INTRODUCTION. 



aggregation, while in the latter it follows their compo- 

 sition. 



" But the following question may still be raised concern- 

 ing the order and system of minerals : ' As it is certain that 

 minerals, when their composition changes, are also changed 

 in their exterior, cannot we in this exterior find characters 

 to determine their natural order or sequence, as well as those 

 that are taken from their affinities of composition ?' Here 

 is the answer : We can, it is true, discover the different rela- 

 tions of composition in minerals by their different external 

 characters, when they are both determined beforehand j but 

 we cannot discover the order of these conformities, because 

 nature employs indifferently sometimes one character, some- 

 times another, to indicate the interior difference, that is to 

 say, the composition ; in the second place, because each ex-* 

 terior character sometimes arises from an essential difference, 

 at other times only from an accidental variety. The systems 

 of those who have inclined to arrange minerals by their 

 external characters, may already furnish proof of the incon- 

 veniences of this method, because we there see mineral 

 brought together which are essentially different -, and that 

 those of the same kind are separated by reason of some acci- 

 dental variety. Botanists and Zoologists have this advantage, 

 that in the objects of those sciences they find the conformities 

 of bodies by their exterior -, and that while they endeavour 

 to class them according to the aggregation of their external 

 parts (or organs), they describe also their external characters, 

 and in some measure accomplish these two objects at the 

 same time. The labour of mineralogists is quite different - 3 

 they must determine at once the composition of minerals by 

 their appearances under chemical operations, or otherwise 

 leave it to be determined by the chemists, and consequently 

 class them accordingly. They ought, on the other hand, to 

 seek after their exterior characters, in order to complete the 

 description from them. 



" I shall also remark, in the first place, that mineralogists 

 hitherto seem to me to have been too much attached to the 



