86 THE MICROSCOPIST. 



chiefly, their structure, whether homogeneous, derived 

 from the debris of previous rocks, or from the agency of the 

 organic world. Ordinary mineralogical characteristics, as 

 to hardness, specific gravity, color, lustre, form, cleavage, 

 and fusibility, and above all, chemical composition, may 

 suffice to show the material, but the microscope will give 

 valuable assistance to this end, and is essential to a knowl- 

 edge of structure. 



Crystalline Forms. The laws of crystallography teach 

 that each chemical combination corresponds to a distinct 

 relation of all the angles which can possibly arise from 

 the primary form, so that the angular inclination of the 

 facets of a crystal is a question of importance. This can 

 be ascertained by a microscope having a revolving stage, 

 properly graduated, or by the use of a goniometer, which 

 is a thread stretched across the focus of the eye-lens, and 

 attached to a movable graduated circle and vernier. The 

 eye-piece attached to the polariscope of Hartnack is thus 

 arranged, so as to act also as a goniometer. 



Crystals are assumed to possess certain axes, and the 

 form is determined by the relation of the plane surface to 

 these axes. . Although the forms of crystals are almost 

 infinitely varied, they may be classified into seven crystal- 

 lographic systems. 



1. The Regular Cubic or Monometric System (Fig. 39) 

 These crystals are symmetrical, about three rectangular 

 axes. The simplest forms are the cube and octahedron. 

 Examples, diamond, most metals, chloride of sodium, fluor 

 spar, alum. 



2. The Quadratic or Dimetric System (Fig. 40). Crystals 

 symmetrical, about three rectangular axes, but only two 

 axes of equal length. Examples, sulphate of nickel, tung- 

 state of lead, and double chloride of potassium and cop- 

 per. 



3. Hexagonal or Rhombohedral System (Fig. 41). Crys- 

 tals with four axes ; three equal in length, in one plane, 



