42 



STRUCTURE OF MINERALS. 



facets are not only uniform in number on similar parts of a 

 crystal, but are even fixed in every angle and every edge.* 



COMPOUND CRYSTALS. 



In the preceding pages, we have been considering simple 

 crystals, and their secondary forms. The same forms are 

 occasionally compounded so as to make what have been 

 called twin or compound crystals. They will be understood 



at once from the annexed figures. Figure 72 represents a 

 crystal of snow of not unfrequent occurrence. It consists, as 



What is a twin or compound crystal ? 



* On a preceding page, it has been explained that in monometric cys- 

 tals the axes are equal ; in dimetric and hexagonal crystals the lateral 

 axes are equal, and the vertical is of a different length, shorter or longer. 

 In the other systems, the trimetric and the two oblique systems, the 

 three axes are all unequal. In the above paragraphs it has been shown 

 that the relative lengths of the axes in a fundamental form of a crystal 

 are fixed, and may be determined by simple calculations. These fixed 

 relative dimensions are supposed to be the relative dimensions of the 

 particles or molecules constituting crystals ; that is if the fundamental 

 form of a crystal is twice as long as broad, the same is true of its mole- 

 cules. The molecules of a cube must therefore be equal in different 

 directions ; those of a square prism must be longer or shorter than 

 broad, but equal in breadth and thicknesss ; those of a rectangular prism 



must be unequal in three 

 directions ; and the relative 

 inequality is determinable as 

 just stated. The simplest and 

 most probable view of the 

 forms of molecules is that they 

 are spheres for monometric 

 solids ; and ellipsoids of different axes for the other forms. Figure 1 

 represents a sphere. 4 



Figure 2 represents an ellipsoid with the lateral axes equal, as s*en 

 in the cross section 2a ; it is the form in the dimetric and hexagonal 

 systems. 



Figure 3 represents an ellipso.-d with the lateral axes unequal (fig. 3a), 

 as in the trimetric and oblique systems ; a variation in the length of the 

 axes will vary the dimensions, according to any particular case. 



