MIN'KKAUUJY. 



113 





MINERALOGY. II. 



CRYSTALLOGRAPHY (continued). 



CI.KAVAOK ia that property which some crystals eminently pos- 

 sess of splitting in certain directions. If a piece of calc-spar 

 irk with a hammer, the small fragments will all bear in- 

 ns of having a tendency to become geometrical solids ; 

 :i:nl )>y holding a knife-edge along the line of easy fracture, 

 ami \hon hitting the knife, a perfect six-sided figure may bo 

 obtained. Galena, the ore of lead, will offer another mineral for 

 practice. In this case the crystal 

 is a cube. The crystal does not 

 always exhibit its pianos of cleav- 

 age parallel to its faces, and the 

 solid obtained by cleavage is called 

 the /uncl<imen/a{ form, and may 

 appear very different to the one 

 from which it came, yet it has a 

 close connection with it, as will 

 be shown. The plane surfaces 

 which bound the crystal are its 

 faces; the line where 

 two faces intersect is 

 an edge; and the 

 angle thus formed is 

 a plane angle; the 

 angle of a corner 

 that is, an angle 

 formed by three faces 

 meeting in a point 

 is a solid angle. In 

 producing secondary 

 forms from the pri- 

 mary or fundamental 

 form the edges are 

 replaced that is, a 

 plane cuts away the 

 edge and occupies its 

 place. Fig. 3 shows 

 a cube whose edges 

 have been replaced 

 by the planes rh. The 

 angles are truncated 

 when they are cut off 

 by planes. Fig. 4 

 shows a cube which 

 has been so treated. 

 By these two pro- 

 cesses an innumerable 

 variety of forms may 

 be produced, all, how- 

 ever, directly refer- 

 able to the primary 

 form. There are thir- 

 teen primary forms, 

 which are either prisms, octahe- 

 drons, or dodecahedrons. A prism 

 is a column having from three to 

 any number of sides ; its two ends 

 are its bases. An octahedron is 

 an eight-sided figure ; a dodecahe- 

 dron has twelve sides. Frequent 

 examples of these will be given. 



The law of symmetry is one of 

 the principles of creation. It is 

 observable in every organic con- 

 struction that about a certain plane or planes the body is 

 similarly built up. For instance, a plane which passes through 

 the centre of the human frame would divide the body into two 

 similar halves. So with crystals, they are all arranged sym- 

 metrically about certain imaginary lines. The position of these 

 lines was first indicated by Weiss ; and, by the arrangement of 

 these ares of symmetry, crystals are divided into six classes or 

 systems : 



1. The Monometric, Regular Tessular or Cubic System. 



2. The Dirnetrio, Right-square Prismatic or Pyramidal System. 



3. The Trimetric, or Prismatic System. 



4. The Monoclinic, or the Oblique System. 



138 



5. The Diclinio, or the Doubly Obti<|u Hyttom. 



6. The HexAgon*! or Rhombohodral Syitem. 



Yet with cryntulH, as in the beat-regulated families there arc 

 accidents ; and we sometime* find departure* from the ordinary 

 course. A pieudomorphotu crystal one baring a fata form- 

 that in, a form not belonging to the snbfltanoa of whi< 

 composed, may be produced by the cryitalliiing material occu- 

 pying a cavity, perhaps formed by some other crystal which had 

 been dissolved away. 



Twin crystals and compound crystals exist, as the snow- 

 crystals figured in the last lesson 

 give proof. MacUt seem to be 

 formed as if the crystals were cut 

 in two, one- half then turned upside 

 down and stuck on again. Fig. 5 

 shows this peculiarity, be being the 

 continuation of a b. These excep- 

 tions are uncommon. 



1. The Monometric, Regular 

 Tessular or Cubic System has three 

 axes of symmetry all equal, and all 

 at right angles to 

 each other (Fig. 6). 

 About these lines the 

 crystal is symmetri- 

 cally arranged, so that 

 when heated it ex- 

 panda equally in all 

 directions, and trans- 

 mits light without 

 breaking the rays. 

 The primary figures 

 of the system may 

 be found by causing 

 planes to pass per- 

 pendicularly through 

 the extremities of the 

 axis. This will pro- 

 duce the cube (Fig. 

 7). 



The other promi- 

 nent figure, the octa- 

 hedron, is formed by 

 causing eight planes 

 to pass through the 

 three extremities of 

 the axes. A glance 

 at Fig. 8 will explain 

 this. By combining 

 these two figures in 

 various proportions, 

 a series of crystals 

 may be produced. The 

 gradations through 

 which these pass are 

 represented in Figs. 7, 4, 10, 8. 

 In Fig. 8, if the axis of the octa- 

 hedron be produced and made t 

 coincide with the axes of Fig. 7, 

 then imagine the octahedron to 

 contract and to be able to cut away 

 those parts of the cube which it 

 touched first, the angles which 

 would have obtruded themselves 

 through the faces of the octa- 

 hedron are truncated (Fig. 4). 



This process proceeds until in Fig. 9 the triangular faces meet, 

 forming the cubo-octahedron. Still further, and in Fig. 10, 

 these faces have become hexagons; and finally, in Fig. 8, 

 the whole cube is obliterated, and the octahedron complete. A 

 similar series of changes causes the cube to pass into the r/ioirt- 

 bic dodecahedron, a figure whoso faces are twelve rhombs. This 

 is effected not by the truncation of the angles, but by the re- 

 placement of the edges. The process is commenced in Fig. 3, 

 carried on in Fig. 1 1, and completed in Fig. 12. This crystal is of 

 frequent occurrence in Nature, and is the more remarkable from 

 being the form selected by the working bee for its cell. There 

 is another class of modification founded on the cube, octahedt'on. 



