644 



PORPHYRY PORPORA. 



manner, the air which is confined in the common 

 bfllows iintlcr a moderate pressure, might, by a 

 more violent action, be made to transpire copiously 

 through the boards and the leather. 



The transmission of a fluid through a solid sub- 

 stance shows the existence of pores; but the re- 

 sistance, in ordinary cases, to such a passage, is 

 insufficient, therefore, to prove the contrary. The 

 permeability of translucent substances to the rays 

 of light, in all directions, evinces the most extreme 

 porosity. But this inference is not confined merely 

 to the bodies usually termed diaphanous ; for the 

 gradation towards opacity advances by insensible 

 shades. The thin air itself is not perfectly trans- 

 lucid, nor will the densest metal absolutely bar all 

 passage of light. The whole mass of our atmos- 

 phere, equal to the weight of a column of thirty- 

 four feet of water, transmits, according to its com- 

 parative clearness, only from four-fifths to three- 

 fourths of the perpendicular light, and consequently 

 retains or absorbs from a fifth to a fourth of the 

 whole. But this absorption is greatly increased by 

 the accumulation of the medium. When the sun 

 has approached within a degree of the horizon, and 

 his rays now traverse a tract of air equal in weight 

 to a column of 905 feet of water, only the 212th 

 part of them can reach the surface of the earth. 

 Even gold itself is diaphanous : if a leaf of that 

 metal, either pure or with only an 80th part of 

 alloy, and therefore of a fine yellow lustre, but 

 scarcely exceeding the 300,000th part of an inch in 

 thickness, and enclosed between two thin plates of 

 mica, be held immediately before the eye, and op- 

 posite to a window, it will transmit a soft green 

 light, like the colour of the water of the sea, or of 

 a clear lake of moderate depth. The inferior ducti- 

 lity of the other metals will not allow that fine lami- 

 nation, which would be requisite for showing, in 

 ordinary cases, the transmission of light. But their 

 diaphanous quality might be inferred from the tints 

 with which they affect the transmitted rays, on be- 

 ing alloyed with gold. 



Other substances, though commonly reckoned 

 opaque, yet admit, in various degrees, the passage 

 of light. The window of a small apartment being 

 closed by a deal board, if a person within shut his 

 eyes a few minutes to render them more acute, he 

 will, on opening them again, easily discern a faint 

 glimmer issuing through the window. In propor- 

 tion as the board is planed thinner, more light will 

 be admitted, till the furniture of the room becomes 

 visible. Writing paper transmits about a third part 

 of the whole incident light, and, when oiled, it often 

 supplies the place of glass in the common work- 

 shops. The addition of oil does not, however, ma- 

 terially augment the diaphanous quality of the pa- 

 per, but renders its internal structure more regular, 

 and more assimilated to that of a liquid. The rays 

 of light travel, without much obstruction, across 

 several folds of paper, and even escape copiously 

 through pasteboard. Combining these various facts, 

 it follows that all bodies are permeable, though in 

 extremely different degrees, to the afflux of light. 

 They must, therefore, be widely perforated, and in 

 every possible direction. 



PORPHYRY is a compound rock, having a basis 

 in which the other contemporaneous constituent 

 parts are imbedded. The base is sometimes clay- 

 stone, sometimes hornstone, sometimes compact 

 feldspar, jade, pitchstone, pearlstone, and obsidian. 

 The imbedded parts are commonly feldspar and 

 quartz; the former in more or less distinct crystals. 

 There are porphyries of different ages. One variety 

 is found graduating into granite and gneiss ; but 

 this does not possess the characteristics of the rock 



in the highest perfection ; another is found in over- 

 lying strata, and unconformable to other rocks, which 

 is tile true porphyry. Its colour is often red ur 

 green, and, when polished, is valuable for ornamen- 

 tal work, being superior to marble, on account of 

 its great hardness. This rock abounds in Egypt, 

 the northern parts of Europe, in Mexico and South 

 America. There is still another variety found in 

 connexion with extinct volcanoes, which is sonic- 

 times distinguished by the name of clay-porphyry. 

 It abounds in the neighbourhood of the Mediter- 

 ranean. 



PORPHYRY ; a celebrated philosopher, of ti.e 

 school of the New Platonists. See New Platonics. 



PORPOISE (delphinus phoccena). Several 

 species of the genus delphinus are known by sea- 

 men under the name of porpoise, but the D. phocccna 

 is that to which this designation is generally applied. 

 The term porpoise, porpesse, or porpus, is derived 

 from the Italian porcopesce, or hog fish, from (lie 

 supposed resemblance of its projecting snout to that 

 of the hog ; the French and German appellations 

 for it have a similar signification. The porpoise is 

 distinguished from the dolphin by the superior thick- 

 ness of its head and its smaller size. The back is 

 generally black, and the belly white. Both jaws 

 are furnished with teeth ; these are compressed, 

 sharp, black above and white beneath. The spout- 

 hole is upon the crown of the head, of a semilunar 

 shape, and divided internally by a cartilaginous 

 membrane. The whole body is covered with a coat 

 of fat, nearly an inch in thickness, beneath which 

 the flesh appears red and muscular, resembling that 

 of the hog. They are very abundant on the Ameri- 

 can coast, and, before a storm, may be seen gambol- 

 ling and tumbling, as it is termed, in large shoals. 

 The porpoise feeds on small fish, such as the herring 

 and mackerel, of which they destroy great num- 

 bers. It is said that they act in concert, when in 

 pursuit of their prey, urging them from one bay or 

 estuary to another, deterring them from the shallow 

 water, and driving them towards each other's am- 

 bush, with all the art of an experienced dog. The 

 flesh was formerly considered as a great delicacy, 

 but is now seldom eaten ; the liver, however, is 

 still esteemed by seamen, and is said to resemble 

 that of the hog in taste. 



PORPORA, NICOLO (called, by the Italians, the 

 patriarch of harmony), was born at Naples in 1685. 

 His first opera, Ariana e Teseo, was brought out at 

 Vienna (1717). The operas which he composedfor 

 Rome, Naples and Venice, before and after his visit 

 to England, amount, according to doctor Burney, 

 to more than fifty. In 1730, several cantatas by 

 him were also published. In 1726, he brought for- 

 ward his Si/ace, at Venice, at tlie same time that 

 Vinci had his Siroe performed on another stage in 

 the same city. At first he found it difficult to sustain 

 the contest, but, up to 1729, he had brought out five 

 operas at Venice, which were received with general 

 applause. In that year he went to Dresden, where 

 the king of Poland appointed him his chapel-master. 

 He was treated with the greatest distinction at 

 court, and Hasse (q. v.) and his wife could not con- 

 ceal their jealousy of Porpora and the celebrated 

 Mingotti (q. v.), whom he had instructed. In 1731, 

 Porpora returned to Naples, and established a sing- 

 ing school, which produced the greatest singers of 

 the eighteenth century, Farinelli, Caffarelii (see 

 Maforano), Salimberti, Uberti, Gabrielli, &c. In 

 1732, he went with Farinelli to London, whither he 

 had been invited on account of the disputes between 

 Handel (q. v.) and the directors of the opera. His 

 twelve cantatas for a single voice procured him this 

 invitation. But his theatre sustained itself only 



