1873.] Mr. Greville Williams on Emeralds and Beryls. 



421 



density of minerals to be less dependent on the temperature at which 

 they are produced than upon their crystalline or amorphous state. 



One crystalline gem (the ruby) has undoubtedly been produced in 

 nature at a high temperature. I have frequently repeated Gaudin's* 

 experiment on the artificial formation of this stone, and can confirm 

 most of his results. I did not, however, find the density to be quite the 

 same as the native ruby or sapphire, which is, in different specimens, 

 from 3*53 to 3*56. Artificial rubies of the finest colour made by me by 

 Gaudin's process had a specific gravity of 3*45, which is not three per 

 cent, lower than that of the ruby. The reason for this close approxima- 

 tion will be found in the fact that fused alumina crystallizes on cooling. 

 The crystallization, however, is confused and imperfect, which causes 

 the resulting product to be only partially transparent, and to have a 

 slightly lower specific gravity than the natural gem. It is, consequently, 

 scarcely correct to call the fused stones made by Gaudin's process 

 " artificial rubies." 



I have convinced myself that rubies have been formed in nature at a 

 temperature equal, or nearly equal, to that of the f using-point of alumina, 

 from the circumstance that the reaction between chromic oxide and 

 alumina, which results in the development of the red colour of the gem, 

 is not effected at low or even moderately high temperatures, but requires 

 a heat as high as that of the oxyhydrogen blowpipe. It is not necessary 

 that the chromium should be presented to the alumina in the form of 

 chromic acid. It appears, therefore, that the red colour of the ruby 

 is not caused by the presence of chromic acid ; it is, in fact, a reaction 

 sui generis between alumina and chromic oxide, which, as far as my expe- 

 riments have gone, only takes place at very elevated temperatures. 



In my next communication I propose to give the results of a compara- 

 tive study of two of the processes most generally employed for the 

 analysis of emeralds, beryls, and other minerals containing glucina and 

 alumina, namely the carbonate-of -ammonia process of Vauquelin, and the 

 caustic-potash method devised by the same chemist, but modified by' 

 Gmelin, and generally associated with his name. These studies are 

 already far advanced. 



Specimens of various beryl glasses, cut and uncut, accompany this 

 paper. 



* Anu. Pharm. vol. xxiii. p. 234. 



VOL. XXI. 



2 L 



