POKDAUK 



PORPHYRY 



prove this statement. Occasionally the spine* end 

 in a peculiar cup shai>ed extremity. The armature 

 of MOMI is "if tin- greatest viiliu- to thr poreopue, 

 though their use is entirely for fafaoairfl mir|M>*e*. 

 It is hardly neceary to ileny the popular MUM 

 that the animal can shoot out 11- .;nilU like DO 

 many arrows ; the notion has arisen from the fact 



Common Porcupine (Hyttrit rrittata). 



that when the animal erect* ite spines loose one*. 

 Himet imcs fall out. See ECHIDNA, GLOBE-FISH. 



Pordage. See PHILADEI.PIIIAXS. 



PordenonCj II-, a name for the religious 

 painter Giovanni Antonio Licinio ( 1483-1539), oorn 

 near Ponlenone in Venctia. 



Porifera. SeeSpoM. 



Porism i defined hy Simson as a proposition 

 to demonstrate that some one thing or more things 

 are given, to which, as also to each of innumerable 

 other things, nut indeed given, hut having the 

 name relation to those which are given, it is to he 



shown that there la-longs Mime ( mon affection 



described in the pnijHisition. Playfoir dflflaed a 

 porism to be a proposition allirming the possibility 

 of finding such conditions as will render a certain 

 problem capable of innumerable solutions. Owing 

 t<i the loss of Kin-lid's three lKx>ks on porisms, and 

 the obscurity of the account given by Pappus of 

 their content*, there has been much discussion 

 among geometers as to the nature of a porism. 

 The two most important liooks on the subject are 

 Siinsoii's Itr I'orisnuttiliii.i in his Opera Reliqua 

 (1776), and Chasles's La troia livres <lr Porismet 

 (TEticlide (1860). Chasle* is of opinion that the 

 porisms were closely allied to the modern theories 

 >f anharmonic ratio, homographic division, and 

 involution. 



Pork. Sec 1'i'i for the animal from which |Mirk 

 is proi-iiM-il ; DIKT and FOOD for the pro|-rties of 

 pork as an article of food; and THICHINOSIS for 

 one of the gravest of the diseases affecting the 

 pig. The great headquarters of the trade in pork 

 i the I'niied Slates. This is partly indicated by 

 the figure* given at CHICAGO and HAM ; but it 

 will IN- shown more clearly hy the following figures. 

 In IH'.Hi tin- total number of pigs in the Imited 

 Kingdom was J.TT^.tiO'.l ; in the I'nited States it 

 in tin' -nine year "i 1,602,780, with a value of 

 9243.-llx.33ti. While Britain import* nigs and pig 

 products (ham*i, bacon, pork, and lard) to a large 

 extent (mainly from America), the I'nited State* 

 export* on a vast scale. In the fiscal year 1889-90 

 lln> export* were as follows: hogs, 91,148; bacon, 

 631,899,677 lb. ; hams, 7<i,a91.!27!) Ib. ; Sn-\t pork. 

 *79,403 Ib. : pi, -I led |M>rk, 79,788,868 lb. ; lard, 

 47 1 ,0*3,598 lb. The value of these pig- prod ucta in 

 that one year 1889-90 was over 967,070,000. 



Porosity. Ky thin term we express the e\peii 

 mental fact that no kind of matter completely tills 

 the (pace it occupies. On the atomic theory- it is 



obvious that this must he the case if the atoms of 

 matter arc spherical, or, indeed, if they have any 

 form save one or two special ones, such as en lien or 

 rhombic dodecahedrons. The Florentine Acade- 

 micians, in their attempts to compress water, 

 prove.! the porosity of silver by flattening a sp. re 

 of that metal, filled with water and soldered. I 

 water escaped through the silver, and stood in tin 

 drops on ite surface. The |irosity of liquids i 

 easily shown by mixing alcohol and water. The 

 bulk of the mixture is considerably lees than the 

 MINI of the hulks of the components. 

 Porous Jars. See KKFKIUERATION. 

 Porph) rite, one of the crystalline igneous 

 rocks. It coiisi-ts principally of phtgioclasc. The 

 ground-mass of the rock is composed of microlite* 

 and minute roil-like crystals of plagioclase. inter- 

 screed amongst which may occur crystalline 

 granules of hornblende, augitc. rhombic p\ roxene, 

 mica, ilmenite, magnetite, \c. Sometimes trace* 

 of a glassy or devitritied base can he detected. 

 Throughout this ground-mass are scattered ]K>r- 

 phyritically larger crystals of plagiiiclase, accom- 

 panied by one or more of the following minerals: 

 hornblende, augite, rhombic pyroxene, mica, \e. 

 The rock shows all varieties of colour, and range* 

 in texture from vitreous and cryptocrystallinc up 

 to coarsely crystalline. It is otten vesicular and 

 amygdaloulal. It occurs abundantly in Scotland 

 ( where it forms many of the hill ranges of the Low- 

 lands), Itoth in the form of lava-flows and intrusive 

 sheets, dykes, and mosses. Porphyrite is so closely 

 allied to Andesite (q.v.) that it may l>e considered 

 as merely an altered variety of that rock. 



PorphyroseilHllSlGr., 'Ix.rn in the purple'), 

 a title given to the Byzantine emperor Constantino 

 VII. (912-959). 



Porphyry (Gr., 'purple'), a term originally 

 confined to an Kgyptian ro<-k nseil in sculpture and 

 known as jxirjiiiorossoantii'ii. It occurs as a dyke or 

 vein some 65 to S."i feet thick in the granite of Jebel 

 Dokhan (formerly callexl Mont Porphyrite*) in 

 Kgypt, between Siout and the Ked Sea. It is com- 

 posed of a felspathic Iwvse, in which are disseminated 

 crystals of oligoclase felspar, with some plates of 

 dark hornblende, and grains of an iron oxide. The 

 beautiful pink or red colour of the porphyritic fel- 

 spar and the line-grained l-ase is due to the diffusion 

 of the red variety of epidote, called \Vithamite or 

 Piedmontite. The term jMirphyry is not now used 

 to denote any particular rock, hut is applied by 

 architects and others to any igneous rock which, 

 like the porfido rosso antico, has a homogeneous, 

 compact uase or fine-grained ground-mass, through 

 which are scattered distinct crystals of one or 

 more minerals. l!y geologists the term porphyry 

 is seldom used without some descriptive word 

 bracketed with it, as quart/ porphyry, orthoclase- 

 ]M>rphyry, augite -porphyry, &c. 



Porphyry, one of the greatest Neoplatonist 

 phikMOpbers, wa* bom at Tyre, or at llatanea, in 

 the year '233 A.I). His original name \\as MalchuB- 

 (HeK Mr/rr/i. 'king'); and 1'nrpliurius ( ' one clad 

 in purple') is but a kind of playful synonym for 

 this royal name. He is said by Socrates the 

 historian and by St Augustine to have been origin- 

 ally a Christian ; but this seems improbable, 

 although it is rriiain that in his youth M wo* a 

 hearer of Origen, or at least held some intercourse 

 with him at (Vsarca in Palestine. What is more 

 certain is that he passed at a later time to Athens, 

 where he studied rhetoric under I.ongimiB, the 

 well known author of the treatise On the AwUM, 

 It was at Koine, however, whither he repaired 

 alMiut 263, that he found the master who per- 

 manently moulded his life. Here he became the 

 most t rusted of the disciples of the Neoplatonist- 



