PSI 



PTO 



PRCXIMUM GENUS. In Logic, the 

 nearest or least remote genus to which a 

 species can be referred. See Genus. 



PRUSSIAN BLUE. Berlin Blue. 

 The sesqui-ferrocyanide of iron ; a beau- 

 tiful deep blue powder, used in the 

 arts. 



PRUSSIAS. A prussiate ; a name now 

 exploded, except in commerce, when it 

 denotes a cyanide : what is termed the 

 yellow prussiate of potash, is a ferro- 

 cyanide of potassium. 



PRUSSIC ACID. A designation of 

 hydrocyanic acid, from its being an in- 

 gredient in Prussian blue. 



PRYING and LIFTING. In the 

 common use of a lever of the first kind, 

 the force is gained by bearing down the 

 long arm of the lever, which is called 

 prying. In the second kind the force is 

 gained by carrying the long arm in a 

 contrary direction, or upward, and this 

 is called lifting. 



PSEUDO-BULB. A term applied to 

 the enlarged aerial stem of orchidaceous 

 plants. It resembles a tuber. 



PSEU'DO-LITE. A mineral having a 

 close affinity to the pseudomorphous 

 crystals of steatite. 



PSEUDOMALACHITE. Prismatic 

 phosphate of copper, from Rheinbreiten- 

 bach, where it occurs with quartz which 

 sometimes passes into chalcedony. 



PSEUDO-MORPHOUS (i/.€ud^?, false, 

 lxop(pr], form). A term applied to sub- 

 stances which, not possessing a crystal- 

 line structure, are found in the form of 

 regular crystals. These accidental forma- 

 tions must be considered as merely casts 

 of the crystals they represent. 



PSEUDO-STRATA. A term proposed 

 by Macculloch for those extended plates 

 of rocks, not divided into parallel la- 

 minae, and commonly called table-layers. 

 Some geologists make a distinction be- 

 tween a stratum and a bed, meaning by 

 the latter what is called by Macculloch a 

 pseudo-stratum. 



PSEUDO-ZOA'RIA. A term proposed 

 by Blainville to include vegetables, many 

 of which have been ranked with the 

 Polypiaria. They are subdivided into 

 two classes, viz. Calciphyta, principally 

 composed of the genus Corallina, and 

 Nematophyta, including byssus, con- 

 ferva, oscillatoria, &c. 



PSILOMELANE. An ore of manga- 

 nese containing a considerable quantity 

 of barytes, occurring in Devonshire, 

 Cornwall, in the Hartz, and most manga- 

 nese mines. 

 2?5 



PSITTA'CIDiE ipsittacus, the parrot). 

 The Parrot tribe ; a family of birds, 

 generally referred to the Scansores, or 

 Climbers, with which, however, they 

 correspond in little else than the struc- 

 ture of the foot, and this is adapted rather 

 for grasping than for climbing. 



PSY'CHICAL {yhxtno?, pertaining to 

 the psyche, or soul). Relating to the 

 phenomena of the soul, and to analogous 

 phenomena in the lower animals. Psy- 

 chology, literally, the doctrine of the 

 soul, denotes a description of the intel- 

 lectual and moral faculties. 



PSYCHRO'METER (xl^uxpor, cold, 

 fxerpov, a measure). An instrument for 

 measuring degrees of cold. Its action is 

 the same as that of the hygrometer, and 

 depends on the combination of heat 

 which accompanies the evaporation of a 

 fluid, and the consequent deposition of 

 the vapour contained in the atmosphere. 



PTERODA'CTYL {nrepov, a wing, 

 ddKTuAof, a finger). Wing-fingered; the 

 name of an extinct genus of flying rep- 

 tiles, species of which have been found 

 in the Oolite and Muschelkalk. Some 

 of the finger joints are lengthened, so as 

 to serve as the expansors of a membran- 

 ous wing. 



PTEROPI'NiE. The single family of 

 the frugivorous or omnivorous Cheiro- 

 ptera, of which the pteropus Javanicus 

 may be considered the type. They 

 abound in warm climates, and include 

 some of the largest species of the order. 



PTERO'PODA (TTTepov, a wing, Troi/r, 

 a foot). The tenth class of the Mollusca, 

 consisting of small, soft, floating, marine 

 animals, which swim by the contractions 

 of two lateral musculocutaneous fins, 

 but are unable to fix themselves or creep 

 in the sea, from the want of feet, as the 

 clio, cleodora, &c. 



PTERY'GIANS (Trrepuf, a wing). A 

 name given by Latreille to a group of the 

 Mollusca, corresponding to the cepha- 

 lopods and pteropods of Cuvier, both of 

 which have locomotive organs, consisting 

 of wing'like expansions of the skin. 



PTOLEMA'IC SYSTEM. A system 

 of astronomy so named from the famous 

 geographer Claudius Ptolemaeus, who 

 flourished in Egypt during the reigns of 

 the Roman emperors Hadrian and Anto- 

 ninus Pius. He taught that the earth 

 was at rest in the centre of the universe, 

 and that the heavens revolved round it, 

 from east to west, in twenty-four hours, 

 carrying all the heavenly bodies with 

 them. This system was believed, and 

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