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 POEOSITY PORPOISE 719 POROSITY (Gr. Trfyof, a passage), the condi- tion of open structure in which the particles of matter are arranged in all bodies, leaving between them pores or interstices that are supposed to be vacant or filled with air. The existence of such spaces even in the most solid bodies is proved in various ways. When wood or stone of the most compact structure is im- mersed in water under the receiver of an air pump and the air is exhausted from the sur- face, that contained in these bodies immediately makes its appearance rising in a cloud of bub- bles. Under great pressure water is forced through the pores of cast iron, even of 4 in. thickness. The porosity of this material is evidently increased by dissolving out the car- bon disseminated throughout its substance, by which it becomes malleable iron without change of form, but the change of texture thus indi- cated is not apparent to the eye. Density, which is the opposite condition to porosity, is increased in most metals by pressure and ham- mering. Liquids are also supposed to be po- rous from the fact that mixtures are sometimes made which occupy less space than the sum of the volumes of the ingredients when separated. This is the case with alcohol and water. (See COMPRESSIBILITY.) PORPHYRY (Gr. nopfipa, purple), a rock so named from the prevalent color of the varie- ties used by the ancients, as the rosso antico or red porphyry of Egypt. This variety con- sists of a ground or paste of reddish feldspar in which are disseminated rose-colored crystals of the feldspar called oligoclase, with some plates of blackish hornblende and grains of peroxide of iron. This in general is the char- acter of porphyry ; but the paste may be green, red, purple, or black, and the interspersed crys- tals may present various shades, usually lighter than the ground. They may be also of horn- blende, quartz, augite, olivine, and other min- erals. The rock is very hard. On the smooth surface the crystals appear as blotches. Vari- ous rocks of an earthy or compact base with distinct interspersed crystals are termed por- phyritic. Granite is so called when it presents distinct feldspar crystals, and so are green- stone, trachyte, &c. The principal uses of porphyry are in architecture and ornamental articles, and in slabs and mullers for grinding hard powdered substances to extreme fineness. No material is more durable, and none retains better the sharp lines and high polish which it receives. In modern times it is most success- fully worked by the Swedes and Russians. In the Swedish royal porphyry works of Dalecar- lia vases, tazze, &c., of immense size are made, one of the latter exceeding 11 ft. in diameter. A vase of pink granitoid porphyry 6 ft. high and 4 ft. 4 in. in diameter was sent to the great exhibition in London in 1851. In the United States porphyry is met with in granitic regions. PORPHYRY (Gr. Hopfbpios, i. e., a wearer of purple), a philosopher of the Neo-Platonic school, born in Batanea or in Tyre about A. D. 233, died in Rome about 305. He was of dis- tinguished family, and was originally named Malchus, the Greek form of the Syro-Phce- nician Melech, signifying king; but he re- ceived from his preceptor Longinus, in allusion to the meaning of that word, the name by which he has ever since been known. He studied under Origen at Csesarea, under Apollonius and Longinus at Athens, and at Rome under Ploti- nus, with whom he remained six years, at the end of which period he went to Sicily, where he wrote his treatise against the Christian reli- gion. He subsequently returned to Rome, and taught there for many years. The philosophi- cal doctrines of Porphyry were essentially those of Plotinus, which he regards as identical with those of Plato, and substantially also with those of Aristotle. His doctrine is distinguished from that of Plotinus by its more practical and religious character. The worship of the na- tional gods of a people seems to have been up- held by him, on the ground that respect should be shown to the ancient religious usages of a nation ; but he acknowledged one absolute su- preme Deity. He wrote expositions of Plato's "Timaeus" and "Sophistes" and of Aristotle's CategoTice and De Interpretatione, and a still extant isagogical work on Aristotle, which is usually printed at the beginning of the Orga- non. Of his 56 different works mentioned, only 19 are extant. The most celebrated of his lost works is that "Against the Christians," which was publicly destroyed in 435 by order of the emperor Theodosius II. It was in 15 books, and treated both the Jewish and Chris- tian Scriptures very minutely. In it he de- nied the doctrine of the divinity of Christ, and maintained that the prophecies of Daniel were written after the events. His Vita Plo- tini, composed shortly before his death, ap- peared first in the Basel edition of the En- neads (1580). The most recent commentators on Porphyry's life and works are Brandjs, Wolff, Bernays, Schafer, and Baltzer. PORPOISE, the common name of the small ce- tacean mammals of the genus phoccena (Guy.). The snout is short, uniformly rounded, wide from the breadth of the more horizontal inter- maxillaries and maxillaries, without the pro- longed beak, separated from the forehead by a distinct furrow, which characterizes the dol- phin, to which family it also belongs. The name is evidently a corruption of the French porc-poisson (hog fish) ; it is called Meerschwein by the Germans, marsouin by the French, and sea hog and puffing pig by the English and Americans. Though it is an air-breathing mammal and not a fish, the shape of the body is fish -like and adapted for progression in the water; the jaws are armed with minute coni- cal teeth; the blow-hole, on the top of the head, is transverse, crescentic, with the con- cavity forward. (For its anatomy see DOL- PHIN.) There are several species in different parts of the world, some of which have a very wide geographical distribution ; they are very