On Crystallography. 355 . 



aUained its greatest dimensions decrease successively,) 

 was already, as it were, displayed in the growing crystal. 



A great deal remains to be done before we can terminate 

 the theory of crystallization. We have only given the laws 

 of the structure of crystals, and we are now enabled to un- 

 ravel those of their formation. The affinity of the mole- 

 cules for each other, the nature of the liquid in which cry- 

 stallization takes place, its degree of density, temperature, 

 and other similar circumstances, would be so many ele- 

 ments which we ought to take into our calculation, and 

 the solution of the problem would determine the law of 

 decrement which oueht to take place in each particular case 

 in virtue of the same circumstances, and the form of the 

 secondary crystal whicli would result from that law. 



We can very well conceive in general, that stony, me- 

 tallic or other molecules, suspended in a liquid, and dis- 

 posed to reunite in order to form a crystal, are attracted 

 at the same time by each other, and even by the molecules 

 of the liquid ; and it is because their mutual affinity is 

 stronoer than that of the liquid that their union is produced. 

 Now'the attraction of the liquid vanes on account ot the 

 circumstances which we have mentioned ; and thus us de- 

 ference from the mutual attraction ot the molecules, which 

 is alwav§ the same, oushl also to undergo variations which 

 influence crystalline forms in their diversity. Anu if there 

 are heterogeneous matters in the liquid, they will act on their 

 part in order to modify the action of the liquid on the true 

 molecules. It would seem that we have a proof ot this in 

 certain crystals of axinite, one part of which is of a violet 

 colour wiih manganese, and the other green, in conse- 

 quence of chlorite being present. The former presents ad- 

 ditional facets, which are not observed lu the latter, which 

 is besides more regularly formed, and has not a striated sur- 

 face, like the violet part. .... 



A superabundant portion of some of the essential princi- 

 ples which would be, as it is said, in exctss, might also 

 have an influence on the form of the crystal, by adding its 

 own particular action to that of the liquid. For it cati 

 hardly be doubted that there is for every substance a fixed 

 nroporiinn of principles, which constitute its true nature ; so 

 ihat all which exceeds the limit given by this proportion 

 ought to be regarded as accidental, and assimilated to a he- 

 temeencous substance. 



But the above are only slight hints, by no means sufli- 



cieiit to clear up the bubject. Our acquircmeuts have more 



'^ Z C progress 



