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OBEA'TE SPHE'KOID. A spheroid flat- 

 tened at the poles is called an oblate 

 spheroid : such is the form of the 

 earth and planets. When, on the 

 contrary, a spheroid is drawn out 

 at the poles instead of being flat- 

 tened, it is called a prolate spheroid. 



OBLI'QUE. (olliquus, Lat. oblique, Fr. 

 olll'icOj It.) Not direct; not per- 

 pendicular ; not parallel. 



In botany, applied to the position 

 of leaves, and implies that one part 

 of the leaf is horizontal and the other 

 vertical. 



In conchology, applied to the 

 whorls of spiral univalves which 

 commonly are in an oblique direc- 

 tion in reference to the axis of the 

 shell. The term is also applied to 

 bivalves when they slant off from 

 the umbones. 



OBLONG, (ollongus, Lat. oblong, Fr.) 

 Having greater length than breadth; 

 longer than broad. 



In botany, applied to leaves 

 several times longer than broad. 

 The term is chiefly used to dis- 

 criminate a leaf whose form does 

 not accurately come under the de- 

 nominations oval, linear, or round. 



O'BLONG O'VATE. Oblong egg-shaped ; 

 between oblong and egg-shaped. 



OBO'VATE. In botany, applied to leaves 

 having the form of an egg, with the 

 broad end forming the base, and the 

 pointed the apex of the leaf. 



OBSI'DIAN. The Obsidienne of Brong- 

 niart; Lave vi f reuse obsidienne of 

 Haiiy. Vitreous lava, a volcanic 

 production, of a dark green colour 

 approaching to black. An analysis 

 of obsidian from Mount Hecla, by 

 Yauquelin, gives the constituents 

 as follows, silica 78, alumina 10, 

 potash 6, lime 1, soda 1*6, oxides 

 of iron and manganese 1 . 



Obsidian has been divided into 

 two kinds, the vitreous and pearly ; 

 these may be distinguished by their 

 fracture, which is either vitreous 

 or pearly. Professor Jameson di- 

 vides obsidian into two subspecies, 



namely, transparent obsidian and 

 translucent obsidian. 



Vitreous obsidian bears a strong 

 resemblance to the glass of wine- 

 bottles. Its fracture is conchoidal, 

 showing frequently large cavities. 

 Lustre vitreous. Specific gravity 

 from 2-34 to 2-90. It generally 

 occurs in large amorphous masses, 

 when it appears almost black ; it is 

 sometimes found in rounded grains. 

 Pearlstone, the Obsidienne perlee 

 of Brongniart ; this variety has a 

 granular structure, and is traversed 

 by fissures in all directions. It is 

 consequently very brittle. Its frac- 

 ture is uneven or granular, and, as 

 before mentioned, pearly. When 

 moistened by the breath it fre- 

 quently returns an argillaceous 

 odour. It occurs amorphous only. 

 Before the blow-pipe both varie- 

 ties intumesce, but the vitreous 

 alone fuses into a globule. Obsi- 

 dian bears indisputable characters 

 of having once been in a state of 

 fusion. 



OBTTJ'SE. (obtusus, Lat. obtus, Fr.) 

 An angle which is more than ninety 

 degrees, or that of a right angle. 



O'BVOLTJTE. In botany, applied to 

 leaves, when their margins alter- 

 nately embrace the straight margin 

 of the opposite leaf. 



OCCI'PITAL. (from occiput, Lat.) 

 Pertaining to the back part of the 

 head. 



O'CCIPTJT. (occiput, Lat.) The back 

 part of the head : the fore part is 

 called sinciput. 



O'CEAN. (wKeavos, Gr. oceanus, Lat. 

 octan, Fr. ocedno, It.) That vast 

 body of water which covers more 

 than three-fifths of the earth's 

 surface. The average depth of the 

 ocean has been very variously esti- 

 mated. Laplace considered, in 

 order to account for the height of 

 the tides according to the laws of 

 gravitation, the depth to average 

 ten miles ; others rate it at five 

 miles. The present cannot be con- 



