A THIRD, in some part vitrify'd. Of the Burning and Eruptions of this Mountain we have a copious History given us by J. Alph. Borelli.

4. A parcel of Sal Armoniac, brought away from Sicily, where it had been gather'd in the late fiery Eruption of Mount Ætna, having been there found copiously, some days after that the fire was extinguisht, upon the surface of that (6161) ferrunginous matter which was left of the burnt minerals. This Salt, he saith, was some of it as yellow as saffron, some like citron-colour, some white, and some greenish; which colours though they may seem to come from the several sorts of Mines of Iron, Brass, &c. whence the Salt issues; yet considering the Experiment made with it by Signor Borelli in his History of the late Burning of Ætna (of which an Account was given in Numb. 75. of these Tracts,) it was a Factitious Salt, such as is sold in shops, being a concrete of Niter, Sulphur and Vitriol burnt and sublimed. For, it seems, when he found so great a plenty of this Salt, and had heard, that the force of Gun- powder was highly increased by the mixture of Sal Armoniac, and thence conceived, that this Salt might have much contributed to the conflagration of this Mountain,and to the fusion of the fabulous, and the fluxing of the vitreous matter; he, for a tryal, added some of this Sal Armoniac to pulverised sulphur and niter; but found, to his amazement, that it was so far from being kindled by fire, that it manifestly hindred the accension of the Brimstone and Salt-peter, which were even extinguish't by it as if water had been powr'd on them: And the same happn'd, upon the addition of powder'd coals, wont to be mix'd in common gun-powder. Which Experiment, he adds, made him suspect, that this Sal Armoniac, found about Ætna, had not been existent in those caverns from the beginning, but that 'tis factitious, as was hinted above.