I 



as a Branch of Natural History^ ^c. 233 



the mineralogist a most important consideration, which has been 

 our chief guide in the formation of the divisions and subdivi- 

 sions, from the first or more general, down to the lowest or 

 most particular. 



§ 39. There are two great and distinct points of view in 

 which minerals may be considered, or in other words, there 

 exist two distinct systems of mineralogical characters, the che- 

 mical and the physical, in each of which systems there are more 

 or less important characters. Now, a certain number of mineral 

 individuals possessing at the same time the same important phy- 

 sical characters and the same important chemical properties or 

 analogies, may be said to be very closely and naturally allied to- 

 gether. If these important chemical and physical characters 

 which are common to such minerals are found at the same 

 time to be distinctive characters, and to belong only to those very 

 same individuals, and not to others, then the groups comprising 

 all those minerals will be found to be very natural groups ; and 

 as our purpose is to establish as many as we possibly can of 

 these natural groups, we will in all cases make a strict rule never 

 to assemble in the same group, whether large or small, minerals 

 which shall not have in common the same important physical 

 and chemical characters. 



§ 40. But as it happens, though happily in a small number 

 of cases, that minerals distinguished from all the others by their 

 physical characters, may be found to exhibit no chemical differ- 

 ence from those belonging to some other group, and vice versa, 

 as for instance arragonite, which differs so much in its crystalline 

 system as well as in hardness, and in the effect of the blowpipe, 

 from calcareous spar, though a real chemical difference has not 

 yet been satisfactorily found between those two minerals ; and 

 as the same thing takes place in regard to a much more import- 

 ant group, namely, the metallic sulphurets of zinc, mercury, 

 arsenic, and of antimony and silver (red silver ore), comprised 

 in the order Blende of Mohs, so different in their translucidity 

 and their semivitreous semimetallic appearance, from all the 

 other opaque and true metallic sulphurets, it is necessary to 

 point out what is to be done in such cases. 



Undoubtedly, such minerals ought first to be separated from 

 those which exhibit a difference with regard to one or other of 



