PORETGN CORRESPONDENCE. 
487 
yellow. It is found in the pegmatite-granites of North America, or 
as rolled crystals in the sands of the Brazils and Ceylon. According 
to Messrs. Deville and Carou, 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 iu the natural miaeral. 
8. Gahnite. — 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 North 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 
iu a platinum cupel, and the operation conducted as before described. 
In this experiment gahnite is deposited on difl'erent parts of the 
apparatus, where it crystallises in well-defined and bi'illiant 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 Geologist, that according to the 
recent interesting experiments of Lewy and Daubree, it is extremely probable 
that water has played an important part iu the formation of this beautiful mineral 
species.— T. L. P. 
