196 On Crystallography. 



From these considerations it appears to me that we 

 may define a species in mineralogy to be, A collection of 

 bodieSy the integrant molecules ofuhich are similar., and com- 

 posed of the saine elements united in the same proportion. 

 This last condition generalizes the dcfinijion, and extends 

 it to substances which, ha\'ing their molecules of the same 

 configuration, differ essentially in the principles which 

 compose these molecules. 



Some mineralogists are of opinion that these collections?- 

 which I have called species ought rather lo be regarded as 

 genera. But where then would be the species which 

 would Subdivide the genus? Would these be crystals of 

 cUfTercnt forms? It seems to me that these modifications, 

 which in truth are the results of so many determinate laws, 

 butTvhich after all belong only to local circumstances, such 

 as the density or other qualities of the liquid, — do not fur- 

 nish a sufficient reason for establishing specific distinctions 

 between them. Thev do not touch the substance, and are 

 confined to the giving of different envelopes to one and the 

 same nucleus. Besides, on the supposition in question we^ 

 should be embarrassed with unshapen masses, which surely 

 do not deserve to be erected into species. The same an- 

 swer applies to the hypothesis on which the species would 

 form groups, one of which, for example, would compre- 

 hend Bodies regularly crystallized; a second, concretions, 

 ^c. But let us conceive that the molecules which pro- 

 dviced the concretion had been freely suspended in a tran- 

 quil liquid ; they would have assumed another arrange- 

 ment, and might have formed crystals. The idea originated 

 by the word species goes straight down .o the bottom of 

 the substance, and dues not stop at simple windings. 



We should not be better founded in regarding as so many 

 species the mixtures of a substance with accidental prin- 

 ciples, which only modify the principal species, but do not 

 transform it into another which may be really distinguished 

 from it. Even these mixed bodies only belong to the 



ture, &c. Thus we fin4 at mount Vesuvius crystals of pyroxene (Auglt of 

 Weruer), the surface and fracture of which have a very shining appearance, 

 ■whereas amongst those of i^Iorway several liave a rough and dull surface, 

 and their interior is not very shining. Nevertheless both are divided under 

 the same angles, and have forms eitiier similar, or which may be referred 

 to the same molecule; s) that the species to which they belong pieserves its 

 unitVi in spite of the ditlerences of aspect which we have mentioned } and 

 consequently these differences are all of them 3t mo^^t the index of a varia- 

 tioain the principles which concur in 'any given way to the composition 

 of the subsiance, but they do not anuounce any variatiou in the essential 

 iind truly coiiotiluent priucif le». 



' principal 



