PER 



PER 



applied to that mode of conversion of a 

 proposition in which the quantity is 

 changed, or limited from universal to 

 particular. This might fairly be named 

 conversion by limitation, but is com- 

 monly called conversion per accidens. 



PERCE'PTION. The mental faculty 

 4)y which we hold communication with 

 the actual world, 1. It is thus distin- 

 guished from conception, which relates to 

 that which has no reality : we perceive 

 the present order of things, we conceive 

 of the future. Thus the ancients dis- 

 tinguished the a\(TOt]Ta, or objects of per- 

 ception, from the vot)Ta, or objects of 

 conception. 2. Perception differs from 

 consciousness, in the subjects of the 

 former being external, those of the latter 

 internal : we perceive a mountain, we are 

 conscious of a thought. 3. It differs 

 from remembrance, in respect to the sub- 

 jects being present or past : we remember 

 a former object of perception, but we do 

 not perceive it until it is again present. 

 4. But the term " perception " is vaguely 

 applied by some writers to all operations 

 and states of mind, comprising the pas- 

 sions, and all the phenomena of memory 

 and of imagination. 



PE'RCIDiE iperca, the perch). The 

 Perch tribe ; a family of Acanthopterygi- 

 ous, or spiny-finned fishes. In the 

 thoracic species, the ventral fins are 

 placed under the pectoral ; in the jugular, 

 upon the throat; in the abdominal, on 

 the abdomen. 



PERCU'SSION {percutio, to strike). 

 A forcible stroke given to a resisting ob- 

 ject by a moving body. There is one 

 point of the moving body in which the 

 whole force of the stroke is concentrated, 

 and the resistance to which would neu- 

 tralize the blow. This point is termed 

 the centre of percussion, and it always 

 coincides with that of oscillation ; and if 

 all parts of the percutient body be carried 

 forward with the same celerity (which is 

 not the case of the pendulum), the centre 

 of percussion is the same with the centre 

 of gravity. The force of percussion is mo- 

 mentum. See Momentum. 



PERDICI'N^ {perdix, a partridge). 

 Perdicine birds, or partridges; a family 

 of the I Rasores, or Scratching birds. 

 There is little essential difference be- 

 tween the large, long-tailed birds, called 

 Cocks and Pheasants, and the small, 

 short-tailed species, called Partridges and 

 Quails. Their habits also are essentially 

 similar. 



PERE'NNIBRA'NCHIATE (peren- 

 253 ^ 



nis, perpetual, branchice, gills). A divi- 

 sion of Amphibious animals, which pre- 

 serve their branchiae through the whole 

 period of their lives, as the proteus, the 

 siren, &c. 



PERFECT NUMBER. A number 

 which is equal to the sum of all its divi- 

 sors. Thus 6 is a perfect number, for its 

 divisors are 1, 2, and 3, and the sum of 

 these is 6, See Number. 



PERFOLIATE. The designation of 

 a leaf which, by union of its margins, 

 encloses the stem, which thus seems to 

 pass through it, 



PERFORATED (perforo, to pierce 

 through). Bored, or pierced through, as 

 by an awl; a term applied to the ear- 

 shells, 



PERI-, PER- (Trept). A Greek prepo- 

 sition, signifying around, about, ex- 

 pressing the relation of circumference to 

 centre, and so strictly different from afi<pi. 



1. Peri-anth {av6of, a flower). A collec- 

 tive term for the calyx and corolla com- 

 bined, the limits of which are undefined, 

 so that they cannot be satisfactorily dis- 

 tinguished from each other, as in tulip, 

 orchis, &c. 



2. Peri-carp (Kapiro^, fruit). A botani- 

 cal term denoting all the parts of a ripe 

 fruit which are on the outside of the real 

 integuments of the seed, except the aril. 

 These parts are, severally, the epicarp, 

 the sarcocarp, and the endocarp. 



3. Peri-cladium (xXa^or, a young 

 branch), A term sometimes applied to 

 the lower part of a petiole, when this 

 part sheaths the branch, as in Apiaceae. 



4. Peri-chcetium (xairt), seta, a hair). 

 A terra applied to the peculiar leaves 

 which surround the base of the seta, or 

 stalk of the sporangium, or seed-vessel 

 of mosses. 



5. Peri-clinium [KKivt], a couch). The 

 name given byCassini to the involucrum 

 of composite plants. It is to be lamented 

 that botanists cannot agree in employing 

 the same term for an organ. Besides the 

 above designation of the involucrum, 

 Linnaeus called it calyx communis, 

 Necker, perigynandra communis, and 

 Richard periphorantkium. 



6. Per-enchyma. In the nomenclature 

 of vegetable tissues propounded by 

 Morren, this term is applied to the 

 amylaceous granules contained within 

 the tissue of plants. 



7. Peri-gee (-y^j, the earth). That point 

 in a planet's orbit in which it is nearest 

 to the earth. Its apogee is that point in 

 which it is furthest from the earth. 



