EXC 



EXO 



EVERGREEN. A terra applied to 

 plants which have persistent or perennial 

 leaves ; i. e. leaves which remain perfect 

 upon the plant beyond a single season, as 

 holly, common laurel, &c. 



EVERLASTING FLOWERS. A po- 

 pular designation of certain flowers which 

 retain their colour for many months after 

 they have been gathered, owing to the 

 hardness of their tissue, and the small 

 amount of moisture which they con- 

 tain. 



E'VOLUTE {evolutus, unrolled). A 

 curve from which any given curve may 

 be supposed to be formed by the evolu- 

 tion or unlapping of a thread from a sur- 

 face having the same curvature as the 

 first curve. See Curve. 



EVOLUTION {evolvo, to roll out). A 

 term applied to a theory of non-sexual 

 generation, according to which the first 

 created embryos of each species must 

 contain within themselves, as it were in 

 miniature, all the individuals of that spe- 

 cies which shall ever exist ; and must 

 contain them so arranged, that each gene- 

 ration should include not only the next, 

 but, encased within it, all succeeding 

 generations. Hence this theory has also 

 received the name of the emboitement 

 theory. Compare Epigenesis. 



EVOLUTION (in Algebra). The ex- 

 traction of roots, or the method of find- 

 ing a quantity, which, raised to a pro- 

 posed power, will produce a given quan- 

 tity. It is just the reverse of in- 

 volution ; and to perform the operation, 

 we must inquire what quantify multi- 

 plied into itself, till the number of fac- 

 tors amounts to the number of units in 

 the index of the given root, will gene- 

 rate the quantity whose root is to be ex- 

 tracted. 



EXCENTRICITY (^f, out, Kt-vrpov, 

 centre). The ratio which the distance 

 between the centre and focus of an ellipse 

 or hyperbola bears to the whole semi- 

 axis. Half the distance between the 

 foci of an ellipse is called its excen- 

 tricity, and sometimes its ellipticity. It 

 is in this way that we speak of the excen- 

 tricity of the orbits of the planets which 

 are supposed to move in ellipses ; the cir- 

 cle which circumscribes the elliptic orbit 

 of a planet being called the excentric. 

 Hence, the excentric anomaly is the arc 

 of the excentric between the perihelion 

 of the orbit, and the straight line drawn 

 through the centre of the planet perpen- 

 dicular to the major axis. See Anomaly. 



EXCHANGES, THEORY OF. A 

 131 



theory introduced by Prevost for explain- 

 ing the equilibrium of temperature of 

 any body. It is founded on the suppo- 

 sition that the quantity of heat which 

 the body diffuses by radiation, is equal to 

 that which it receives by radiation from 

 surrounding objects, and which it either 

 wholly or in part absorbs. 



EXCrPULUS. That part of the thaUus 

 which forms the rim and base of the 

 shields of lichens. 



EXCITATION OF ELECTRICITY. 

 The disturbance of the electric equili- 

 brium by friction, elevation of tempe- 

 rature, contact, &c. Bodies have been 

 distinguished into conductors and non- 

 conductors, according to the facility with 

 which the electric influence passes, or is 

 conducted, along their surfaces. 



EXCITO-MOTORY. A designation of 

 that function of the nervous system, 

 discovered by Dr. Marshall Hall, by 

 which an impression is transmitted to a 

 centre, and reflected so as to produce con- 

 traction of a muscle, without sensation 

 or volition. 



EXCU'RRENT {excurro, to run out). 

 A term applied to that mode of ramifica- 

 tion in plants, in which the axis remains 

 always in the centre, all the other parts 

 being regularly disposed around it, as in 

 pinus abies. 



EXCURTRFCES {excurro, to rush 

 hastily). Snatchers ; a name given by 

 Macgillivray to an order of Birds, inter- 

 mediate between the flying and walking 

 tribes, and belonging to the Insessores of 

 other writers. They include the shrikes 

 and fly-catchers, the thick-bills and the 

 rollers. 



EXHALATION. A general terra for 

 all the effluvia or steams raised from the 

 surface of the earth in form of vapour. 

 Some distinguish exhalations from va- 

 pours, expressing by the former all stearas 

 emitted from solid bodies, and by the 

 latter, the steams raised from water and 

 other fluids. 



EXFNTINE. A term applied by 

 Fritzsche to a third membrane situated 

 between the extine and the inline, in 

 the pollen of yew, juniper, cypress, &c. 



EXO- (effo). A Greek adverb, signify- 

 ing without, on the outside, &c. 



1. Exo-genous {^eivoixai, to be pro- 

 duced). Outside-growing; increasing in 

 diameter by deposition to the exterior : a 

 term applied to the structure of the axis 

 of dicotyledonous plants, in which the 

 newest-formed fibres are deposited on 

 the exterior of those previously formed, 

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