1820.] Royal Academi/ nf Sciences. 385 



but almost always single, and of a remarkable sharpness and regu- 

 larity of form, and they do not undergo any variations, but those 

 which riiay result from the chemical action of the substance form- 

 ing the deposit. 



The variations that take place in crystals formed in a chemical 

 mixture ; that is to say, in a solution of another substance, are 

 very numerous, even when this substance cannot be united with 

 them. The above-mentioned phenomena are repeated in differ- 

 ent forms : common salt crystalhzed in a solution of borax 

 acquires truncations at the solid angles of its cubes ; and alum 

 crystallized in muriatic acid takes a form which M. Beudant has 

 never been able to obtain in any other manner. 



If the foreign substance dissolved in the liquid can be united 

 in any proportion whatever with the crystal of another substance 

 that is formed in it ; and nevertheless the crystal, by its superior 

 enei'gy, determines the form of the constituent molecule, as we 

 saw last year in the case of sulphate of iron, the matter in the 

 solution will exercise in its turn some influence upon the second- 

 ary form of the crystal, and this influence usually consists in 

 simplifying it, and causing the additional facets to disappear. 



Thus 30 or 40 per cent, of sulphate of copper may be united to 

 the rhomboidal crystallization of sulphate ol' iron, but it reduces 

 this sulphate to a pure rhomboid without any truncation either of 

 the angles or of the edges. 



A small portion of acetate of copper reduces sulphate of iron 

 to the same simple rhomboidal form, notwithstanding that this 

 formis so disposed to become complicated with additional surfaces. 



Other mixtures simplify in a less degree : thus the sulphate of 

 alumine brinos that of iron to a rhomboid with the lateral angles 

 only truncated, or what M. Haiiy calls his vurietc unitaire ; and 

 whenever this variety of green vitriol is found in the market, 

 where it is very common, we may be sure, according to M. 

 Beudant, that it contains alumine. 



Lastly, the different proportions between the base and the 

 acid, or, in double salts, between the two bases, produce very 

 sensible effects upon the secondary forms, without altei'ing the 

 primitive form in the least. This has been already exemplified 

 in respect to alum, and M. Beudant has verified it in many other 

 salts. 



The author of these researches has made some ingenious appli- 

 cations of these facts to the phenomena afforded by different 

 crystalline mineral substances, upon which direct experiments 

 cannot, in the present state of tlie science, be made ; and he has 

 exhibited some striking analogies : thus natural crystals mixed 

 with foreign substances are in general more simple than others, 

 as is shown in a specimen of axinite,or violet schorl, ofDauphine, 

 one extremity of which, being mixed with chlorite, i< reduced to 

 its primitive form; while the other end, which is puc, is varied 

 by many facets produced bv rliflc rent decrements. 



Vol. XV. N° V. 2 B 



