12 TERMINOLOGY. 



. 21. PLANES OR FACES. 



The surfaces which limit a crystal are termed its planes 

 or faces. 



The surface of crystals are not always perfectly plane, being some- 

 times slightly spherical, as in some particular modifications of the 

 Diamond; although in crystallography they are considered as per- 

 fect planes. 



as the Diamond, the Emerald, and the Topaz, as well as the ores of certain 

 metals, and the many earthy minerals occurring in metallic veins, as 

 Quartz, Heavy-spar and Fluor were among the instances of crystallized 

 minerals the most familiar to mankind. Notwithstanding, several of 

 these forms were known to be constant in their shape, besides being- 

 identical with some of the regular solids of geometry, still they contin- 

 ued until the time of Linnaeus, to be regarded as the results of mere ac- 

 cident or chance. This philosopher appears to have been the first to 

 consider them as the products of fixed laws, and to imagine that the study 

 of their relations might be of utility in the recognition of minerals. In 

 1772, Rome de Lisle published the first general treatise upon crystallog- 

 raphy; in which he made known a great number of crytals never be- 

 fore noticed by naturalists. He determined the angles of their planes, 

 and established the important fact, that the angles are invariable 

 among the individuals of the same variety. Afterwards, Bergmann and 

 Hatty made a contemporaneous observation relative to the internal struc- 

 ture of crystals, from which they inferred, that the direction of the 

 cleavage was constant in all crystals of the same substance, whatever 

 might be their form. From this fact Bergmann contented himself with 

 forming the supposition that the various forms assumed by the different 

 individuals of any one substance, might be conceived to flow from a sin- 

 gle parent or derivative form, through the operation of certain decre- 

 ments, upon its edges and angles. But Hatty verified the observation 

 concerning cleavage, in every species in which it was possible to dis- 

 cover the natural joints, and by connecting with it, his ingenious theory 

 relating to the forms and dimensions of the molecules, of which he con- 

 ceived the primary forms were composed, he proceeded to unfold, math- 

 ematically, the laws of decrernent by which the secondary forms might 

 be produced ; and thus elevated crystallography to the rank of a geomet- 

 rical science. 



