POD 



PO I 



PLOMB GOMME. Hydrated alu- 

 minate of lead, found in the French de- 

 partment of Cote du Nord. 



PLOPOCA'RPIUM. A term applied 

 by Desvaux to a form of fruit, consisting 

 of several follicles united in a single 

 flower, as in nigella and delphinium. 



PLUMBA'GO. Carburet of iron; a 

 mineral, also known as graphite and 

 blacklead, occurring in rounded masses 

 deposited in beds in the primitive forma- 

 tions, particularly in granite, mica-schist, 

 and primitive limestone. Borrowdale in 

 Cumberland is a celebrated locality of 

 graphite, and affords the only specimens 

 which are sufficiently hard for making 

 pencils. 



PLUMBER'S SOLDER. An alloy of 

 one part of tin and two of lead. 



PLUMBOCA'LCITE. A mineral sub- 

 stance in which the oxide of lead occurs 

 in the form of carbonate of lime; an 

 isomorphism by which the protoxide of 

 lead is connected with the magnesian 

 oxides. 



PLUMULE {plumula, a little feather). 

 Gemmule. The ascending part of the 

 axis of the embryo of a seed. 



PLUTO'NIC ACTION. The influence 

 of volcanic heat and other subterranean 

 causes under pressure. 



PLUTONIC ROCKS. A designation 

 of granite, porphyry, and other igneous 

 rocks, supposed to have been consoli- 

 dated from a melted state at a great 

 depth from the surface, and named from 

 Pluto, the fabled God of the infernal 

 regions. The term umtratified is also 

 applied to this, the first class of rocks. 



PLUVIO' METER {pluvius, rain, 

 fxerpov, a measure). A rain-gauge; a 

 vessel for catching falling rain, in order 

 to determine what quantity of rain has 

 fallen in a given period. 



PNEUMA'TICS (iTvedna, wind). That 

 branch of natural philosophy which treats 

 of the mechanical properties of air and 

 other compressible fluids. The term 

 pneumatoloffy was once employed for 

 metaphysics. 



PNEUMOBRANCHIA'TA (Trveu/xa, 

 air, /Spa7X'a» gills). Under this term, 

 Lamarck arranges several genera of 

 Molluscous animals into a section, which 

 connects his second order Hydrohranchia 

 •with his third order Tracheli-podes. The 

 genera are onchidium, parmacella, limax, 

 testacellus, and vitrina. 



PODE'TIUM {novv, 7ro56p, afoot). A 

 little foot; the stalk-like elongation of 

 the thallus, which supports the fructifi- 

 264 



cation of the Cenomyce, a plant of the 

 order of the Lichens. 



PODOGY'NIUM (ttovp, TTodof, a foot, 

 yvvi], a female). A term applied to the 

 stalk upon which the ovary is seated in 

 certain plants, as the Passiflora, Tac- 

 sonia, &c. It is also called gynophore, 

 and thecaphore. 



PODOPHTHA'LMIA ( Trovf , 7ro36f, 

 a foot, o(p0a\u6i, the eye). A group of 

 the malacostracous Crustacea, which have 

 moveable pedunculated eyes. It contains 

 two orders; the Decapods and the Sto- 

 mapods. See Edriopthalmia. 



PODOSPE'RMIUM (Troi/r, irodo^, a 

 foot, o-7rep)ua, seed). A term applied by 

 some writers to the funiculus or umbili- 

 cal cord, by which the ovule of plants is 

 connected with the placenta. 



PODU'RIDiE. A family of wingless 

 insects, belonging to the order Thysa- 

 noura, in which the extremity is pro- 

 longed into a forked tail, as in the 

 podura. 



PCECILO'PODA (TTotKi'Xop, various, 

 TToi/f, a foot). A designation of the low- 

 est forms of the Crustacea, from the an- 

 isopodous character of their segments, 

 the feet of different segments being pre- 

 hensile, or natatory, or branchial, or 

 ambulatory. 



POIKILI'TIC (TToiKt'Xor, variegated). 

 A term sometimes applied, in geology, to 

 the New Red Sandstone formation, owing 

 to the varieties of colours which it pre- 

 sents. 



POINT. A physical point is the 

 smallest magnitude perceptible by the 

 senses. If such a point be supposed to 

 be infinitely diminished, an idea is 

 formed of a mathematical point, of which 

 we may say that it has "no parts." As 

 a mathematical line may be conceived to 

 proceed from the motion of a mathema- 

 tical point, so a physical line may be 

 conceived to be generated by the motion 

 of a physical point. 



Point of Contrary Flexure. A point 

 at which a curve changes its curvature 

 with respect to any given external point, 

 being concave on one side and convex 

 on the other. 



POINTS OF THE COMPASS. Besides 

 the cardinal points. North, South, East, 

 and West, intermediate directions are 

 adopted by seamen and travellers, 

 amounting in all to thirty-two primary 

 directions, each of which is called a 

 point of the compass, and thence the 

 interval between them, or the angle from 

 point to point, is also called a point. 



