a compound rock. The art of cutting porphyry, practised by the ancients, appears to have been lost. Indeed it is difficult to conceive what tools they used for fashioning those huge columns and other porphyritic works in some of the ancient buildings in Rome. Modern tools will scarcely touch porphyry. Dr. Lister therefore thinks, that the ancients had the secret of tempering steel better than we; and not, as some imagine, that they had the art of softening the porphyry, since it is probable that time and air may have contributed to increase its hardness. Mr. Addison says, he saw a workman at Rome cutting porphyry; but his advances were extremely slow and almost insensible. The Italian sculptors work the pieces of old porphyritic columns still remaining (for the porphyry quarries are long since lost) with a brass saw without teeth. With this saw, emery, and water, they rub and wear the stone with infinite patience. Many persons have endeavoured to retrieve the ancient art, and particularly Leon Baptista Alberti, who, searching for the necessary materials for temper, says, he found goat's blood the best of any; but even this availled not much; for in working with chisels tempered with it, sparks of fire came much more plentifully than pieces of the stone. The sculptors were thus, however, able to make a flat or oval form; but could never attain to any thing like a figure.

In the year 1555, Cosmo de Medicis is said to have distilled a water from certain herbs, with which his sculptor, Francesco Tadda, gave his tools such an admirable hardness and so fine a temper, that he performed some very exquisite works with them, particularly our Saviour's head in demi-relief, and Cosmo's head with that of his duchess. The very hair and beard, how difficult soever, are here well conducted; and there is nothing of the kind superior to it in all the works of the ancients; but the secret appears to have died with him. The French have discovered another mode of cutting porphyry, namely, with an iron saw without teeth, and with grez, a kind of free-stone pulverized, and water. The authors of this invention say that they could form the whole contour of a column, if they had matter to work on. Others have proposed to harden tools so as to cut porphyry, by steeping them in the juice of the plant called bear's breech or brankusine. (See Birch's Hist. R. S. vol. i. p. 238. vol. ii. p. 73. &c.) Mr. Boyle says, that he caused porphyry to be cut by means of emery, steel saws, and water; but he observes, that in his time the English workmen were ignorant of the manner of working porphyry, and that none of them would undertake to cut or polish it. (See his Works abr. vol. i. p. 11.)

Da Costa supposes, and perhaps with reason, that the method used by the ancients in cutting and engraving porphyry was extremely simple, and that it was performed without the aid of any scientific means. He imagines, that, by unwearyed diligence, and with numbers of common tools, they rudely hewed or broke the stone into the intended figure, and by continued application reduced them into more regular designs; and that they completed the work by polishing it with great labour, by the aid of certain hard sands found in Egypt. And he thinks, that in the porphyry quarries there were layers of grit or loose disunited particles, analogous to the porphyry, which they carefully sought for, and used for this work. See GEOLOGY AND MINERALOGY.