280 SALTS. 



What is said 700 ' FoRMS OF CRYSTALS.AS every 

 of the variety flower has its own distinctive form of 



of forms in a . , . 



single sub- leaves and petals, so every substance has 

 its own form or set of forms from which it 

 never essentially varies. Among these, or its combi- 

 nations, it is, as it were, left free to choose in every 

 crystal which it builds. The mineral quartz, which 

 caps its prismatic palace with a hexagonal pyramid, is 

 an example. Its common form represented in Fig. 4, 

 is a combination of the prism and double six-sided py- 

 ramid, which commence the series. 



701. A form similar to the double six- 



D escribe some 



forms of a sided pyramid, with faces corresponding to 

 single set. -^ twe j ve converging edges, belongs to the 

 same set. Double pyramids similar to each of these, but 

 of one-half or one-third their relative height, or differing 

 from them by some other simple ratio, also belong to 

 the same set of forms. Fig. 3 represents a form com- 

 posed of two of these pyramids. Fig. 5 represents 

 another form in which one of them is modified by two 

 faces of a prism. To all of these and certain other in- 

 timately related forms, the imaginary privilege of se- 

 lection and combination, above referred to, extends. 

 But most substances, like quartz, as above described, 

 affect some particular shape or combination in which 

 they usually appear. 



702. MODIFICATIONS OF CRYSTALS. 



What modifi- _ ... 



cations of the Whatever the form or combination may be, 



may occur? ^ * s su 806 ?^ 6 of variation, in any degree, 



so long as its angles correspond to those of 



the perfect shape. Thus the mineral quartz, in its 



