86 



That gradual transitions between these forms can be imagined 



with variation of the 

 values of h, k, and / 

 from zero to every ar- 

 bitrary integer num- 

 ber, is obvious on com- 

 paring the different 

 simple forms with each 

 other. 



In this way we have 

 made clear the general 

 method of description 

 of crystallographical 

 polyhedra, and their 

 definition with respect 



Fig> 89> to their symmetry- 



properties. By the ap- 

 plication of this principle to the other possible classes we shall 

 arrive at all possible 

 simple forms of crys- 

 tals, and thus the 

 consequent develop- 

 ment of these ideas 

 is best left to treatises 

 on crystallography, 

 as this book deals 

 with general aspects 

 of the subject only, 

 rather than with its 

 applications to a 

 special science 1 ). 



7 ^A^f 3 thprpfnrp 

 it v\^ HH-.1 vj.vji t; 



prefer to draw atten- 

 tion to some other 

 subjects relating not 

 only to the symmetry 



of crystalline matter, 



Fig. 90. 



but to that of physi- 



1) It may be remarked that the same views hold in the case where not 





