PRI 



PRI 



loglsm are sometimes called together 

 the antecedent. 



PREPOSITION {prcepositus, placed 

 before). A word which connects two 

 words together, in such a manner as to 

 indicate the relation which the things or 

 ideas signified by them bear to each 

 other. The name of this part of speech 

 has been derived from the accidental 

 circumstance of its being placed imme- 

 diately before the object related to the 

 other thing named ; but the essence of 

 the preposition is, to signify relative po- 

 sition. *' Every preposition," says Adam 

 Smith, " denotes some relation con- 

 sidered in concrete with the correlative 

 object." 



PRESSURE. The application of force 

 to a resisting body, when that force is in 

 continued contact with the body on which 

 it is exerted. The centre of pressure is 

 that point at which the whole amount of 

 pressure may be applied with the same 

 effect as it has when distributed. 



PRIMARY. A term applied, in the 

 astronomical division of the sphere, to 

 the great circle traced iipon its surface, 

 half way between the poles. All the 

 smaller circles, which intervene between 

 the primary and the poles, are called 

 parallels. 



PRIMARY LIMESTONE. A modi- 

 fication of limestone, with a crystalline 

 texture, occurring under many different 

 aspects, and in every member of the 

 series, so that no particular position can 

 be ascribed to it. Its colours are 

 various. 



PRIMARY ROCKS. The lowest se- 

 ries of the stratified rocks, characterized 

 by a crystalline texture, and by the ab- 

 sence of organic remains. This term 

 has no reference to priority of formation, 

 but expresses the order of their occur- 

 rence, they being the first after granite. 

 These rocks were termed by Mr. Lyell 

 metamorphic, from the supposition that 

 they were deposited in a sedimentary 

 form, and were afterwards altered by the 

 action of heat. 



PRIME NUMBER. Any number 



which cannot be separated into factors, 



f^ as 7; 11, 13, 17, &c., is called a prime 



f ' number. Every number which can be 



so separated, as 6, 8, 9, 12, &c., is called 



a composite number. When two numbers 



have no common measure but unity, 



they are said to be prime to each other : 



thus 3 is prime to 7 ; 13 to 31 ; and so 



on. A prime number is one which is 



prime to every other number. 



271 



PRIMINE {primus, first). The first 

 or outermost sac of the ovule in plants. 



PRIMITIVE WORDS. In grammar, 

 these are words not derived from any 

 other word in the language, as school. 

 See Derivative Words. 



PRIMULA'CEiE. The Primrose tribe 

 of dicotyledonous plants, peculiarly dis- 

 tinguished by the stamens being oppo- 

 site to the lobes of the gamopetalous 

 corolla, and by having a superior capsule 

 with a free central placenta. 



PRIMUM MOBILE. A term given 

 by the old astronomers to an imaginary 

 sphere, by the motion of which diurnal 

 motion was given to the heavens. 



PRINCE'S METAL. Prince Rupert's 

 Metal. A species of copper alloy, in 

 which the proportion of zinc is greater 

 than in brass. 



PRISM (7rptV/ua, from Trpt'eo, to saw). 

 A prism is defined, in Geometry, to be a 

 solid figure contained by plane figures, 

 of which two that are opposite are equal, 

 similar, and parallel to one another, and 

 the others parallelograms. The name is 

 derived from the property exhibited by 

 the glass prism of separating a ray of 

 light into its constituent parts. 



Prism, Achromatic. A prism through 

 which objects are viewed refracted in- 

 deed, so that they are not seen in their 

 true position, but yet free from a prisma- 

 tic fringe about their edges. It con- 

 sists of two prisms fitted together, the 

 one being of fiint, the other of crown- 

 glass. The vertical angle of the flint- 

 glass prism must be less than that of the 

 crown-glass, and their sizes must be in- 

 versely as their dispersive powers. la 

 English flint-glass the vertical angle of 

 the flint-glass prism must be § that of 

 the crown-glass, but in Fraunhofer's It 

 must be ^ the size. 



PRISMATIC CRYSTALS. The square 

 prism differs from the cube only in having 

 its lateral edges either longer or shorter 

 than the terminal. The right rhombio 

 prism is a right prism whose base is a 

 rhomb. The oblique rhombic prism differs 

 from the preceding in being oblique in 

 the direction of one of the diagonals of 

 the base. All other prismatic crystals, 

 which do not possess any of the degrees 

 of symmetry which characterize the 

 other prisms, are comprehended in the 

 class of doubly oblique prisms. In all 

 the varieties of prisms, the crystals 

 are supposed to rest on one of the bases 

 or terminal planes of the prism, the sur- 

 faces which bound the prism lengthwise 

 N4 



