CRYSTALLISATION. CRYSTALLOGRAPHY. 85 



chemical characters, these last being determined by two 

 kinds of analysis, tne qualitative and the quantitative, the 

 former being again divisible into examination by the blow- 

 pipe, or the dry mode, as it is sometimes called, and the 

 action of tests and re-agents, or the humid method. Of 

 these we shall confine our attention to the external cha- 

 racters, and the examination by the blowpipe, referring the 

 student for information on those departments of the investi- 

 gation which have reference to chemistry, to. the works 

 of Thomson, Brande, Turner, Eownes, &c. 



CRYSTALLISATION. The particles of liquid and gaseous 

 bodies, during the formation of solids, sometimes cohere to- 

 gether in an indiscriminate manner, and give rise to shape- 

 less masses ; but they occasionally attach themselves to each 

 other in a certain order, so as to constitute solids possessed 

 of a regularly limited form. Such bodies are called crystals, 

 and the process by which they are formed is termed crystal- 

 lisation. Certain crystalline forms are peculiar to certain 

 substances ; and though these forms are modified to a con- 

 siderable extent, yet we can invariably, by a careful dis- 

 section of the crystal, extract from it a nucleus which has 

 constantly the same form in the same mineral species. 

 Thus, whatever be the shape of a crystal of calcareous spar, 

 we can always obtain from it an obtuse rhomboid ; while 

 every cube of fluor spar, by cautiously dissecting off" the 

 angles, yields for a nucleus a regular octohedron. Such a 

 nucleus is called a primary form, while the secondary forms 

 are deducible from the primary by certain laws of- decrement. 

 So constant are these forms, that calcareous spar never crys- 

 tallises in cubes, nor fluor spar in rhomboids. 



CRYSTALLOGRAPHY. A crystal may be defined to be a 

 symmetrical mineral solid contained within plane or curved 

 surfaces. 



These surfaces are called planes, or faces, as a, b, c (fig. 4). 



An edge is formed by the meeting of two planes, as d. 



Crystals may sometimes be split in directions parallel to 

 their natural planes, and frequently in other directions. 



The splitting of a mineral in any direction, so as to 

 obtain a new plane, is termed cleaving it ; and the crystal is 

 said to have a cleavage in the direction in which it splits. 



The planes produced by cleaving a crystal, are termed its 



