8 



MINERALS AND GEOLOGY 



Crystals originate in almost all cases in which matter passes from a 

 gaseous or liquid into a solid state ; but if the process take place too 

 quickly, or the matter solidify without free space for expansion, 

 crystalline masses, in place of regular crystals, will result. If a small 

 fragment of arsenical pyrites, or native arsenic, be heated at one end 

 of an open and narrow glass tube, the arsenic, in volatilizing^ will 

 combine with oxygen from the atmosphere, and form arsenious acid, 

 which will be deposited at the other end of the tube, in the form of 

 minute octahedrons (Fig. 3, below). In like manner, if a few par- 

 ticles of common salt be dissolved in a small quantity of water, and 

 a drop of the solution be evaporated gently (or be left to evaporate 

 spontaneously) on a piece of glass, numerous little cubes and hopper- 

 shaped cubical aggregations will result. Boiling water, again, satu- 

 rated with common alum, will deposit octahedral crystals on cooling: 

 the cooled water not being able to retain in solution the full amount 

 of alum dissolved by the hot water. Finally, it may be observed 

 that bismuth, antimony, and many other bodies, crystallize by slow 

 cooling from the molten state. Although, as explained above, crystals 

 usually originate when matter passes slowly from the gaseous or 

 liquid condition into the solid state, crystallization and solidification 

 are not actually identical. Various substances, such as silica in cer- 

 tain conditions, its hydrate (constituting the different opals), gums, 

 many resins, &c., appear to resist altogether the action of crystal- 

 lization. 



The crystal forms and combinations met with in Nature, exclusive 

 of those produced by the chemist in his laboratory, are exceedingly 

 numerous, many thousands being known to exist. By the help of 

 certain laws, however, and, more especially, by the aid of one, termed 

 "the Law of Symmetry," we are enabled to resolve these multitudinous 

 combinations into six groups or systems. The forms of the same group 

 combine together, and may be deduced mathematically from each other; 

 whilst those of distinct groups are unrelated. Thus, although the cube, 

 the rhombic dodecahedron, and the regular octahedron (Figs. 1, 2 & 3) 

 appear at first sight to be unconnected forms, their co-relations may 



re-announcement, or rather re-discovery of the law in 1772, by the French crystallographer, 

 Rome de 1'Isle. Many of the contemporaries of the latter amongst others the celebrated 

 Buffon attempted to deny its existence ; but being susceptible of practical proof, its truth 

 was soon established. 



