PO R 



POR 



grouped together three classes of Radi- 

 ated animals, on account of their external 

 resemblance to the many-armed cuttle- 

 fishes, which were so denominated by 

 the ancient Greek naturalists. The term 

 now denotes a class of Radiated animals, 

 furnished with numerous prehensile or- 

 gans radiating from around the mouth. 



29. Poly-scope {anoTreio, to view). A 

 lens plane on one side, and convex on 

 the other, the latter being formed of seve- 

 ral plane surfaces, or facettes, for the 

 purpose of multiplying any object. 



30. Poly-sepalous. A designation of 

 that form of calyx, in which the sepals 

 are separate from one another, as distin- 

 guished from the monosepalous or gamo- 

 sepalous calyx, in which the sepals co- 

 here. 



31. Poly-spermousia-TrepfiatSeed). Hav- 

 ing many seeds ; a botanical term applied 

 to certain fruits, which contain many 

 seeds, and distinguished from oligo- 

 spermous, or few-seeded. 



32. Poly-synthetic {avvOea-n, a putting 

 together). The name applied by Du 

 Ponceau to the class of languages spoken 

 by the Indian tribes of America, from 

 the manner in which words are abbre- 

 viated and combined to express ideas. 



33. Puly-thalamaceans {Oa\afio9, a 

 chamber). The name given by De Blain- 

 ville to an order of Cephalopods, includ- 

 ing ah those which inhabit many-cham- 

 bered shells. 



34. Poly-zonal {^wvrt, a belt). Lite- 

 rally, what is composed of many zones, 

 or belts. The term is applied to certain 

 lenses, composed of pieces united in 

 rings, which are therefore called poly- 

 zonal lenses. 



POMA'CEiE {pomum, an apple). The 

 Apple tribe of dicotyledonous plants. 

 Trees or shrubs with leaves alternate; 

 flowers polypetalous ; stamens perigyn- 

 ous ; fruit 1 to 5-celled. 



POME {pomum, an apple). A fruit 

 consisting of two or more inferior carpels 

 united together, the pericarp being 

 fleshy, and formed of the floral envelope 

 and the ovary closely adhering. 



PO'NDERABLE MATTER {pondus, 

 •weight). A term applied to all bodies 

 possessing weight, as metals, gases, &c., 

 as distinguished from light, heat, and 

 electricity, which, having no appreciable 

 weight, are termed imponderable agents. 



PORCELAIN CLAY. A species of 

 clay with various shades of white, con- 

 sisting, according to Wedgwood, of 60 

 parts alumina and 40 silica. It probably 

 268 



originated in the decomposition of fel- 

 spar. 



PORCELAIN JASPER. A variety 

 of jasper, produced by the action of sub- 

 terraneous fire on clay slate. 



PORI'FERA {porus, a pore, fero, to 

 bear). A class of the Radiata, consisting 

 of soft, gelatinous animals, which have 

 their body traversed internally by nu- 

 merous anastomozing canals, commencing 

 from superficial minute pores, and termi- 

 nating in large open vents. 



PO'RISM {iToptajxa, a corollary). 

 Something deduced from a previous 

 demonstration. In the original Greek 

 of Euclid's Elements, the corollaries to 

 the propositions are called porisms ; but 

 this scarcely explains the nature of po- 

 risms, as it is manifest that they are 

 different from simple deductions from 

 the demonstrations of propositions. Prof. 

 Play fair defines a porism to be " A pro- 

 position affirming the possibility of find- 

 ing such conditions as will render a cer- 

 tain problem indeterminate or capable 

 of innumerable solutions." This defi- 

 nition represents a porism as almost the 

 same as an indeterminate problem. Every 

 indeterminate problem containing a 

 locus may be made to assume the form of 

 a porism, but not the converse. Porisms 

 are of a more general nature than inde- 

 terminate problems which involve a 

 locus. 



PORO'SITY. That property of matter, 

 in consequence of which it does not com- 

 pletely fill the space which bodies occupy ; 

 and this is owing to the presence of 

 pores, or interstices, which occur among 

 the particles of bodies, even the densest. 

 The mass and density of bodies are esti- 

 mated according to their various degrees 

 of porosity: the greater the mass, the 

 less porous will be the substance ; and, 

 of two bodies, that is the more dense, 

 which with equal bulk contains the 

 greater mass. 



POR'PHYRY (7rop0upa, purple). An 

 unstratified or igneous rock. This term 

 was applied, in the time of Pliny, to a 

 red rock, with small, angular, white 

 bodies, diffused through it, which are 

 crystallized felspar, brought from Egypt ; 

 and hence it has been applied to every 

 species of unstratified rock in which 

 detached crystals of felspar, or other 

 mineral, are diffused through a base of 

 other mineral composition. Thus, clay- 

 stone with crystals of felspar, is called 

 claystone porphyry. 



PORTLAND LIMESTONE. Port- 



