CRYSTALLOGRAPHY 



3 



Tin- ft >nvs surrounding tin- molecule in a solid may radiate 

 equally in every ilireetion as if from a center, .-uch a group of 

 molecules would form an amorphous solid; or again, the lin 

 force may vary with the direction, when the solid would be crys- 

 talline. The physical properties, as elasticity, hardness, trail- 

 mission of light, conductivity of heat, will be the same for all 

 directions in amorphous solids. Glass, which is an amorphous 

 substance, will expand equally in all directions upon heating. 

 If a sphere of glass is heated and measured at various temperatures 

 it will at each measuring l>e a true sphere ; while all its diameters 

 have increased in length with the rise in temperature, they all 

 have increased by exactly the same amount. Amorphous solids 



38 no regular 

 outward form 

 bounded by plane 

 3, but are ir- 

 regular, globular, 

 or rounded masses, 

 Fig. 2. The physi- 

 cal properties of 

 crystalline solids 

 are the same along 

 parallel directions, 

 but not necessarily 

 so along directions 

 that are not par- 

 allel. The direc- 

 tional variation of the physical properties is a primary character 

 of crystals, and all compounds to be crystalline must possess it; 

 as will be shown later, this is caused by the regular arrangement of 

 the molecules, assumed as the substance passes from the liquid or 

 gaseous state to the solid. 



Crystals are generally bounded by plane faces, but the smallest 

 fragment of a crystal will possess this directional quality of its 

 properties, and by it may be identified as crystalline. The 

 tendency to form crystals, or the crystalline force, varies with the 

 substance, being very strong in quartz or calcite, which are almost 

 never found but in the crystalline condition; while in others, as 

 chrysocolla and turquoise, 1 upon which crystal faces have never been 



FIG. 2. Hyalite from Waltsch, Bohemia : An 

 Amorphous Mineral. 



1 Turquoise has lately been found in crystals. 



