76 TERMINOLOGY. 



The primary form which is selected, must always allow of the 

 derivation of all the forms belonging to the series of crystallization, 

 agreeably to the rules of derivation explained in . 51. 



. 67. DIRECTIONS FOR ASCERTAINING THE PRIMARY 

 FORMS OF CRYSTALS. 



The rules for discovering the primary forms of crystals, 

 depend upon the indications of cleavage, where it is observ- 

 able, and upon analogy, where it is not. 



1 . Crystals possessed of Cleavage. 



(a.) If a crystal has the form of one of the primary solids, either 

 entire or altered by modifications, that form is its appropriate pri- 

 mary, unless the cleavages it possesses are incompatible with such 

 a form ; in which case, the particular solid developed by mechanic- 

 al means, is the true primary. For example, if the crystal be a Cube 

 of Galena, on making a trial to learn its cleavages, we perceive they 

 are three in number, and that they take place parallel to the faces 

 of the crystal with the same facility in each direction; the Cube is 

 therefore inferred to be the primary form of the crystals of Galena. 

 Again, if we seek for the cleavages in a cubic crystal of Fluor, we 

 do not find them, as in the former example, parallel to the faces of 

 the crystal, uui m an oblique direction, to the number of four, and 

 leading to a solid having the form of a regular Octahedron, which is 

 the primary form of the species. In the hexagonal prism of Corun- 

 dum also, we find no cleavages coinciding with the faces of this form, 

 but three which tend to the developement of a Rhomboid, the pri- 

 mary form in this instance, &c.* 



* In consequence of the irregularity of crystals, (. 53,) considerable 

 embarrassment will occasionally be experienced in recognising the 

 planes of a primary solid, in a crystal where they exist. The secondary 

 planes, although symmetrically disposed, are nevertheless very often 

 disproportionably extended, so as greatly to disguise the true character 

 of the crystal. A little experience in reading crystals, aided by a few 

 general rules, will soon overcome the difficulty. 



Several of the forms are not liable to give rise to perplexity, in their 

 determination. Such are the Tetrahedron and the rhombic Dodecahe-> 



