CLASSIFICATION OF MINERALS. 255 



mineralogical classifications, and, it is generally 

 believed, with advantage to the science : How is 

 this consistent with what has been said ? 



To this the answer is, that when this has been 

 done with advantage, the authority of external cha- 

 racters, as well as of chemical constitution, has been 

 brought into play. We have two sets of properties 

 to compare, chemical and physical ; to exhibit the 

 connexion of these is the object of scientific minera- 

 logy. And though this connexion would be most 

 distinctly asserted, if we could keep the two sets of 

 properties distinct, yet it may be brought into view 

 in a great degree, by classifications in which both 

 are referred to as guides. Since the governing 

 principle of the attempts at classification is the 

 conviction that the chemical constitution and the 

 physical properties have a definite relation to each 

 other, we appear entitled to use both kinds of evi- 

 dence, in proportion as we can best obtain each; 

 and the general consistency and convenience of our 

 system will then be the security for its containing 

 substantial knowledge, though this be not presented 

 in a rigorously logical or systematic form. 



Such mixed systems of classification, resting 

 partly on chemical and partly on physical charac- 

 ters, naturally appeared as the earliest attempts in 

 this way, before the two members of the subject 

 had been clearly separated in men's minds; and 

 these systems, therefore, we must first give an 

 account of. 



