The melted matter that boils up in the crater, and flows down the mountain, is called Lava. I dare say most of you have seen some pieces of this substance when polished and worked into ornaments. It is found in great variety, and is sometimes black, porous, and light like cinders; sometimes it consists of crystallized particles of quartz, felspar, and other minerals, so as closely to resemble granite; and not unfrequently it is a solid dark-coloured mass, heavy and hard as the stone that our streets are paved with. It issues from the crater in a melted state, bubbling and boiling like water in a tea-kettle, but you must not therefore suppose that it runs down the declivity of the mountain like water. On the contrary, its motion is mostly very slow, seldom being faster when it gets at some distance from the crater, than four miles an hour, which is about as fast as a man can walk. When it has run still further from its source it does not travel more than a few yards in a day. The motion of a stream of Lava is very peculiar, for the surface exposed to the air is immediately formed into a crust, and hence it constantly moves with a crackling noise, and when the stream is quite fresh no light is seen except in the cracks that are constantly being formed at the extremity. In this way a current will sometimes go drawling on for months after the eruption which gave rise to it has ceased. A very curious effect is produced when the lava runs in a certain state of fluidity down a steep descent. A thick, strong crust forms on the outside, and it is one of the qualities of lava when it has become hard, like most other stony substances, to present great opposition to the passage of heat. In consequence of this the liquid lava in the inside of the current is kept hot, and continues to run on for a long time after the supply from the crater has ceased, and leaves the crust in the form of an arched passage. From what I have told you about lava streams, you will see that there is not much danger from them to living creatures, who may always get out of their way fast enough: but sometimes houses and even towns are enveloped in them. [Illustration: PLATE VI. VESUVIUS N^o. 2] However, from the peculiar mode in which they travel there is often a way of preventing this, and on one occasion it was resorted to, and the town of Catania thereby saved. A current of lava from Mount Etna was making its way straight towards the town, but a body of fifty bold strong fellows went out to meet it, armed with crow bars; with these they broke great holes in the crust at the side, and thus the stream was turned into another course, and pursued its way on one side of the town. Throughout an eruption, a great quantity of dust is produced by the rubbing of the stones against each other which are thrown out from the crater, and often fall back and are thrown up again several times.