a compound rock, essentially consisting of some base or ground, in which are interspersed crystals of some other substance, as when an argillaceous stone, or a pitchstone, has crystals of feldspar or quartz interspersed in it, and hence is denominated an argillaceous or pitchstone porphyry. See GEOLOGY Index. Porphyry is still found in immense strata in Egypt. The hard red lead coloured porphyry, variegated with black, white, and green, is a most beautiful and valuable substance. It has the hardness and all the other characters of the oriental porphyry; and even greatly excels it in brightness, and in the beauty and variegation of its colours. It is found in great plenty in the island of Minorca; and is well worth importing, being greatly superior to all the Italian marbles. The hard, pale-red porphyry, variegated with black, white, and green, is of a pale flesh colour; often approaching to white. It is variegated in blotches from half an inch to an inch broad. It takes a high polish, and emulates all the qualities of the oriental porphyry. It is found in immense strata in Arabia Petraea, and in the Upper Egypt; and in separate nodules in Germany, England, and Ireland.

Ficoroni takes notice of two exquisitely fine columns of black porphyry in a church at Rome. In Egypt there are three celebrated obelisks or pillars of porphyry, one near Cairo and two at Alexandria. The French call them aquilias, and in England they are called Cleopatra's needles.

The art of cutting porphyry, practised by the ancients, appears now to be lost. Indeed it is difficult to conceive what tools they used for fashioning those huge columns and other porphyry works in some of the ancient buildings in Rome. One of the most considerable of these, still entire, is a tomb of Constantia, the empress Constantine's daughter. It is in the church of St Agnes, and is commonly called the tomb of Bacchus. In the palace of the Tuileries there is also a bust of Apollo and of twelve emperors, all in porphyry. Some ancient pieces seem to have been wrought with the chisel, others with the saw, others with wheels, and others gradually ground down with emery. Yet modern tools will scarcely touch porphyry. Dr Lister therefore thinks*, that the ancients had the secret of tempering* Philosophical steel better than we; and not, as some imagine, that they had the art of softening the porphyry; though it is probable that time and air have contributed to increase its hardness. Mr Addison says, he saw a workman at ii. p. 560. Rome cutting porphyry; but his advances were extremely slow and almost insensible. The Italian sculptors work the pieces of old porphyry 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 Battista Alberti; who, searching for the necessary materials for temper, says, he found goats blood the best of any; but even this avails 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 anything 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-relievo, and Cosmo's head and his duchess's. The very hair and beard, how difficult however, 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, viz. with an iron saw without teeth, and gresz, a kind of free stone pulverized, and water. The authors of this invention say, that they could form the whole contour of a column hereby 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 brankurtfune. 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; and observes, that in his time the English workmen were ignorant of the manner of working porphyry, and that none of them would undertake. Porphyry undertake to cut or polish it. See his Works abr. vol. i.

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 that are now lost. He imagines, that, by unwearied diligence, and with numbers of common tools at great expense, 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 particular hard lands 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 Hist. Nat. of Fossils, p. 285.

PORPHYRY-Shell. See Murex, Conchology Index.