Corundum, Ruby, and various Crystallized Silicates. 49 



coated with silicate of lead, which we remove in various ways — 

 either by the action of fused oxide of lead, or by fluorhydric 

 acid, or by potass in fusion, or by prolonged calcination in 

 hydrogen, and afterwards by the action of alkalies and acids ; 

 but in certain cases we find in the geodes some nearly pure 

 crystals, which then exhibit all the characters of the natural 

 corundums and rubies — possessing their composition, adaman- 

 tine brilliance, hardness, specific gravity, and crystalline form. 



Our rubies, in fact, scratch quartz and topaz ; their specific 

 gravity is 4-0-4 # l. They lose, like natural rubies, their rose- 

 colour when strongly heated, and resume it on cooling. Sub- 

 mitted to lapidaries, they have been found as hard as, and often 

 harder than, natural ones. They rapidly wear away the best 

 grindstones of hardened steel. M. Jannettaz has kindly sub- 

 mitted our rubies to crystallographic observations ; with the 

 Amici microscope they present a black cross in their interior 

 and coloured rings upon the margins. 



The crystals which we have had cut, and now exhibit to the 

 Academy, have not yet the brilliance demanded by commerce, 

 because they did not present to the lapidary faces favourable 

 for cleavage and cutting ; but here are some crystalline masses 

 weighing several kilograms, among which we shall doubtless 

 find some that can be easily cut. 



We will now describe the method which has enabled us to 

 produce the fine specimens of crystallized silicates which we 

 lay before the Academy. The experiments about to be de- 

 scribed are connected with the preceding ; for they have fre- 

 quently given us crystals of corundum together with crystallized 

 silicates. 



It was by means of fluorides that we produced the crystal- 

 lized bodies, of which we have still to speak. In carrying 

 out these researches we have had the opportunity of apprecia- 

 ting all the accuracy of the observations of M. Daubree, who 

 first demonstrated the important part played by fluorine, as a 

 mineralizer, in the formation of mineral beds and of silicates. 

 Those views are confirmed anew by our experiments. 



Guided by the classic writings of M. Henri Sainte-Claire 

 Deville, we have ascertained that, of all the mineralizers, 

 perhaps the most active is the fluoride of aluminium. Sub- 

 mitting a mixture of equal weights of silica and fluoride of 

 aluminium to a red heat during several hours, we verified that 

 by the mutual reaction of the two substances fluoride of 

 silicium is liberated, and a crystallized body is obtained which 

 appears to be kyanite — that is, silicate of alumina. According 

 to the determinations of M. Jannettaz, this body ocaurs in 

 doubly refracting acicular crystals which extinguish light 



Phil. Mag. S. 5. Vol. 5. No." 28. Jan. 1878. E 



