22 



CHEMICAL PHYSICS. 



same substance under the same conditions always assumes the same 

 shape. Substances capable of assuming in solidifying two or more 

 different shapes or conditions, are said to be dimorphous and poly- 

 morphous, respectively. When substances of different kinds crystallize 

 in exactly the same form we call them isomorphous (magnesium sul- 

 phate and zinc sulphate). Also, a crystal of one kind of matter must 

 have the power of growing in the solution of another kind before 

 the two kinds of matter are considered isomorphous. If two iso- 

 morphous substances be contained in one solution, they will crystallize 

 together, and the crystals be made up of particles of both substances, 



Crystal Systems. The study of crystals forms an extensive field, known 

 as crystallography. The limited scope of this book forbids any detailed study 

 of crystals, and the reader must be referred to the large works on chemistry or 

 works on crystallography for such information. But a brief description of the 

 classification of crystals may not be out of place here. 



All crystals are referred to axes or imaginary lines drawn through the cen- 

 ter. The great variety of forms of crystals depends upon the number and 

 length of these axes and their relative inclination that is, the angles at which 

 they intersect. All crystal forms have been divided into two large groups, the 

 orthometric and the clinometric, and these have been further subdivided into six 

 systems. Orthometric refers to the fact that the axes intersect at right angles, 

 while clinometric means that the axes intersect at oblique angles. 



FIG. 1. 



FIG. 2. 



The cube. Regular octahedron. 



The orthometric group includes the following systems : 



(1) Kegular system, also known as the monometric, cubic, octahedral, or 

 tessular system. 



The crystals have three axes of equal length and intersecting at right angles. 

 The fundamental forms of this system are the cube and the octahedron (Figs. 

 1 and 2). Some substances crystallizing in this system are alum, phosphorus, 

 arsenic trioxide, diamonds, alkali iodides, chlorides, fluorides, and cyanides, 

 and many metals and their sulphides. 



(2) Quadratic system, also known as the dimetric, square prismatic, or tet- 

 ragonal system. 



