OPIUM 459 



OOX.ITIC LIMESTONE. Limestones of the Oolitic series of rocks, such as 

 Bath stone, Portland stone, and Caen stone. See LIMESTONE. 



OOST, or OAST. The provincial name of the stove in -which picked hops are dried. 



OP AXi. An ornamental stone. The following are the more important varieties of 

 the opal : -The precious opal, exhibiting a play of rich colours. Fire opal or girasol, 

 with hyacinth-red and yellow reflections. Common opal, semi-opal ; non-opalescent 

 varieties. Hydrophane; non-transparent, but becoming so by immersion in water. 

 Cacholong ; nearly opaque, of a bluish-white colour. Hyalite ; colourless, pellucid, or 

 white. Opal jasper, wood opal ; and several others. All these are composed of 

 silica in the gelatinising or colloidal state, with more or less water, and occasionally, 

 as accidental admixtures, other bodies in small proportions. By analyses the following 

 results have been obtained as regards the silica : The precious opal of Hungary 

 contains 92 per cent, of silica ; the fire opal of Mexico, 92 ; the fire opal of Faroe, 

 8873 ; semi-opal of Hanau, 8275; semi-opal of Kaschau, 92-16 ; and the cacholong 

 of Faroe, 95-82. 



Opal may be regarded as an uncleavable quartz. Its fracture, conchoidal ; lustre, 

 vitreous or resinous ; colours, white, yellow, red, brown, green, grey ; lively play of 

 light. Hardness, 5'5 to 6'5; specific gravity, 2-091. It occurs in small kidney-shaped 

 and stalactitic shapes, and large tuberose concretions. The phenomena of the play 

 of colours in precious opal have not been satisfactorily explained. Haiiy attributes 

 the play of colours to the fissures of the interior being filled with films of air, agreeably 

 with the law of Newton's coloured rings. Mohs, however, thinks this would produce 

 iridescence merely. Brewster concludes that it is owing to fissures and cracks in the 

 interior of the mass of a uniform shape. 



The precious opal stands high in estimation, and is considered one of the most 

 valuable gems, the size and beauty of the stone and the variety of the colours 

 determining its value. The so-called ' mountain of light,' an Hungarian opal in the 

 Great Exhibition of 1851, weighed 526 J carats, and was estimated at 4,QQ(jl. sterling. 



In Vienna is a precious opal weighing 17 oz. ; and it is said a jeweller of Amsterdam 

 offered half a million of florins for it, which was refused. 



Hydrophane, or Oculis mundi, is a variety of opal without transparency, but acquiring 

 it when immersed in water, or in any transparent fluid. 



Hungary has long been the chief locality of precious opal, where it occurs near 

 Kaschau, along with common and semi-opal, in a' kind of porphyry. Fine varieties 

 have, however, been discovered in the Faroe Islands ; and most beautiful ones, some- 

 times quite transparent, near Gracios-a-Dios, in the province of Honduras, America. 

 Precious opal has also been recently found in Queensland, and to a less extent in New 

 South Wales. The red and yellow bright coloured varieties of fire opal are found 

 near Zimapan, in Mexico. In modern times, fine opals of moderate bulk have been 

 frequently sold at the price of diamonds of equal size ; the Turks being particularly 

 fond of them. The estimation in which opal was held by the ancients is hardly 

 credible. Nonius, the Roman senator, preferred banishment to parting with his 

 favourite opal, which was coveted by Mark Antony. Opal which appears quite red 

 when held against the light, is called girasol by the French ; a name also given to 

 the sapphire, or corundum asteria, or star-stone. 



OPEN-CAST. A miner's term, signifying that the mineral is obtained by open 

 workings, and not by mining. 



OPERAXVXETER is the name given to an apparatus invented by Samuel Walker, 

 of Leeds. It consists of a train of toothed wheels and pinions enclosed in a box, 

 having indexes attached to the central arbor, like the hands of a clock, and a dial- 

 plate ; whereby the number of rotations of a shaft projecting from the posterior part 

 of the box is shown. If this shaft be connected by any convenient means to the 

 working parts of a gig-mill, shearing-frame, or any other machinery of that kind for 

 dressing cloths, the number of rotations made by the operating-machine will be ex- 

 hibited by the indexes upon the dial-plate of this apparatus. 



A similar clock-work mechanism, called a counter, has been for a great many years 

 employed in the cotton-factories, and in the pumping-engines of the Cornish and 

 other mines, to indicate the number of revolutions of the main-shaft of the mill, or of 

 the strokes of the piston. A common pendulum, or spring-clock, is commonly set up 

 alongside of the counter; and sometimes the indexes of both are regulated to go 

 together. 



OPIUM is the juice which exudes from incisions made in the haids of ripe poppies 

 (Papavcr somniferum}. rendered concrete by exposure to the air. The best opium 

 which is found in the European markets comes from Asia Minor and Egypt; what is 

 imported from India is reckoned inferior in quality. This is the most valuable of all 

 the vegetable products of the gum-resin family, and very remarkable for the com- 

 plexity of its chemical composition. 



