1873.] Mr. Greville "Williams on Emeralds and Beryls. 409 



XV. " Researches on Emeralds and Beryls. — Part I. On the 

 Colouring-matter of the Emerald." By Greville Williams, 

 F.R.S. Received May 9, 1873. 



A considerable amount of discussion has taken place at various times 

 regarding the cause of the colour of the emerald. Klaproth concluded 

 from his earlier analyses that it was due to iron*; but the results of his 

 later experiments f , made after he became aware of Vauquelin's discovery 

 of the presence of chromium in emeralds J, confirmed the observations of 

 that chemist. 



Erom the time of Vauquelin's analyses, the colour of the emerald was 

 always regarded as due to the presence of oxide of chromium, until the 

 publication of the memoir of Lewy§, who, having burnt emeralds in 

 oxygen in a similar apparatus to that employed by M. Dumas in his re- 

 searches on the atomic weight of carbon, ascertained that they contained 

 that element, and concluded that the colour was due to the presence of 

 some organic substance. Lewy also affirmed that the deepest-tinted 

 emeralds contained the most carbon. The small quantity of chromium 

 contained in emeralds he considered to be insufficient to account for 

 the colour. Wohler and Rose||, on the other hand, having exposed 

 emeralds to a temperature equal to the fusing-point of copper for one 

 hour without their losing colour, and also having fused colourless glass 

 with minute quantities of oxide of chromium and obtained a fine green 

 glass, considered chromium and not organic matter to be the cause of the 

 colour. 



Boussingault^T, in the course of an investigation of the " morallons "**, 

 arrived at the same conclusion as Wohler and Rose ; and although ad- 

 mitting them to contain carbon, denied that it was the cause of their 

 colour, inasmuch as they endured heating to redness for one hour with- 

 out loss of colour. This result has been confirmed by Hofmeister f f , who 

 found that an emerald endured a red heat for hours without destruction 

 of the colour, except at the edges, and concludes this small bleaching to 

 arise from the destruction of the crystalline character of the stone. I have 

 carefully repeated and extended these experiments. The emeralds em- 

 ployed were canutillos from Santa Ee de Bogota ; they were kindly given 

 to me by Professor Church. The following values were obtained in a 

 determination of their specific gravity : — 



* Klaproth, Chem. Essays, vol. i. p. 325 (London, 1801). 



t Klaproth, loc. cit. vol. ii. p. 172 et seq. (1804). 



J Vauquelin, Ann. de Chim. vol. xxvi. [1] p. 262 (1798). 



§ Comptes Eendus, vol. xlv. p. 877 (1857). 



|| Chem. News, vol. x. p. 22. 



% Comptes Eendus, vol. lxix. p. 1249 (1869). 



** The emeralds from the mines of New Granada are divided, according to Boussin- 

 gault, into classes, two of the most important being the " canutillos," or finely crystal- 

 lized, and the " morallons," or amorphous emeralds. 



tt Journ. fur prakt. Chem. vol. lxxvi. p. 1 (1859). 



