Porphyry. Belongs to that class of minerals, which, occurring in great masses, mineralogists have considered as properly designated by the general term rock: a precious kind of stone or marble. It is of a brownish red, green, or black colour frequently interspersed with white stains; formerly brought from Egypt, and exceeding all others in hardness. The ancient porphyry quarries are long since lost, and the art of cutting it, as practised by the Egyptians, is also lost. The modern tools will scarcely touch it. Either the ancients, therefore, had the art of tempering steel better than we, or they had the art of softening the porphyry. Though it is probable that time and air have contributed to increase its hardness. Mr. Addison saw a workman at Rome employed in the cutting of porphyry, but his advances were exceedingly slow, and almost insensible. The Italian sculptors work it with a brass saw without any teeth, together with emery and water. Leon Bap-tista Alberti, searching for the necessary temper, says he found goat's blood the best of any. Cosmo de Medicis is said to have distilled a water from certain herbs, with which his sculptor gave his tools such an admirable hardness and temper, as that he performed some fine works with them, particularly our Saviour's head, in demi-relievo. Even the very hair and beard, how difficult soever, are here well conducted, and there is nothing of the kind better in all the works of the ancients. But the secret seems to have died with him. The French pretend to have found another method, with an iron saw without teeth, and freestone pulverized with water. The Porphyries are composed of felspar in little fragments, of short, and a kind of cement of the nature of jasper, on which the colour of the porphyry generally depends. The felspar and short which enter into the composition of porphyries, contain silex, alumina, magnesia, baryta, and a little iron.