the Pumice-stone, a substance frequently thrown out of volcanoes, though there are many which are never known to throw it out. It is very full of pores and blisters; in consequence of which it is specifically very light, and resembles the frothy slag produced in our iron furnaces. It is of two colours, black and white; the former being that which it has when thrown out of the volcano; the latter, as Crested conjectures, being perhaps faded and bleached. M. Magellan considers it rather as a volcanic ejection than a volcanic production; and describes it as of a white, reddish-brown, grey, or black colour. It is of a rough and porous consistence, being made up of slender fibres parallel to each other, and very light, so that it swims on water. It strikes fire with steel, though with difficulty, and seems originally to have been an asbestos decomposed by the action of fire; but, on observing the appearance of that glairy slag produced in the iron-furnaces, which entirely resembles the pumice-stone, and is produced from the calcareous fluxes used to promote the fusion of the ore, our author is of opinion that the formation of pumice may be rather attributed to that kind of froth which must be formed at the top of the melted matters in the volcanic crater. An hundred parts, according to Bergman, contain from 6 to 15 of magnesia, with a small proportion of calcareous earth, and the greatest part filax. Another kind of pumice, which seems to be a ferruginous granite altered by fire, has been discovered by Dolomieu at Stromboli.

Pumice-stone is used in some mechanical arts; as for rubbing and smoothing the surface of metals, wood, plateboard, and stone; for which it is well fitted by reason of its harsh and brittle texture; thus scouring and carrying off the little inequalities from the surfaces just mentioned.