OnCrystalhgraphy* 193 



tirlginal bodies of one and the same species. It certainly 

 has its obscure sides, and there are circumstances in which 

 it disappears. Uut wherever it shows itself, there is a ray 

 of light against which we ought not to shut our eyes. 



I shall add, that, with a hitle expertness in applying cal- 

 culation to theory, we may decide if a given form enters 

 into, or ought to be excluded from^ any given species* 

 Thus we shall find that the cube which has been quoted 

 among the varieties of carbonated lime is foreign to the 

 crystalHzation of this substance*. Now it is easy to see 

 of what service this adaptation of calculation must be, in 

 order to separate the crystallized minerals into their re- 

 spective species, assigning to each what belongs to it, and 

 tVeeing it from what it nught have usurped. 



All that precedes leads us to an interesting consideration 

 relative to the chemical composition of minerals : viz. that 

 the principles which concur to form their integrant mole- 

 cules must, as I think, be constant, both as to their quan- 

 tities and their qualities, in such a manner that the sub* 

 stances which cause a variation in the products of the 

 analysis are foreign to the molecules, and merely interposed 

 among them in the mass of the mineralf. We may com- 

 pare a substance mixed with these additional principles to 

 t^ertain salts with which other salts are accidentally united, 

 as is the case with nitre of the first formation. When 

 We make this salt undergo successive solutions and cry- 

 stallizations in order to purify it, the liquid in no respect 

 alters the figure of its molecides ; it only separates them 

 from each other, and frees them from those of the other 

 salts which were associated with them, and which had 

 gone for nothing in their composition. In the same way 

 the principles on which ihe differences between the analyses 

 of various pieces of one and the same mineral depend, 

 merely form with the substance peculiar to the latter a 

 simple mixture, from which the integrant molecules would 

 come out untouched, if power were not given to us to re- 

 gulate, it' we may be allowed the expression, their de- 

 parture];. 



From 



• Vide under the head Carljonated Lime, the variety which we have 

 called ciiloid. 



t 1 even think that, irt the case in which we say that there is an excess of 

 One of the principles in other respects ess&ntial to the composition of a 

 mineral, the superabundant pait goes for nothing in the formation of the 

 molecule, and ought to be ranked among heterogeneous and purely acci- 

 dental princi|)les. 



^ These are the accidental principles wl^ich produce a variation in cer- 

 tain eiteri^l characters; such as colour, lustre of external surface, of frac- 



N 2 tur^, 



