"Est etiam genus pulveris, quod efficit naturaliter res admirandas. Nascitur in regionibus Baianis, et in agris municipiorum, quae sunt circa Vesuvium montem, quod commixtum cum calce et caemento non modo caeteris aedificiis praestat firmitates, sed etiam moles, quae construuntur in mari, sub aqua solidescunt. Hoc autem fieri hac ratione videtur, quod sub his montibus et terra ferventes sunt fontes crebri, qui non essent, si non in imo haberent, aut de sulfure, aut alumine, aut bitumine ardentes maximos ignes: igitur penitus ignis, et flammae vapor per intervenia permanans et ardens, efficet levem eam terram, et ibi, qui nascitur tophus, exugens est, et sine liquore. Ergo cum tres res consimili ratione, ignis vehementia formatae in unam pervenerint mixtionem, repente recepto liquore una cohaerescunt, et celeriter humore duratae solidantur, neque eas fluctus, neque vis aquae potest dissolvere." About Baia, Puzzole, and Naples, we have an opportunity of remarking the truth of these last words. Several of the piers of the ancient harbour of Puzzole, vulgarly called Caligula's bridge, and which are composed of bricks joined with this sort of cement, are still standing in the sea, though much exposed to the waves; and upon every part of the shore you find large masses of brick-walls rounded and polished by friction in the sea, the brick and mortar making one body, and appearing like a variegated stone. Large pieces of old walls are likewise often cut out into square pieces, and made use of in modern buildings instead of stone. Soon after the first quotation, Pliny says, "Si ergo in his locis aquarum ferventes inveniuntur fontes, et in montibus excavatis calidi vapores, ipsaque loca ab antiquis memorantur pervagantes in agris habuisse ardores, videtur esse certum ab ignis vehementia ex topho terraque, quemadmodum in fornacibus et a calce, ita ex his ereptum esse liquorem. Igitur dissimilibus, et disparibus rebus correptis, et in unam potestatem collatis, callida humoris jejunitas aqua repente satiata, communibus corporibus latenti calore confervescit et vehementer effecit ea coire, celeriterque una soliditatis percipere virtutem." [30] Scipione Falcone, a very good observer, in his _Discorso naturale delli cause et effetti del Vesuvio_, says, that he saw, after the eruption of Vesuvius in 1631 (which was attended with hot water), the mud harden almost to a stone in a few days; his words are these--"fatta dura a modo di calcina e di pietra non altrimenti di cenere, perche dopo alcuni giorni vi ci e caminato per sopra e si e conosciuta durissima che ci vogliono li picconi per romperla." This account, with other circumstances mentioned in this letter, make it highly probable, that all the _tufas_ in the neighbourhood of Vesuvius have been formed by a like operation. [31] This piece is now in the Museum of the Royal Society, together with other specimens, mentioned in this and in the following letter. M. M. [32] Letter IV.