenderer plants did not seem to be affected. The circumstance of this _mofete_, added to that of the frequent earthquakes felt lately at Rhegio and Messina, makes it probable that an eruption of Mount Etna is at hand.