448 APPENDIX ON 



a simple forms ) as well as their combinations, must 

 " each be always distinct from the (other).''' 



If therefore in the hands of Dr. Brewster, the use 

 Of optical characters cannot at present be relied upon 

 for the determination of a mineral species, it may be 

 doubted whether they can be successfully employed 

 by less accurate and less intelligent observers. 



The proposed arrangement in the following alpha- 

 betical list, is, as it has been already observed, to a 

 certain extent, chemical ; several species, to which 

 separate specific names have been given, being ar- 

 ranged frequently under one head or genus, in the 

 alphabet. And there are, probably, many other 

 species which now stand singly in their alphabetical 

 order, which in the opinion of some of my readers 

 might, with equal propriety, be collected into other 

 chemical groups. 



This collection of species into groups or genera, 

 has not been regulated by any very precise rule. 

 The leading principles, however, upon which they 

 have been formed, are either the simplicity of com- 

 position of the species of which they consist, or the 

 apparent certainty with which that composition has 

 been determined; some few species may, however, 

 be considered as rather arbitrarily included under 

 particular genera. 



In most of the genera, the first and second of these 

 principles are apparent; an example of the third may 

 be seen under the head of Cerium, where the Yttro- 

 cerite is placed, although it contains a greater pro- 

 portion of Yttria than it does of the oxide of Cerium. 



The species which are left in their alphabetical 

 order, are generally those which are denominated 

 earthy minerals, and are composed of Alumine, Lime, 

 Magnesia, Silex, &c. in various proportions, which 



