ON THE APPLICATION OF THE 



belongs, is, as we have already seen, to determine 

 the class of primary forms to which it belongs. And 

 if the individuals of that class differ from each other 

 in the mutual inclination of some of the primary 

 planes, the goniometer must be resorted to for deter- 

 mining that inclination in the crystal we are ex- 

 amining. 



It happens, however, not unfrequently, that all the 

 primary planes of a crystal are obliterated, and that 

 the secondary form consists of an entirely new figure. 



In this case the observer will encounter a difficulty 

 in his attempt to deduce the characters of the pri- 

 mary form from the secondary crystal, unless the 

 secondary crystal can be referred to one of those 

 entire secondary forms described in the table of second- 

 ary forms ; but in any case this difficulty will be in 

 some decree overcome, by a habit of examining and 

 comparing crystals with each other, although it pro- 

 bably cannot be entirely removed. For if any rules 

 could be given for determining a primary form from 

 the inspection of a secondary form, on which no trace 

 of the primary planes remain, and where no assistance 

 is afforded by cleavage, they must be too numerous 

 and complicated to be serviceable to the young 

 mineralogist, for whose use these pages are princi-! 

 pally designed. 



In the few rules, therefore, which I propose to give, 

 I shall suppose, generally, that the secondary crystal 

 which is to be examined retains some portion of the 

 natural primary planes, or of cleavage planes which 

 are parallel to these.* 



* The planes of the regular tetrahedron, and of all the octahedrons, 

 and of the rhombic dodecahedron, may be easily recognised from the 

 figures in the preceding tables, and they will not therefore be par- 

 ticularly noticed here. And the different varieties of octahedrons are 

 distinguishable from each other by the angles at which their several 

 planes respectively meet. 



