63 THE GEOLOGIST. 



stowed away with the greatest care for a few days. As to the 

 came of this hardening of stones recently extracted from their natural 

 beds we shall have occasion of referring to it presently. 



M. Lewy has shown also in his interesting paper {Researches on the 

 Formation and Composition of Emeralds), that the beautiful green 

 colour, so much admired in the stones of which we are speaking, 

 and without which they would, in all probability, be valueless, is owed 

 to an organic substance somewhat similar to that called chlorophylle, 

 which colours the leaves of plants, f The green colour of emeralds 

 has always been attributed to a slight quantity of chromic oxide, which, 

 although it certainly does play an important part in the coloration of 

 other minerals, has positively nothing to do with that of emeralds. 

 Analysis has furnished only an exceedingly slight quantity of chromic 

 oxide ; so small, indeed, that the distinguished chemist whose work we 

 are analysing could not weigh it separately. Our readers have, perhaps, 

 heard of, or seen, a mineral called ouwarovite, a sort of chromiferous 

 garnet, whose green colour (which withstands heat, whilst that of the 

 emerald does not) is exactly that of the emerald ; but ouwarovite 

 furnishes 23 and-a-half per cent, of oxide of chrome, whereas in 

 emeralds, as we have seen, there is only a slight vestige of this 

 green oxide. This is certainly a new and unexpected result. 



A certain number of facts related in his memoir lead M. Lewy to 

 affirm that emeralds have been deposited from water. It would, perhaps ^ 

 be more rational to say that water has been active in their production J 

 — First, besides the organic colouring matter, emeralds contain about 

 2 per cent, of water. In the next place, the black white- veined lime- 

 stone in which they are found contains fossil ammonites. This lime- 

 stone, perfectly freed from the microscopic emeralds with which it is 

 Btrewed, by digestion with dilute hydrochloric acid, gives in analysis a 

 To oiTo th part of glucina. 



Great uncertainty has prevailed as to the mineral constituents of the 

 emerald and its true chemical formula ; this has determined me to give 

 hure the numbers obtained by M. Lewy, and to corroborate them by 



• Presented to the Academy of Sciences, Nov. 15th, 1857. 



t It is, perhaps, as well to observe that M. Lewy has not ascertained if this 

 orjjijvnic substance in the emerald contains oxygen or not. He seems to consider 

 it JiH a carburet of hydrogen. 



t Sec further, the phenomena recently observed by M. Daubree. 



