306 MR HAIDINGER on the Determination of the Species in 



others, in order to constitute a species, is produced by exa- 

 mining every property by itself, which minerals present, and join- 

 ing those individuals which either possess the same properties, 

 or in which the gradations of the differences in their characters 

 may be considered as producing continuous series. For ex- 

 ample, if it be impossible to unite two varieties of form, or two 

 degrees of specific gravity, within a single series, it is evident 

 that the individuals under consideration themselves cannot be 

 comprehended within the limits of the same series. 



The fundamental proposition upon which, in natural history, 

 we ground this reasoning, is, that two individuals, which do not 

 differ in any of their properties, are identical, and may be sub- 

 stituted for one another, in every inquiry carried on according to 

 the principles of that science ; so that, if we have found one of 

 them to belong to a certain class, to a certain order, genus or 

 species, the other individuals also must belong to the same as- 

 semblage of natural productions. Two hexahedral crystals of 

 Fluor, possessing the same violet-blue tint of colour, the same 

 degree of transparency, and agreeing exactly in hardness 

 and specific gravity, may serve as examples of identical mi- 

 nerals. 



But if, instead of one of these crystals, we compare the vio- 

 let-blue hexahedron with another individual of Fluor, possessing 

 the same form, but a honey-yellow colour, or the same colour, 

 but the form of a regular octahedron, the two individuals will 

 evidently be different, even though the rest of their properties 

 should be critically the same. A difference will also exist, if, in- 

 stead of the second individual, we take a hexahedron of Gold or 

 of Rock-salt ; and the difference between the latter individuals 

 and the individuals of Fluor is no doubt much greater than that 

 among the above-mentioned crystals of Fluor themselves. From 

 these considerations we infer, that the difference among several 

 individuals is not the same in every instance, but that various 



