PUMICE, a highly porous light mineral substance of volcanic origin, resulting from the solidification of foam or scum formed by the escape of steam or gas on the surface of molten lave. It is principally found of a whitish or clear grey coulour, more rarely of a slaty blue or reddish tint. In composition it is allied to the obsidians, containing in every 100 parts about 72 of silica, 17 of alumina, 2 of iron oxide, and 9 of soda an potash; and under the blow-pipe it fuses to a white enamel. Its porosity renders it so exceedingly light that in the dry condition it floats readily on the surface of water, sinking only when thoroughly saturated. Owing to this property it is found very widely diffused over the ocean-bed, even at points far removed from volcanic vents, considerable blocks been brought up in the dredgings of the "Challenger" at all the points of its sea-bottom exploration. It is obtained for industrial purposes in the regions of recent volcanoes—the Lipari Islands, Iceland, Auvergne, Teneriffe, &c.—and is highly valued as a smoothing and polishing material for the metals, marbele, horn, wood bone, ivory, and leather. For some purpose it is reduced to the condition of a fine powder, and used either or spread upon paper or linen, as glass or emery-paper. A toilet soap is prepared containing a proportion of powdered pumice. An artificial pumice is made from a mixture of calcined and pulverized quartz and alumina baked in the form of a porous brick.