Iti TERMINOLOGY. 



CONSIDERATIONS UPON THE CONNEXION AMONG CRYSTALS, 

 AND THE RELATIONS UPON WHICH IT DEPENDS. 



OBSERVATIONS. 



. 29. Certain mineral species affect peculiar crystalline 

 forms. 



Numerous observations have fully established the fact, that cer- 

 tain geometrical forms are constantly found in connexion with cer- 

 tain mineral substances, while others are never seen in such con- 

 nexion. Thus the Cube, as found in Common Salt, the hexagonal 

 Prism in the Emerald, the rhombic Prism in Heavy Spar, and the 

 Rhomboid in Carbonate of Lime : while the hexagonal Prism is never 

 found in Common Salt, the Rhomboid in Emerald^ or the Cube in 

 Carbonate of Lime. 



. 30. Several different forms are frequently found in 

 the different individuals of the same species. 



In this observation, (which limits . 29,) it must be understood, 

 that the slightest deviation between two crystals, such as would 

 arise out of the absence of the smallest perceptible portion of an 

 edge, or of an angle, and its replacement by a regular face, would 

 require these crystals to be considered as different inform. While, 

 however, many of the differences alluded to in the present observa- 

 tion, are of this sort, or such as may be considered modifications of one 

 predominating form, there exist others, which are more obvious. An 



An oblique angled parallelogram^ Fig. 9, Fig. 9. 



has its opposites sides, as a b and dc parallel ; & <* 



but its adjacent sides, as aft and &c, and adja- \ \ 



cent angles, as b and c unequal. \ \ 



The term rhombic is applied as an adjective c $ 



to the planes of such solids as present planes 



of the figure of the rhomb, and that of rectangular to such as refer tQ 

 the rectangle. 



