P O R 



[209 ] 



POT 



are consequently destined to an existence 

 as completely stationary as that of plants, 

 yet such is not the condition of the earlier, 

 and more transitory stages of their deve- 

 lopment. On the gemmule the power of 

 locomotion is conferred, until it has found 

 for itself a proper habitation ; this chosen, 

 it there fixes itself and there continues 

 for the remaining period of its existence. 



PORO'SITY. (porosite, Fr. porosita, It.) 

 The quality of having pores ; of being 

 porous. 



PO'ROUS. (poreux, ~Fr.poroso, It.) Hav- 

 ing small spiracles, interstices, or pas- 



PO'ROUSNESS. The quality of being porous. 



PO'RPHYRY. (from iroptyvpa, purple, Gr. 

 porphyrites, Lat. porphyre, Fr. porfiro, 

 It. porphyr, Germ.) Porphyry has been 

 so called in reference to the purple, or 

 reddish, colour so commonly perceptible 

 in it. Generally, any form of rock in 

 which one or more minerals are scat- 

 tered through an earthy or compact base. 

 Porphyry has generally a compact texture. 

 Sometimes it is composed of tabular, 

 columnar, or globular distinct concre- 

 tions ; and not unfrequently it is tra- 

 versed by numerous seams and rents. 

 There are many varieties of porphyry, 

 named according to the base of each, as 

 Petrosiliceous Porphyry, Felspar Por- 

 phyry, Clinkstone Porphyry, Argillaceous 

 Porphyry, &c. Geologists have described 

 four formations of porphyry ; but it is 

 generally agreed that there is much un- 

 certainty with respect to the situation of 

 these formations. The porphyry which 

 occurs regularly imbedded in granite, or 

 which appears to be formed by a mere 

 change of structure in that rock, may 

 properly be classed with primary rocks ; 

 it is not considered to be an extensive 

 formation. Porphyry also occurs in enor- 

 mous masses ; at the head of Glen 

 Ptarmagan, a cliff of porphyry fifteen 

 hundred feet in height, in shape resem- 

 bling an oblique truncated pyramid, passes 

 through granite. In some instances por- 

 phyry is, beyond all question, a volcanic 

 formation. Near Christiana, in Norway, 

 an immense mass of porphyry, from 

 1600 to 2000 feet in thickness, covers 

 beds of gneiss, limestone, and grey- 

 wacke. Dykes of porphyry cutting 

 through the subjacent rocks indisputably 

 prove the volcanic character of this im- 

 mense mass. Porphyry and basalt often 

 cover the primary mountains in the 

 Andes ; resembling immense castles lifted 

 into the sky. 



PORPHYRI'TIC. > Resembling porphyry ; 



PORPHYRA'CEOUS. J containing porphyry ; 

 composed of a compact homogeneous 

 rock, in which distinct crystals or grains 



are imbedded : the compact stone is called 

 the base, and sometimes the paste. The 

 base, or paste, is generally felspar. 



PO'RTLAND BEDS. A marine formation, 

 occurring in the Isle of Portland and in 

 Wiltshire. These beds consist of coarse 

 shelly limestone, fine grained white lime- 

 stone, and compact limestone, all pos- 

 sessing an oolitic structure ; and beds of 

 chert. The Portland beds lie imme- 

 diately under the Purbeck beds, and 

 above the Kimmeridge clay. They con- 

 stitute the uppermost members of the 

 oolite group, and abound in ammonites, 

 trigonise, &c. 



PO'RTLAND LI'MESTONE. ^ One of the mem- 



PO'RTLAND STONE. $ bers of the 



Portland beds ; a marine oolitic forma- 

 tion, obtained principally from Portland, 

 whence the name, and used in building. 

 This limestone is soft when quarried, 

 but hardens on exposure to the at- 

 mosphere. 



PO'TASH. An alkali obtained by the inci- 

 neration of vegetables, or the woody parts 

 of plants that do not grow near the sea. 

 The water in which the ashes are washed 

 is evaporated in iron pots, from which 

 circumstance it was called potash. Al j 

 though potash is very widely dissemi- 

 nated in the earth's crust, its amount, 

 collectively considered, is greatly inferior 

 to that of silex and alumine. There are 

 few, if any, of the inferior stratified rocks 

 without potash; and, viewing them in 

 the mass, potash may be considered as 

 constituting five or six per cent, of the 

 whole. Potash may be regarded as con- 

 stituting between six and seven per cent, 

 of granites, greenstones, and rocks of 

 that class. 



POTA'SSIUM. A metal discovered by Sir 

 H. Davy in 1808. At a temperature of 

 32 potassium is hard and brittle, with a 

 crystalline texture; at 50 it becomes 

 malleable, with a lustre like that of po- 

 lished silver; and at 150 it fuses. Po- 

 tassium is lighter than water, its specific 

 gravity being 0'85 : to preserve it un- 

 changed, it should be kept in a phial 

 with pure naphtha. Sir H. Davy ob- 

 tained potassium by submitting solid 

 hydrate of potash to the action of vol- 

 taic electricity ; it has, however, subse- 

 quently been procured by other means, 

 particularly by those described by Gay 

 Lussac and Thenard. 



PO'TSTONE. The Lapis ollaris of Pliny ; 

 Toftstein of Werner ; Talcollaire of Haiiy. 

 A variety of steatite, nearly equal in 

 hardness to common steatite ; it is, how- 

 ever, more tenacious, and though it may 

 be turned with the lathe, it breaks with 

 difficulty. It is smooth and unctuous to 

 the touch. It is usually of a greenish- 



