Corundum, Ruby, and various Crystallized Silicates, 51 



plane of the optic axes is parallel to tbese intersections ; they 

 cut one another at angles of 60° 42' and 119° 18'. 



There is therefore produced in this curious reaction corundum 

 and a crystallized double silicate. These two crystalline sub- 

 stances result from the following transformations : — 



In the calcination of the mixture of alumina and fluoride of 

 barium there are evidently formed fluoride of aluminium and 

 baryta. The fluoride of aluminium, once produced, must have 

 acted in two different ways. Decomposed by the gases from 

 the hearth, it formed fluorhydric acid and corundum, which 

 crystallized under the influence of the vapours. Acting be- 

 sides upon the silica of the crucible, it gave rise to silicate of 

 alumina, which, combining with the baryta, produced the fine 

 crystals of double silicate of alumina and baryta which we 

 exhibit to the Academy. Such, in our opinion, is the theory 

 of the reaction. 



Permit us now to dwell on the conditions which have de- 

 termined the crystallization of the two substances, corundum 

 and the double silicate. Looking at the specimens we here 

 exhibit, and which present such well-defined crystals, one is 

 struck with the place which they occupy in the crucibles : 

 they seem to have been volatilized ; and yet we have ascertained, 

 by exposing them to the highest temperatures of our furnaces, 

 that they are absolutely fixed. It is because the fluorides are 

 not merely powerful mineralizers ; they are also compounds 

 which, as was formerly said, give wings to the least-volatile 

 substances. Do we not recollect, indeed, that remarkable 

 formation of orthose felspar, produced artificially and found 

 in the upper part of a copper-furnace at Mansfeld? The 

 employment of fluoride of calcium in the melting-bed of the 

 furnace which produced that felspar permits the belief that the 

 fluorine intervened in that case as a transporting agent. It 

 was evidently this which presented itself in our experiments, 

 as in those which have been so often performed by M. H. 

 Sainte-Claire Deville : the agents of the transport and crystal- 

 lization of the corundum and the silicate were likewise the 

 fluorine compounds which we employed. 



It was to be presumed that this action of fluoride of barium 

 upon alumina in presence of silica, forming a crystallized double 

 silicate, would reappear as a general phenomenon connected 

 with the decomposition of the. fluorides by different bases. This 

 we have, in fact, proved ; in another communication we will 

 describe some crystallized double silicates produced under 

 the tame conditions as the double silicate of alumina and 

 baryta ; and then we shall give the general formulae of these 

 compounds. 



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