ON THE USE OP SYMBOLS. 



whole number or fraction, expressive of the particu- 

 lar law of decrement, in reference to each plane 

 respectively, as it is supposed to have been ascer- 

 tained by calculation. 



If the two planes occur on the same crystal, they 

 would be denoted by the two symbols being used to- 

 gether, thus, P H H p . 



If it should be required to describe any decrement 

 acting upon an edge or angle of the lower plane of 

 the crystal, upon which the small letters are supposed 

 to be placed, the crystal is imagined to be turned 

 with that plane upwards, the edge or angle on which 

 the decrement has taken place is to be brought the 

 nearest to the eye, and we are then to describe the 

 plane or planes in the manner already directed, only 

 using the small letter, instead of the capital, to indi- 

 cate the edge or angle which is modified. And if it 

 should be necessary to describe a decrement upon 

 the back planes of the crystal, we are supposed to 

 pass round it, and to substitute small letters in the 

 symbol for the capitals which designate the cor- 

 responding front planes. 



3 



The preceding explanations will render sufficiently 

 intelligible the general method of representing the 

 secondary planes of crystals by means of symbolic 

 characters. Before we proceed, however, to apply 

 this method to the different classes of primary forms, 

 it will be necessary to separate the secondary forms 

 of the crystals to be represented into three principal 

 classes. 



1. Those which are strictly symmetrical, as modifi- 

 cation , , c, d, e, or f, of the cube, where 

 similar decrements take place on similar edges 

 or angles, and proceed along similar planes. 



