CRYSTALLOGRAPHY. 



2. Descriptions of Crystals. 



In describing crystals there are two subjects for considera- 

 tion : First, Form ; and secondly, Structure. 



A. Form. — Under form come up for description, not only 

 the general forms of crystals, but also — 



(1.) The systems of crystallization, that is, the relations ol 

 all crystalline forms, and their classification. 



(2.) The mutual relations of the planes of a crystal as ascer- 

 tained through their positions and the angles between them. 



(3.) The distortions of crystals. The perfection of symmetry 

 exhibited in the figures of crystals, in which all similar planes 

 are represented as having the same size and form, is seldom 

 found in nature, and the true form is often greatly disguised by 

 this means. The facts on this point, and the methods of avoid- 

 ing wrong conclusions need to be understood, and these are 

 given beyond. With all such imperfections the angles of crys- 

 tals remain essentially constant. There are irregularities also 

 from other sources. 



(4.) Twin or compound crystals. With some species twins 

 are more common than regular crystals. 



(5.) Crystalline aggregates, or combinations of imperfect 

 crystals, or of crystalline grains. 



Explanations of Terms. 



The following are explanations of a few terms used in connection 

 with this subject : 



1. Octahedron. — A solid bounded by eight equal triangles. They are 

 equal equilateral triangles in the regular octahedron (Fig. 2, p. IT) ; 

 equal isosceles triangles in the square octahedron i Fig. IT, p. 32) ; equal 

 inequilateral triangles in the rhombic octahedron (Fig. 8, p. 3T). 



2. Double six-sided pyramids. Double eight-sided pyramids. Double 

 twelve-sided pyramids. — Solids made of two equal equilateral six-sided, 

 or eight-sided, or twelve-sided, pyramids placed base to base (Fig. 20, 

 p. 32, and 6, 10, pp. 4(5, 4T). 



3. Right prisms. Oblique prisms. — Right prisms are those that are 

 erect, all their sides being at right angles to the base. When inclined, 

 they are called oblique prisms. 



4. Tnterfaciat angle. — Angle of inclination between two faces or planes. 



5. Similar planes. Similar angles. — The lateral faces of a square 

 prism (Fig. 2, p. 14) are equal and have like relations to the axes, and 

 hence they are said to be similar. Solid angles are similar when the 

 plane angles are equal each for each, and the enclosing planes are sev- 

 erally similar in their relations to the axes. 



6. Truncated. Bevelled. — An edge of a crystal is said to be truncated 

 when it is replaced by a plane equally inclined to the enclosing planes, 

 as in Fig. 13, p. 19 ; and it is bevelled when replaced by two planes 



