FORETGN CORRESPONDENCE. 



487 



yellow. It is found in the pegmatite-granites of I^'orth America, or 

 as rolled crystals in the sands of the Brazils and Ceylon. According 

 to Messrs, Deville and Caron, it can be artificially produced by the new 

 method above described. Fluoride of aluminium and fluoride of 

 glucinium are mixed in their equivalent proportion, and their vapours 

 are decomposed by vapours of boric acid. The crystals of cymophane 

 thus obtained are completely similar to the fine samples that come to us 

 from America. They actually present the striae and the peculiar heart- 

 shaped make which are so characteristic in the natural mineral. 



8. Gahiite. — Gahnite, a rare mineral (which must not be confounded 

 with garnet), named after the illustrious Swedish mineralogist Gahn, 

 professor and friend of the afterwards celebrated Berzelius, is an 

 aluminate of zinc, which hitherto has only been found in Sweden, and 

 perhaps also near the town of Franklin, in Korth America. To ob- 

 tain this mineral artificially, Messrs. Deville and Caron made use of an 

 iron crucible, into which they introduced a mixture of fluoride of 

 aluminium and fluoride of zinc. The boric acid employed was placed 

 in a platinum cupel, and the operation conducted as before described. 



In this experiment gahnite is deposited on diff'erent parts of the 

 apparatus, where it crystallises in well-defined and brilliant octahedrons. 

 They are rather strongly coloured by iron-oxide, which they obtain in 

 all probability from the iron crucible ; otherwise, the crystals resemble 

 the natural specimens, which, moreover, are mostly grey or of a greenish 

 tint. 



9. Staurotide, Sfc. — If in the foregoing experiments we put the 

 vapour of a metallic fluoride (for instance, fluoride of aluminium) in 

 contact with silica, by placing the latter in the little cupel instead of 

 boric acid, we shall find that certain silicates may be obtained in a 

 crystalline state. The authors, whose paper we have before us, have 

 thus succeeded in obtaining a crystalline substance possessing the 

 aspect of staurotide, of which it has also many of the physical and 

 chemical properties. 



The same substance is very easily obtained by heating alumina to a 

 high temperature in a current of gaseous fluoride of silicium. In this 

 operation the amorphous alumina is transformed into a mass of crystals 

 which possess, at least, the composition of staurotide. 



A silicate of zircone has been obtained in the same manner ; but 

 the authors have not completed their investigations of the last-named 

 substances. 



In the foregoing methods of experimentation, when fluoride of 

 silicium is decomposed by oxides, the compounds that result from this 

 decomposition can only contain a comparatively small amount of silica ; 

 it is therefore difficult, if not impossible, to obtain in this manner 

 silicates which are known to contain a large proportion of silicic acid. 

 Thus, for example, the authors endeavoured to obtain emerald by 

 the action of fluoride of aluminium and fluoride of glucinium on silica. 

 But the experiment failed.^'' A substance was obtained that crystal- 



* We have already shown in the pages of the Gteologist, that according to the 

 recent interesting experiments of Lewy and Daubree, it is extremely probable 

 that water has played an important part in the formation of this beautiful mineral 

 species.— T. L. P. 



