Note 48 — LETTER VI.[45]
Reading edition
Having heard the same remark with respect to the lava's of Vesuvius, I determined, during an eruption of that Volcano, to watch the progress of a current of lava, and I was soon enabled to comprehend this seeming phænomenon; though it is, I fear, very difficult to explain. Certain it is, that the lava's, whilst in their most fluid state, follow always the law of other fluids; but when at a great distance from their source, and consequently incumbered with scoriæ and cinders, the air likewise having rendered their outward coat tough, they will sometimes (as I have seen) be forced up a short ascent, the fresh matter pushing forward that which went before it, and the exterior parts of the lava acting always as conductors (or pipes, if I may be allowed the expression), for the interior parts, that have retained their fluidity by not having been exposed to the air.