444 APPENDIX ON 



widely in their chemical character, may really belong 

 to one species. 



These considerations appear to confirm the pro- 

 priety of the Abbe Haiiy's definition of a mineral 

 species, as given in p. 6. For although it might have 

 been sufficient, theoretically, to comprise within the 

 terms of this definition, such individual minerals as 

 are composed of similar particles united in equal 

 proportions, yet in the present imperfect state of our 

 knowledge of the true constituent elements of many 

 minerals, it appeared practically necessary to super- 

 add to this definition the condition that, if they be- 

 long to the same species, the form of their molecules, 

 or, which is the same thing in effect, their primary 

 forms should also be similar. 



Hence when we find different minerals agreeing in 

 their crystalline forms, and varying in their chemical 

 composition, we shall probably determine their spe- 

 cies more accurately from their crystalline than from 

 their chemical characters. 



I say probably, for the 6rystallographical cha- 

 racter has its uncertainties also. The natural planes 

 of crystals are generally too imperfect to give mea- 

 surements which may be said to agree very nearlv 

 with each other; the differences among such as belong 

 to the same species of mineral, amounting frequently 

 to nearly a degree ; and the cleavage planes, which 

 generally afford better corresponding results, cannot 

 always be obtained; but if they could, the angles 

 of mutual inclination even of those, are not always 

 alike, owing probably to an interposition of foreign 

 matter between the laminae of the crystal, and being 

 there unequally dispersed. Nor do we know that 

 the difference of the angles under which the primary 

 planes of different species of minerals meet, is not 

 Jess than our best goniometers can distinguish. I( 



