MINERALOGY. 483 



3, The Prismatic, in which the crystallization assumes the form 

 of prisms : And 4, the Hexahedral or tessular form. It has been 

 observed, that crystallization could not have taken place, if the in- 

 tegral atoms of a solid had not been free and moveable upon one 

 another. Many general causes determine the circumstances which 

 favour crystallization, or this geometrical and regular union of 

 solid particles, such as repose, precipitation, loss of caloric, 

 decomposition, and consequently new chemical combinations. It 

 is probable that in each species of bodies the integrant parti- 

 cles have determinate and constant forms, which, by their dis- 

 position upon one another, produce the different figures assum- 

 ed by the crystals. 



2. CLEAVAGE. To have an exact idea of a crystal, it is ne- 

 cessary to ascertain its structure by a kind of dissection, or as 

 it is termed Cleavage. This is the property which minerals 

 possess of splitting in certain determinate directions. The faces 

 or planes thus obtained, termed the faces of the cleavage, are 

 more or less smooth and shining, and represent members of 

 the aggregated crystallization characteristic of the mineral. 



3. HARDNESS. Another important external character in mi- 

 neral bodies is their comparative Hardness. This is generally 

 expressed in numbers, formed from a scale derived from a series 

 of substances of different qualities in this respect. The most 

 precise scale hitherto proposed, is, according to Professor Jame- 

 son, that of Mohs, in which a -series of mineral substances of 

 varying hardness serves the purpose of comparison. Thus 



No. 1. denotes the hardness of common and Venetian talc. 

 No. 2. is the hardness of a variety of prismatoidal gypsum with 



imperfect cleavage and transparency. Varieties perfectly 



transparent and crystallized are too soft. 

 No. 3. Hardness of a cleavable variety of calcareous spar. 

 No. 4. Hardness of fluor spar. 

 No. 5. Hardness of apatite. 

 No. 6. Hardness of prismatic felspar. 

 No. 7 Hardness of rhomboidal quartz. 

 No. 8. Hardness of prismatic topaz. 

 No. 9. Hardness of rhomboidal corundum. 

 No. 10. Hardness of octahedral diamond. 



A series of specimens of the minerals now named and thus 

 numbered, is used as a comparative scale ; and the hardness of 



