NUT 



[ 183] 



O C E 



hence, the star a Lyrse will come within 

 5 of the celestial pole, and become the 

 polar star of the northern hemisphere. 

 NUT-GALL. An excrescence which grows 

 on some species of oaks. These excres- 

 cences are produced by the Cynip quercus 

 folii, of Linnaeus, a small insect which 

 deposits its egg in the tender shoots of 



the quercus infectoria, a species of oak 

 abundant in Asia Minor. When the 

 maggot is hatched, it produces a morbid 

 excrescence of the surrounding parts, and 

 it ultimately eats its way out of the nidus 

 thus formed, and makes its escape. The 

 best galls are imported from Smyrna and 

 Aleppo. Brande, Man. of Pharmacy. 



O 



OBCO'RDATE. In botany, an epithet for an 

 inversely heart-shaped leaf, petal, or le- 

 gume. 



OBLA'TE. (oblatus, Lat.) Flattened or 

 depressed at the poles ; generally applied 

 to spherical bodies, flattened at the poles ; 

 of the shape of an orange. 



OBLA'TE SPHE'ROID. A spheroid flattened 

 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 flattened, it is called a prolate 

 spheroid. 



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

 oblico, It.) Not direct ; not perpendi- 

 cular ; not parallel. 



In botany, applied to the position of 

 leaves, and implies that one part of the 

 leaf is horizontal and the other ver- 

 tical. 



In conchology, applied to the whorls of 

 spiral univalves, which commonly 'are in 

 an oblique direction in reference to the 

 axis of the shell. The term is also ap- 

 plied to bivalves when they slant off from 

 the umbones. 



OBLI'Q,UITY. (from obliquitas, Lat. obli- 

 quite, Fr. obliquita, It.) Deviation from 

 parallelism or perpendicularity. 



O'BLONG. (oblongus, 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 discriminate a leaf whose 

 form does not accurately come under 

 the denominations 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. Vitreous lava, a volcanic pro- 

 duction, of a dark green colour approach- 

 ing to black. An analysis of obsidian 

 from Mount Hecla, by Vauquelin, gives 

 the constituents as follows, silica 78'0, 

 alumina lO'O, potash 6'0, lime TO, soda 



1*6, oxides of iron and manganese 

 1-0. 



Obsidian has been divided into two 

 kinds, the vitreous and pearly ; these 

 may be distinguished by their fracture, 

 which is either vitreous or pearly. 



Vitreous obsidian bears a strong re- 

 semblance to the glass of wine-bottles. 

 Its fracture is conchoidal, showing fre- 

 quently 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 fracture is uneven or granu- 

 lar, 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 varieties in- 

 tumesce, but the vitreous alone fuses into 

 a globule. Obsidian bears indisputable 

 characters of having once been in a state 

 of fusion. 



OBTU'SE. (obtusus, Lat. obtus, Fr.) An 

 angle which is more than ninety degrees, 

 or that of a right angle. 



O'B VOLUTE. In botany, applied to leaves, 

 when their margins alternately embrace 

 the straight margin of the opposite leaf. 



OCCIDENTAL, (occidentalis, Lat. occiden- 

 tal, Fr. occidentale, It.) Western, as 

 opposed to oriental. 



OCCIPITAL, (from occiput, the hind part 

 of the head, Lat. occipital, Fr.) Per- 

 taining to the back part of the head. 



O'CCIPUT. (occiput, Lat. from ob and 

 caput, occiput, Fr.) The back part of 

 the head : the fore part is called sinciput. 



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

 ocean, Fr. oceano, 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 ac- 

 cording to the laws of gravitation, the 



