CRY 



256 



CUB 



globe which is accessible to our examina- 

 tion. The greatest depth to which man 

 has hitherto been iible to extend his ob- 

 servations is from eight to ten miles, 

 which lias about the same relation to the 

 magnitude of the earth that the thickness 

 of the paper of a globe a foot in diameter 

 has to the magnitude of that globe. 



CRCS'TA. In gem sculpture, a gem en- 

 graved for inlaying on a vase or other 

 object. 



CRUSTA'CEA, ) A class of articulated 



CarsTA'cEAMS. J animals, with articu- 

 lated feet, respiring by means of branchiae, 

 protected in some by the borders of ?. 

 shell (crusta), and external In others, hut 

 which are not inclosed in special cavities 

 of the body, and which receive air from 

 openings in the surface of the skin. Their 

 circulation is double, and analogous to 

 that of the Mollusca. They are apterous, 

 furnished with compound eyes, and usu- 

 ally with four antennae. They have mostly 

 three pairs of jaws : the two superior ones 

 are called mandibles ; as many foot-jaws, 

 the last four of which, in a great many 

 instances, become true feet; and ten feet 

 properly so called. Their envelope is 

 usually solid, and more or less calcareous. 

 The class is divided into two sections, the 

 JUalacostraca and the Entomostraca. The 

 crabs, crawfish, pernys, &c., belong to 

 the first of these divisions ; the Molucca 

 crab, fish-louse, trilobites, &c., belong to 

 the second. 



CBUSTA'CEAN, \ Covered with a shell; 



CRCSTA'CEOCS. J shelly : C. animals are 

 jointed, while the testaceous are covered 

 by an uninterrupted shell. 



CRFX. In astronomy, the cross: an as- 

 terism containing five stars, four of which 

 are in the form of a cross, the most north- 

 erly and southerly are in a line with the 

 south pole, and are therefore called the 

 pointers. 



CHYOPH'ORUS, from x$'->ot, cold, and 

 qtca, to bear ; the frost-bearer : an instru- 

 ment invented by Dr. AVollaston, to de- 

 monstrate the relation between evapora- 

 tion at low temperatures and the produc- 

 tion of cold. It consists essentially of 

 two bulbs of glass, connected by a tube, 

 as shown in the figure. The water is 

 A 



boiled in one of the bulbs till the in- 

 cluded air is expelled by the steam, 

 through a small aperture in the tube at 

 A, which is then hermetically sealed: 

 when cool, the pressure of the included 

 team is -educed to that due to the te'- 



perature of the surronndinz atmosphere : 

 if the empty ball be now surrounded with 

 a freezing mixture, rapid evaporation 

 takes place from the water in the distant 

 ball, which is soon converted into ice, by 

 the rapid abstraction of its heat of tem- 

 perature. 



CRTPT, from xfwrea, to hide: a subter- 

 ranean place, especially under a church, 

 for the interment of the dead. Crypts 

 were originally places whore the j-rimi- 

 tive Christians performed their devotion, 

 hence the term has been applied to under- 

 ground chapels, as the crypt of St. Paul's. 



CRYPTOGA'MIA, the 24th and last class 

 of the Linna?an system of plants, compre- 

 hending those in which the parts essen- 

 tial to fructification have not been suffi- 

 ciently ascertained to admit of their being 

 referred to other classes : hence the name 

 from x^ros, concealed, and <yct/M>;, 

 marriage It contains four orders, Filiccs 

 (ferns), ytusci (mosses), Algee (sea- weeds), 

 and Fungi. 



CRYPTO PORTICUS. In ancient architec- 

 ture, a concealed portico. 

 CKTS'TAL, Lat. crystallus, Gr. gu<rToXXo<, 

 from xguts, frost, and trnKKu, to set. I. 

 In chemistry and mineralogy, an inorganic 

 body which, by the operation of affinity, 

 has assumed the form of a regular solid, 

 terminated by a certain number of plane 

 and smooth surfaces. Every body sus- 

 ceptible of crystallisation, crystallises in 

 tan* primitive form, as the tetrahedron, 

 cube, dodecahedron, octahedron, six-sided 

 prism, and parallelepipeds. All secondary 

 forms arise from the decrement of parti- 

 cles from the edges and angles of these 

 primitive forms. The transparent crystals 

 of quartz are termed rock-crystal, and 

 those of carbonate of lime, found chiefly 

 and of the finest quality in Iceland, are 

 called Iceland crystal, but more commonly 

 I. spar. 2. A species of glass more per- 

 fect in its composition and manufacture 

 than the common glass. 



CBYs'TALLijiE HLMOUR, ) A solid body, 



LEK s. | of a lenticular 



form, being part of the eye, situated be- 

 hind the aqueous humour, opposite the 

 pupil, and its posterior portion is received 

 into a depression on the fore part of the 

 vitreous humour. It is formed of two 

 segments of spheres of unequal sizes ; the 

 anterior surface is the less convex. 



CRIST ALLIZA'TION, a process in which 

 crystallisable bodies tend to assume a re- 

 gular form, when placed in circumstances 

 favourable to that particular disposition 

 of their molecules. 



CCBE, Lat. cubus, Gr. x'j^ot, A regular 

 solid body, consisting of six square and 

 equal faces, with right, and therefore 

 equal, angles. A die is a small cube. 

 The third power of a quantity is also 



