104 ON THE TABLES OF MODIFICATIONS. 



planes, as explained above, we may immediately dis- 

 cover the class of primary form to which the figure 

 belongs. 



The front planes exhibited in the drawing of any 

 crystal are generally one half the number belonging to 

 it ; hence it is obvious that if these be described, the 

 parallel planes of crystals being always similar to 

 each other, the planes which are parallel to those 

 shewn in the drawing, may be conceived to be des- 

 cribed also. 



It may be remarked that the modifying planes of 

 the series of forms contained in the tables, are pro- 

 duced by cutting off portions of the figure of the 

 primary crystal and thus reducing its bulk. It is 

 almost unnecessary, after what has been already said 

 on the formation of crystals, to observe, that nature 

 proceeds by building up the secondary forms instead 

 of thus truncating the primary. And we may, if we 

 please, imagine, that the secondary figures given in 

 these tables, have been produced by additions to 

 primary crystals of smaller relative dimensions than 

 those placed at the head of the several tables. 



The inclination of any two planes to each other, as 

 a and P, modification a of the cube, or the angle at 

 which they meet, is commonly expressed in this man- 

 ner, a on P so many degrees and minutes. 



