The phenomena of lava and burning mountains have been variously explained by different philosophers. Some, and these men of reputation too, have followed the idea of the ancients, and suppose the center of the earth to be fire, to which they are so extravagant as to imagine volcanos are the chimneys. Others suppose them to be occasioned by the fermentation of firestone, sulphur, and iron, the explosions of nitre, of aqueous steam violently heated, or of air pent up and greatly rarefied. All these doubtless assist, or contribute to the formation of volcanos, but the only inexhaustible source of fire that I am informed of, is the electric fluid, whose powers were not discovered before the middle of this century. This is nothing more than the light of the sun absorbed by the earth, which of course must be again emitted, just as we receive in rain, the water and the damps he exhales from us. Whenever, therefore, the electric fluid is absorbed in larger quantities than the general outlets can return, it acts with uncommon violence, and breaks forth either by an earthquake, or an eruption. When this happens, and the eruption is not very violent, the burning matter rises gradually from the bottom to the top of the crater, and when it has filled it, it runs over like the boiling of an immense caldron. But sometimes it scorns all bounds, and the crater, monstrous as it is, being insufficient to give vent to its violence, the side of the mountain is burst open, and thrown with inconceivable force to an immeasurable height in the air. The lava then rushes forth in an impetuous torrent of liquid fire, overwhelming every thing in its course. That of 1767 was six miles long and two wide, and in some places, where it past over cavities, it was sixty or seventy feet deep. Eruptions are generally preceded and closed by an immense discharge of stones and ashes, which often create more destruction than the lava itself. For some time after the eruption, the lava has the appearance of melted glass, after which the upper part breaks into large pieces of a kind of dross, which, in the course of time crumble into mould, and form the richest soil imaginable, whilst the lower becomes hard stone. But this is the process of many years, and consequently great part of Vesuvius is horridly barren, whilst the rest is most beautifully cultivated.