The expulsive agent is in all cases steam, mingled to a greater or less degree with other vapors or gases. Its operation may be simply illustrated. Pure water does not readily boil over in any open vessel of ordinary dimensions. But if the vessel be very deep in proportion to its width, and the heat is applied at the base only, it boils over more readily. Now, if instead of water, we substitute porridge, thick molasses, or any similar thick or viscid material, the bubbles of steam rise slowly; and if rapidly generated, they force the matter out at the top ere they escape. Such bubbles as reach the top, burst, throwing tiny particles of the mass into the air. How great a portion of the material expelled from volcanoes consists of steam and other gases is not easy to determine. But that the quantity of vapor is enormous is indisputable. Vesuvius is noted for the “pine tree” of vapor that overhangs it. The ascending steam and gases, on reaching an upper atmosphere as light as themselves, spread out horizontally in every direction, thus much resembling in outline the stone-pines that are a prominent feature in the Neapolitan landscape. Some effort has been made to connect volcanic eruptions with atmospheric pressure; for, say the theorists, a fall of two inches in the barometer removes a pressure of over 2,000,000 tons from each square mile. A sufficient answer to this is, that this, after all, is only one pound to the square inch; while the force that can cast up volumes of melted matter from a great depth must needs be many tons to the square inch. Clearly these gentlemen would perch us on a sort of universal fire-box, and poise the lid on a hair trigger. But heavy rainfalls and terrific thunder storms are almost invariable accompaniments of explosive eruptions. That these are the result and not the cause of volcanic action is clear. An electrical machine was invented by Sir William Armstrong, in which electricity was generated by forcing steam at great speed through a narrow orifice. This same principle would produce volcanic thunder storms. The immense volumes of vapor, reaching the open air, must rapidly cool and be precipitated as rain. The Italians dread these torrents, sweeping down immense quantities of mud, more than they do the streams of lava. If an eruption causes an immediate fall of two inches of rain over an area seven miles square, it will be found that such a rainfall amounts to more than seven millions of tons of water. Yet the rainfall often is greater, and the area affected is larger; while it is not to be supposed that the entire volume of vapor cast forth is at once precipitated on the earth. This computation can not be assumed as anything more than a mere illustration of the tremendous forces brought into operation.