This substance has been used for various purposes. It is possible to cut and polish it; but its brittleness and frangibility are so great, that, without much care, it will fly into pieces during the working. The reflectors of telescopes are sometimes formed of it. In Mexico and Peru obsidian is cut into mirrors; and the inhabitants of those countries used formerly to manufacture it into knives and other cutting instruments. Hernandez says that he saw more than a hundred of these knives made in an hour. Cortez, in a letter to the Emperor Charles the Fifth, relates that he saw razors that had been formed of obsidian. The natives of Easter and Ascension Islands use this substance for cutting instruments; and also for pointing their lances and spears, and, in place of flint, for striking fire with. According to the account that has been given by Pliny, the ancients sometimes formed obsidian into mirrors, and ornamental articles of different kinds. The Danish lapidaries, who obtain considerable quantities of it from Iceland, cut it into snuff─boxes, ring─stones, and ear─pendants. Obsidian is found near Mount Hecla, and in other parts of Iceland. Sir George Mackenzie, during his journey through that island, observed an immense mass of this substance, which appeared to him to have been part of a stream that had flowed from a volcano. It is also found in Sicily, and several other islands of the Mediterranean; and in nearly all parts of the world where there are volcanoes. 108. _PUMICE is an extremely light and porous mineral, of somewhat fibrous texture, and of white, grey, reddish, brown, or black colour._ From the texture of this mineral, which is chiefly brought from the neighbourhood of volcanoes, some persons have imagined it to be asbestos decomposed by the action of fire. Its lightness is such that, placed in water, it will float. To mechanics and other artists pumice stone is a very useful mineral. It is employed for cleansing and smoothing the surface of wood, leather, metal, stones, glass, and other substances; and it is used by parchment─makers, curriers, and hat─makers. Hence it forms a considerable article of trade: and is exported from the Lipari Islands, in great quantities, to the different countries of Europe. Sailors in the Mediterranean rub their beards off with pumice, instead of shaving. On account of its porosity, it is used in Teneriffe as a filtering stone. It forms a pernicious ingredient in some kinds of tooth powder; and in Italy is ground and used instead of sand, in the making of mortar. Pumice occurs in Ireland, along with obsidian (107); and it abounds in several islands of the Grecian Archipelago. AZURE STONE FAMILY.