1 873.] New Facts concerning the Diamond. 445 



distinguished from each other, from their mutual brilliancy), 

 lost nothing of its weight after cooling, and retained all its 

 transparency and brilliance of surface. The experiment 

 was several times repeated upon colourless diamonds, or 

 those of a pale yellow tint, but always with the same result ; 

 in an atmosphere free from chemical action upon it, the 

 diamond may be subjected to a white heat for a considerable 

 time without undergoing any change. 



In a superb cut diamond weighing between 6 and 7 carats, 

 the brilliancy of the stone was decidedly increased after the 

 operation. The loss of brilliancy observed by M. Schrotter is 

 a proof, in M. Baumhauer's opinion, that notwithstanding the 

 precautions employed, the diamond had come in contact with 

 the oxygen of the air, or else that at so elevated a tem- 

 perature a reducing action had been effected upon the 

 magnesia by the diamond, which had then been superficially 

 burnt by the oxygen of that earth. 



A diamond which presented to the naked eye an appear- 

 ance of dirty green, was treated in a similar manner ; 

 examination with a lens showed that the colour did not 

 extend to the entire stone, but was confined to small por- 

 tions, which formed small green clouds in the centre of the 

 mass. After heating to a white-heat in hydrogen, the 

 brilliancy of surface remained as before ; the transparency 

 was rather increased than diminished, but the green hue 

 was transformed into pale yellow. Another small diamond, 

 of so dark a green as to approach black, and almost opaque, 

 assumed a violet hue, retaining, however, its brilliancy and 

 becoming much more translucid. A small cubic diamond 

 of light green colour preserved its brilliancy and trans- 

 parency intact, but lost its colour completely : no difference 

 in its weight before and after the operation could be per- 

 ceived. Brown diamonds lose most of their colour when 

 heated to whiteness in hydrogen ; they generally assume a 

 greyish tint, in all cases the shade is much lighter, and on 

 examination with a lens they appear limpid, with black 

 spots. Diamonds of a yellow tint, such as Cape diamonds 

 almost invariably are, scarcely lose any portion of their 

 natural colour. 



Since the last Exhibition at Paris in 1867, opportunity 

 has been afforded of examining a very remarkable diamond 

 belonging to M. Coster. Although almost colourless, upon 

 being heated out of contact with the air (in a magnesia 

 bath) it assumed a deep rose colour, which it retained for 

 some days when kept in the dark ; when exposed to the 

 light, however, particularly that of the sun, the colour 



VOL. III. (N.S.) 3 M 



