O PP 



ORG 



immersed in water, and is called hydro- 

 phane, oculus mundi, or changeable 

 opal. 



2. Sun or Fire Opal, of a hyacinth-red 

 colour, clianjjing to a paler hue by heat ; 

 found only at Zimapan in Mexico, in a 

 particular variety of hornstone porphyry. 



3. Common Opal, a translucent white 

 variety of which, appearing yellow or red 

 when held between the eye and the light, 

 is called girasol. 



4. Semi-opal, agreeing in its principal 

 characters with common opal, and pre- 

 senting spotted, striped, or brown de- 

 lineations. 



5. Mother-of-pearl Opal, or Cacholong, 

 described as a variety of quartz, found in 

 the river Cach in Bucharia. 



6. Jasper Opal, or ferruginous opal, of 

 a scarlet-red and grey colour, referred by 

 some authors to jasper. 



7. Wood Opal, or opalized wood, con- 

 sisting of wood petrified by silicious 

 earth, and acquiring a structure similar 

 to simple mineral opal. 



8. Liver Opal, or menilite, found at 

 Menil-Montant, near Paris, in a bed of 

 adhesive slate. 



OPE'RCULUM {operio, to shut up). 

 A cover or lid. In Malacology, the term 

 operculum denotes the hard lid, either 

 corneous or calcareous, which closes the 

 mouth of many spiral shells, and is car- 

 ried on the posterior part of the belly of 

 the animal. 



OPHl'DIA {o(pii, a serpent). An order 

 of Reptiles, comprising the Serpents, 

 which have no atlantal or sacral extre- 

 mities perceptible externally, and conse- 

 quently no members or appendages of 

 locomotion. 



OPPOSITE TERMS. In logic, those 

 terms are said to be opposite, or incon- 

 sistent, which express qualities inap- 

 plicable to the same object at the same 

 time, as " black and white." Relative 

 terms are opposite, only when applied 

 with reference to the same subject : as 

 one may be both Master and Servant ; 

 but not at the same time to the same 

 person. 



OPPOSITION. Two bodies are in 

 conjunction, when they are at the same 

 point of a circle ; and in opposition, when 

 they are at opposite points. For the ap- 

 plication of these terms in Astronomy, 

 see Conjunction and Opposition. 



OPPOSITION, LOGICAL. Two pro- 

 positions are said to be opposed to each 

 other, when, having the same subject 

 and predicate, they differ, in quantity, or 

 242 



quality, or both. The four kinds of op- 

 position are termed contraries, sub- 

 contraries, subalterns, and contradic- 

 tories. 



O'PTICS (oTTTo/ia*, to see). That 

 branch of natural science which treats of 

 the properties of light and vision ; in its 

 proper acceptation, it relates to direct 

 vision, while catoptrics treats of reflected, 

 and dioptrics of refracted vision. Prac- 

 tical Optics is the application of the phy- 

 sical properties of light, and the mathe- 

 matical laws of optics to the construction 

 of optical instruments. 



O'RBIT {orbita, a track). The course 

 which a star describes around the sun or 

 another star. The Latin word orbis is a 

 circle, or a globe ; and hence the paths 

 of the planets round the sun are termed 

 orbits, and the planets themselves orbs, 

 although the former are now understood 

 to be elliptical, and the latter spheroidal. 

 The earth's orbit is the ecliptic. 



1. Orbit, Plane of. The plane of an 

 orbit is an imaginary surface, passing 

 from one extremity, or side of the orbit 

 to the other. If the rim of a drum-head 

 be considered the orbit, its plane would 

 be the parchment extended across it, on 

 which the drum is beaten. 



2. Orbits, inclination of. If we sup- 

 pose planes to pass through the orbits of 

 the planets, and to be indefinitely ex- 

 tended, all these planes will pass through 

 the sun's centre; but all of them will 

 cut the plane of the ecliptic, though at 

 different angles, which are respectively 

 called the inclination of the orbits. The 

 average inclination of the moon's orbit 

 is about five degrees. 



ORBITE'L^ {orbis, an orb, tela, a 

 web). A family of Spiders, which spread 

 abroad webs of a regular and open tex- 

 ture, either circular or spiral, and remain 

 in the middle, or on one side, in readiness 

 to spring upon an entangled insect. 



ORCEIN. A red colouring matter 

 found in archil, and referred by Dr. Kane 

 to a mixture of two substances, which 

 he terms alpha-orcein and beta-orcein, 

 the latter being produced by oxidation of 

 the former. 



ORCHlDA'CEiE. The Orchis tribe 

 of Monocotyledonous plants, in which 

 the stamens and the style are consoli- 

 dated into a central column, and the 

 ovary is inferior. They correspond with 

 the Gynandria Monandria of Linnaeus. 



ORCIN. A colourless matter procured 



from the lichen dealbatus, assuming a 



[ deep violet colour when exposed to am- 



