P A 



[527 ] 



P H 



stitute the Opal resinite Girasol of 

 the French mineralogists. 4. Semi- 

 opal ; a feebly translucent variety, 

 having a conchoidal fracture ; 

 colours white, grey, and brown. 

 Prof. Ehrenberg states that nodules 

 of semi-opal, which occur in the 

 Poliers-chiefer, are composed of 

 silex derived from infusorial re- 

 mains that have been dissolved and 

 formed into silicious concretions, 

 having dispersed through them 

 numbers of infusorial shields, par- 

 tially dissolved, together with 

 others that are unaltered. Ehren- 

 berg also thinks that he has found 

 indications of microscopic organic 

 bodies of a spherical form in semi- 

 opal from Champigny, and also in 

 semi-opal from the dolerite of Stein- 

 heim, and in precious opal from 

 porphyry of Kaschan. 5. Menilite; 

 a variety occurring in small, irreg- 

 ular, roundish masses, often tuber- 

 ose, or marked with little edges on 

 the surface. The exterior is often 

 bluish or striped, but the interior 

 has a brown or dark grey appear- 

 ance. Fracture conchoidal. It is 

 translucent. 6. Mother of pearl 

 opal or Cacholong. 7. Jasper opal. 

 8. Wood opal. For details of these 

 varieties, see the several words. 

 These varieties consist of silex in 

 various proportion, from 86 to 95 

 per cent., combined with oxide of 

 iron and water. Specific gravity 

 2-09. Not sufficiently hard to 

 give fire with steel. The semi-opal 

 contains about 3 per cent, of 

 alumina. 



OPALE'SCENT. Resembling opal ; ex- 

 hibiting a play of various colours ; 

 displaying iridescence. 



O'PALIZED. Converted into a sub- 

 stance resembling opal. 



O'PALIZED WOOD. This has the form 

 and texture of wood ; the vegetable 

 matter having gradually given 

 place to a silicious deposite posses- 

 sing the characters of semi- opal. 

 Its texture is fibrous j fracture con- 



choidal, with a moderate lustre, 



It does not strike fire with steel. 



Specific gravity between 2-0 and 



2*6. Colours white and grey, often 



shaded with yellow or red, and 



passing into yellow or brown. 



Translucent at the edges. 



OPAQUE. ") Not transparent ; not per- 



OPAKE. j mitting the passage of 



rays of light. 



OPE'BCULAR. Having a lid, or cover, 

 or operculum. 



1. A lid, by means of which many 

 of the molluscous animals close the 

 aperture of their shells. It is in 

 some animals testaceous ; in others, 

 horny or cartilaginous. It is af- 

 fixed to the animal. The opercu- 

 lum of multivalves is composed of 

 two or four pieces. The operculum 

 is calculated for the protection of 

 the animal when it retires within 

 its dwelling, of which it may be 

 termed the door ; it is adapted to 

 the shape of the aperture, which it 

 closes completely. The cartilagi- 

 nous operculum of the common 

 periwinkle is a familar example. 



2. The flap which covers the gill, 

 or organ of respiration in fishes. 



OPHI'DIA. (from o'0ts, a serpent, Gr.) 

 The third order in the class Repti- 

 lia, in Cuvier's arrangement, com- 

 prising three families, Anguina, 

 Serpentia, and Nuda. In the 

 structure of the skeleton of the ser- 

 pents, the first of the true reptiles, 

 we may observe a beautiful illus- 

 tration of the simple means employ- 

 ed in organic structures to accom- 

 plish the most numerous and diver- 

 sified ends, and of the resources of 

 nature in adapting the forms of 

 bones, in all their essential and 

 common parts, to the various uses 

 the animal is to make of them in 

 the living state. "We have here 

 animals destitute of anterior and 

 posterior extremities, destitute of 

 arms and legs, of handi and feet , 



