262 EMERALD 



of chromic oxido which ho found in tho emerald would bo utterly inadequate to pro- 

 duce the intense green colour : moreover, according to his researches, tho gem entirely 

 loses its colour at a dull red heat, whilst those minerals that are truly coloured l>y 

 chromium are unaffected in this respect by temperature. In his experiments tho loss 

 on ignition amounted to nearly 2 per cent., of which 0*12 consisted of a hydrocarbon, 

 derived probably from the associated organic remains ; and since the amount of this 

 hydrocarbon was found to bo greatest in those stones that possess tho deepest colour, 

 Lewy was induced to regard this organic compound as tho true colouring agent in tho 

 gem ; comparing it with chlorophyll, or tho green colouring matter of plants, a com- 

 pound well known to possess intense tinctorial power. 



Soon after its announcement, this opinion was called in question by tho experiments 

 of Hofmeister on the Salzburg emerald ; and it was further controverted by the re- 

 searches of "VVohler and Gustav Hose on the emerald of Muzo. Tho latter chemists 

 showed not only that the colour of the emerald is unaffected by ignition, but also that 

 the amount of chromic oxido present in a given emerald is quite competent to impart 

 an intense green colour to a quantity of glass equal to that of tho mineral from which 

 the oxido was derived. The subsequent researches of Boussingault also confirmed the 

 opinion that the emerald is coloured by oxide of chromium; and the yet more recent 

 investigations of Dr. Greville Williams lead to the same conclusion. The latter 

 chemist has found that certain colourless beryls may contain as large a proportion of 

 carbon as is to be found in the richest-tinted emeralds, and that the Muzo emerald 

 may bear a bright red heat without loss of colour. He also finds that on fusing tho 

 emerald an opalescent green glass is first formed, but that this, on prolonged heating, 

 becomes transparent and almost colourless. The density and hardness of the emerald 

 are diminished by fusion. 



Dr. Greville Williams's experiments are of so much interest that we give his own 

 account of them : 



' I have carefully repeated and extended those experiments (those of Hofmoister). 

 The emeralds employed were canutlllos (finely-crystallised emeralds) from Santa Fe 

 de Bogota. The following values were obtained in a determination of their specific 

 gravities before fusion : 



No. of Experiment W. W. t. st. D. 



1 4-4964 2-8293 16'5 -998921 2-69 



2 4-4961 2-8294 17'0 -998841 2'69 



3 1-6655 1-0486 16'0 -999002 270 

 The formula used was : 



\V is the weight in air ; W the weight in water ; st the specific gravity of water at t ; 

 t the temperature of the water ; D the specific gravity. 



' One of the above emeralds was exposed for three hours in a platinum crucible to a 

 bright reddish-yellow heat. At the end of the operation it was opaque at the edges, 

 but the green colour was not destroyed. This experiment completely confirms those 

 of Wohler and Kose, and Hofmeister. It is, I think, quite evident that no organic 

 colouring matter could withstand such a temperature for so long a time. 



1 The announcement by Lewy, that the depth of colour of emeralds is in proportion 

 to the amount of carbon present, made it appear improbable that beryls contained 

 that element. The power of this colouring matter to resist a red heat having, how- 

 ever, made me inclined to disconnect the question of colour from that of tho presence 

 of carbon, I made experiments to determine whether beryls contained that element, 

 and, if so, to what amount. 



* An experiment made at this stage of tho enquiry showed that tho beryl analysed 

 contained the same amount of carbon as Lowy's emerald. To settle tho question 

 definitely, as to the presence of carbonic anhydride, I burnt 1-2 grm. of beryl in a 

 platinum boat in a current of oxygen. The water produced was absorbed by sulphuric 

 acid ; the carbonic anhydride was received in a Geissler's potash tube containing lime- 

 water. Carefully -purified oxygen was allowed to stream through the lime-water to 

 prove its freedom from carbonic anhydride. The beryl was then heated to redness in 

 the stream of gas. and in a few seconds the limo-watcr became milky ; thus not only 

 proving the presence of carbon in a colourless beryl, but, taken in conjunction with 

 the quantitative determination, showing conclusively that tho depth of colour is not, 

 in this class of stones, in the ratio of the amount of carbon present. 



1 But it was still possible that tho carbon might have been derived from the decom- 

 position of a carbonate in the beryl. To settle this question, I arranged an ;qij':ir:ilus 

 in which, in a current of pure air passing through lime-water, a finely-levigated beryl 

 was boiled with dilute sulphuric acid. No carbonic anhydride was evolved. It was, 



