Die 



DIF 



sisting of seven intervals, viz. five tones 

 and two semitones. 



DIBO'THRIANS {6tt, twice, /36epiop, 

 a pit). A division of tape-worms, includ- 

 ing those bothriocephallans which have 

 only two fossae or pits on the head. 



DIBRA'NCHIA'TA (ai?, twice, /Spa'-y- 

 X«a, gills). An order of Cephalopods, in 

 which the branchiae are two in number, 

 one situated on each side of the body. It 

 embraces all the ordinary genera, and is 

 subdivided into two tribes, viz. the deca- 

 pods, which have the eight ordinary arms 

 and the two longer tentacula, and the 

 octopods, in which the tentacula are ab- 

 sent. See Tetrabranchiata. 



DI'CERATES (6if, twice, Kepa^, a 

 horn). A family of Gasteropods which 

 have two tentacula on the head, in De 

 Blainville's arrangement. The diceras 

 was applied by Lamarck to a fossil genus 

 of bivalves. 



DICHO'TOMOUS (6ixa, doubly, refxvu), 

 to divide). A term applied to stems or 

 branches which bifurcate, or are con- 

 tinually divided into pairs. Hence, to 

 dichotomize, is to cut into two equal and 

 similar parts. 



DrCHROISM (dip, twice, xp««Ma, co- 

 lour). In Optics, the property by which 

 a crystallized body assumes two distinct 

 colours according to the direction in 

 which light is transmitted through them. 



DrCHROITE. A mineral consisting of 

 a silicate of alumina and magnesia. 



DICLE'SIUM (3«\ip, folding two 

 ways). In Botany, a collective fruit, 

 consisting of a pericarp indehiscent, one- 

 seeded, enclosed within an indurated 

 perianth, as in mirabilis. 



DFCOTYLE'DONSCdif, twice, kotu\»,- 

 3a)v, a seed-lobe). A grand division of 

 the vegetable kingdom, comprising all 

 those plants in which the embryo con- 

 tains two cotyledons or seed-lobes. The 

 dicotyledonous embryo coincides with the 

 exogenous development of wood, and, 

 hence. Dicotyledons and Exogens are con- 

 vertible terms. 



DICTUM DE OMNI ET NULLO. 

 A principle established by Aristotle as 

 the basis of his logical system, viz. "that 

 whatever is predicated {i. e. affirmed or 

 denied) universally of any class of 

 things, ipay be predicated, in like manner 

 {viz. affirmed or denied), of any thing 

 comprehended in that class." 



DICTYO'PTERA {diKTvov, a net, tttc- 

 p6v, awing). An order of insects, com- 

 prising the cock-roaches, in which the 

 wings are four in number when they 

 107 



exist ; but they are generally of equal 

 size, and never folded. 



DIDA'CTYLE (5i?, twice, daKTvXov, a 

 finger or toe). Two-fingered, or two- 

 toed ; as applied to various animals which 

 have two digits on their extremities. 



DI'DYM {Sldv/jLOf, twin). The name of 

 a metal recently discovered united with 

 oxide of cerium, and so called from its 

 being, as it were, the iwin-broiher of 

 lantanium, which was previously found 

 in the same body. 



DIDYNA'MIA (3if, twice, duvafxn, 

 power). The fourteenth class of plants 

 in the system of Linnaeus, characterized 

 by the presence of four stamens, of which 

 two are long, two short. This condition 

 occurs in the natural order Labiatae. 



DIELE'CTRIC. A body which has 

 the power of transmitting the electric 

 influences through itself, — a property 

 observed in the most perfect non-con- 

 ductors of electricity, whether solid, 

 liquid, or aeriform. See Induction. 



DIERESI'LIS {diaipeaii, separation). 

 A general term applied by Mirbel to 

 those fruits which spontaneously sepa- 

 rate into several cocci, when ripe, as in 

 the mallow. The term is almost syn- 

 onymous with carcerule. 



DI'FFERENCE. DIFFERENTIA. 

 In Logic, the formal or distinguishing part 

 of the essence of a species ; in common 

 discourse, it is the characteristic; when 

 necessarily applicable, it is a property; 

 when contingently, an accident. 



DIFFER'ENTIAL CALCULUS. The 

 term differential, in the higher Mathe- 

 matics, denotes a quantity infinitely 

 small, or less than any assignable magni- 

 tude. The object of the differential cal- 

 culus is to find the ratios of the diflfer- 

 ences of variable magnitudes, on the 

 supposition that these differences be- 

 come infinitely small. It agrees with 

 fluxions in every respect excepting its 

 notation and the manner of its expla- 

 nation. 



DIFFERENTIAL COEFFICIENT. 

 In analysis, the ratio of the differential 

 of any function of a variable quantity to 

 the differential of the variable. 



DIFFRA'CTION OF LIGHT {dif- 

 fringo, to break in pieces). Inflexion of 

 light. The turning of light from a recti- 

 linear course by the interposition of an 

 opaque body. 



DIFFU'SION OF HEAT. A term 

 expressive of the modes by which the 

 equilibrium of heat is effected, viz. by con- 

 duction, by radiation, and by convection. 

 F6 



