DOK 



DOU 



bears to Addition. In Algebra, the sign 

 -^ placed between two quantities means 

 tliat the former of those quantities is to 

 be divided by the latter : thus a-r-b means 

 that a is to be divided by b. This divi- 

 sion is more simply expressed by making 

 the former quantity the numerator, and 

 the latter the denominator, of a fraction, 

 as p which expresses the quotient of a 

 and b. 



DIVISION, FALLACY OF. A fallacy 

 in Logic, occasioned by using the middle 

 term in the major premiss collectively, 

 and in the minor distributively, the term 

 which is first taken collectively being 

 afterwards divided. 



DIVISION, LOGICAL. A metapho- 

 rical expression to signify "the distinct 

 (e. e. separate) enumeration of several 

 things signified by one common name." 

 It is analogous to the real division of a 

 whole into its parts, and is directly oppo- 

 site to generalization, which is performed 

 by means of abstraction. 



DOBEREINER'S LAMP. A method 

 of producing an instantaneous light, by 

 throwing a jet of hydrogen gas upon 

 recently-prepared spongy platinum; the 

 metal instantly becomes red hot, and 

 then sets fire to the gas. The discovery 

 •was made by Dobereiner of Jena, in 1824. 



DODE'CAGON (Swde/ca, twelve, -ywna, 

 an angle). A regular polygon of twelve 

 equal sides. The area of such a figure 

 is, in general, equal to the square of its 

 side multiplied by the constant number 

 11196. 



DODECAGY'NIA {b^heKa, twelve, 

 twri, a woman). The designation of 

 those orders of plants in the Linnaean 

 system, which are characterized by the 

 presence of twelve styles. 



DODECAHE'DRON (3a.3e(ca, twelve, 

 fibpa, a seat). A geometrical solid, con- 

 tained by twelve equal pentagons, which 

 are equilateral and equiangular. 



DODECA'NDRIA {bwbeKa, twelve, 

 uvt](>, a man). The eleventh class of 

 plants in the system of Linnaeus, charac- 

 terized by the presence of twelve stamens. 



DOG-DAYS. The period of the year 

 comprising forty days, twenty before and 

 twenty after the heliacal rising of the 

 dog-star. This period, being the hottest 

 of the year, was considered by the Greeks 

 as the season of fevers, plagues, and 

 death. See Canicular Period. 



DOKIMA'STIC ART {boKifxa^ia, to 



prove by trial). The art of assaying 



minerals and ores, in order to determine 



the quantity of metal which they contain. 



112 



DOLA'BRIFORM {dolabra, an ax^, 

 forma, likeness). Axe-shaped; a term 

 applied to the succulent leaves of a 

 species of Mesembryanthemum, which 

 are thick at one border and thin at the 

 other. 



DO'LERITE. One of the varieties of 

 the Trap rocks, composed of augite and 

 felspar. 



DO'LOMITE. A crystalline lime- 

 stone, containing magnesia, and named 

 after the French geologist Dolomieu. 

 The compact or brown kind is commonly 

 called Magnesian Limestone. 



DOMI'NICAL LETTER. One of the 

 first seven letters of the alphabet, em- 

 ployed in the calendar to denote the 

 Sundays, or days of the Lord (i.e. Domini), 

 throughout the year. If the year con- 

 sisted of exactly 52 weeks, the dominical 

 letter would always be the same ; but, as 

 there is an excess of 1^ day, the Sunday 

 letter falls back one letter each year, ex- 

 cept in leap-year, when it falls back two 

 letters. As every fourth year is bissextile, 

 and as the number of letters employed is 

 seven, the same order of dominical letters 

 will return only in four times seven, or 

 twenty-eight years, whereas, without that 

 intervention, it would return in seven. 



DO'MITE. A white mineral found in 

 the Puy de Dome in Auvergne. 



DO'RSAL {dorsum, the back). Any 

 thing placed on the back. In Malaco- 

 logy, the term is applicable to the valves 

 of bivalves, in which it serves to distin- 

 guish that part of their circumference on 

 which the bosses are placed, from that 

 which is nearest the belly, when the ani- 

 mal is crawling. The upper part of a 

 spiral shell, when the mouth is down- 

 ward, may be termed its back, or dorsal 

 surface. The term dorsad signifies to- 

 wards the back. 



DORSIBRA'NCHIA'TA {dorsum, the 

 back, branchia, gills). An order of Anel- 

 lida, or red-blooded worms, in which the 

 branchial tufts are attached to the back. 

 These were termed by Milne Edwards 

 errantes, from their powers of locomo- 

 tion. In popular language they are 

 termed sea-centipedes, sea-mice, or ne- 

 reids. 



DOUBLE STARS. A term applied 

 to some fixed stars which, though appa- 

 rently single to the naked eye, are found, 

 by means of the telescope, to consist of 

 two or more stars exceedingly near to 

 each other. 



DOUBLER. An instrument employed 

 in electrical experiments, and so con- 



