OPA 



OPA 



^he Table Mountain at the Cape of Good 

 Hope. 



ONONIS, in botany, restharrow, a ge- 

 nus of the Diadelphia Decandria class and 

 order. Natural order of Papilionacese or 

 Leguminosse. Essential character : calyx, 

 five-parted, with linear segments ; banner 

 striated ; legume turgid, sessile ; filaments 

 connate, without a fissure. There are 

 thirty-eight species ; these are herbace- 

 ous plants or under shrubs ; leaves ter- 

 nate, with the leaflets often serrulate ; 

 stipules fastened to the bottom of the pe- 

 tiole ; flowers yellow or purple, one or 

 many flowered. 



ONOPORDUM, in botany, a genus of 

 the Syngenesia Polygamia JEqualis class 

 and order. Natural order of Composite 

 Capitatae. Cinarocephalae, Jussieu. Es- 

 sential character: calyx scales mucronate ; 

 receptacle honey combed. There are se- 

 ven species. 



ONOSMA, in botany, a genus of the 

 Pentandria Monogynia class and order. 

 Natural order of Asperifoliae. Borra- 

 gineae, Jussieu. Essential character: co- 

 rolla, bell-shaped, with the throat per- 

 vious ; seeds four. There are three spe- 

 cies. 



ONYX. See CHALCEDONY. 



OPACITY, in philosophy, a quality of 

 bodies which renders them impervious 

 to the rays of light. It has been suppos- 

 ed that opacity consists in this, that the 

 pores of the body are not all straight. 

 This doctrine, however, is deficient : for 

 though to have a body transparent, its 

 pores must be straight, or rather open 

 every way; yet it is inconceivable how it 

 should happen, that not only glass and 

 diamonds, but even water, whose parts 

 are so very moveable, should have all 

 their pores open and pervious every way; 

 while the finest paper, or the thinnest 

 gold leaf, should exclude the light, for 

 want of such pores. 



So that another cause of opacity must 

 be sought for. Now all bodies have vast- 

 ly more pores or vacuities than are ne- 

 cessary for an infinite number of rays to 

 pass freely through them in right lines, 

 without striking on any of the parts them- 

 selves. For since water is nineteen 

 times lighter or rarer than gold; and yet 

 gold itself is so very rare that magnetic 

 effluvia pass freely through it, without 

 any opposition, and quicksilver is readily 

 received within its pores, and even water 

 itself by compression, it must have more 

 pores than solid parts ; consequently wa- 

 ter must have at least forty times as much 

 vacuity as solidity. The cause, therefore, 



why some bodies are opaque, does not 

 consist in the want of rectilinear pores, 

 pervious every way, but either in the un- 

 equal density of the parts, or in the mag- 

 nitude of the pores, and their being either 

 empty, or filled with a different matter; 

 by means of which the rays of light, in 

 their passage, are arrested by innumera- 

 ble refractions and reflections, till at 

 length falling on some solid part, they 

 become quite extinct, and are utterly 

 absorbed. Hence cork, paper, wood, 

 &c. are opaque ; while glass, diamonds, 

 &c. are pellucid. For in the confines or 

 joining of parts alike in density, such as 

 those of glass, water, diamonds, &c. 

 among themselves, no refraction or re- 

 flection takes place, because of the equal 

 attraction every way ; so that such of the 

 rays of light as enter the first surface, 

 pass straight through the body, except- 

 ing such as are lost and absorbed, by 

 striking on solid parts; but in the border- 

 ing of parts of unequal density, such as 

 those of wood and paper, both with re- 

 gard to themselves, and with regard to 

 the air, or empty space in their larger 

 pores, the attraction being unequal, the 

 reflections and refractions will be very 

 great; and thus the rays will not be 

 able to pass through such bodies, being 

 continually driven about, till they be- 

 come extinct. 



That this interruption or discontinuity 

 of parts is the chief cause of opacity, Sir 

 Isaac Newton argues, appears from hence, 

 that all opaque bodies immediately be - 

 gin to be transparent, when their pores 

 become filled with a substance of nearly 

 equal density with their parts. Thus, 

 paper, dipped in water or oil, some stones 

 steeped in water, linen cloth dipped in oil 

 or vinegar, &c. become more transparent 

 than before. 



OPAL, in mineralogy, a species of the 

 Quartz family, found in many parts of 

 Europe, especially in Hungary. When 

 first dug out of the earth it is soft, but it 

 hardens and diminishes in bulk by expo- 

 sure to the air. The specific gravity va- 

 ries from 1.9 to 2.5. There are four 

 subspecies, viz. the precious, the common, 

 the semi, and the wood opal. Some spe- 

 cimens have the property of emitting va- 

 rious coloured rays, with a particular ef- 

 i ulgency when placed between the eye and 

 the light. The opals that possess this pro- 

 perty are distinguished by lapidaries by the 

 epithet oriental or nobilis. It is esteemed 

 the most beautiful of the gems by Eastern 

 nations ; but in Europejit is not quite so 

 highly valued on account of its liability to 



