DIAMOxND: BEHAVIOUR WHEN HEATED 115 



that soft bar-iron, when heated with charcoal, takes up a certain amount of carbon 

 and becomes converted into steel. In his experiment, Guyton de Morveau replaced 

 charcoal with diamond, and succeeded in converting the soft iron into steel, the 

 characters of which were identical with those of steel produced by the ordinary process. 

 His experiments thus supplied further proof of the chemical identity of carbon and 

 diamond. 



The behaviour of diamond, when raised to a high temperature, varies according to 

 whether it is in contact with air or not. In both cases, however, the stone will be easily 

 cracked or fractured if the rise of temperature is too sudden ; such damage to the stone may 

 be avoided by ensuring that both the heating and the subsequent cooling shall be slow 

 and gradual. 



In a stream of oxygen gas, a crystal of diamond begins to bum at a low red-heat. 

 It will gradually rise in temperature until it reaches a white-heat, and will then burn 

 uninterruptedly with a pale blue flame, even after the source of heat, such as a gas-flame, 

 applied at first for the purpose of raising the temperature to the point of combustion, has 

 been removed. The crystal gradually decreases in size, and finally disappears, the flame 

 at the last moment often flickering brightly like that of an expiring lamp-flame. The 

 combustion of diamond proceeds gradually from the exterior inwards ; it is unattended by 

 fusing, or, indeed, by any great alteration in the general form of the crystal, or in the physical 

 characters of its substance, the material of the inner portion remaining unaltei-ed during the 

 ■combustion of the exterior. 



It has been mentioned above that the combustion of diamond in a current of pure 

 oxygen wiU proceed even when the source of heat is withdrawn ; it is otherwise, however, 

 when the diamond is burning in atmospheric air. Should the source of heat in this case be 

 removed combustion will cease, owing to the fact that the oxygen of atmospheric air is 

 largely diluted with nitrogen, a gas which does not support combustion. In the one case 

 the heat evolved during combustion is sufficient to keep the stone above the temperature of 

 ignibion, while in the other it is not. 



The temperature to which a diamond crystal must be heated in the air before combustion 

 is started, is higher than the temperature of ignition in pure oxygen. According to 

 Lavoisier it is a little lower than the melting-point of silver, this being fixed at 916° C. 

 Moissan has recently determined the temperature of ignition of diamond in oxygen at 690° 

 to 840° C. Small crystals are more easily induced to bum than are larger ones ; according 

 to Petzholdt, small diamonds placed on platinum foil, heated from below with a blowpipe 

 flame, disappear in a very short time, the whole experiment occupying but a few minutes. 

 Diamond dust burns with greater ease and rapidity the greater its fineness ; thus powder of 

 an extreme degree of fineness, when heated on platinum foil over the flame of an ordinary 

 spirit-lamp, bums almost instantaneously with a brilliant glow. Whether in a finely 

 divided condition or not, diamond burns much more easily than does the other crystallised 

 modification of carbon, namely, graphite. 



The oxidation of diamond powder, that is, its chemical union with oxygen, takes place 

 with comparative ease if it is mixed with saltpetre, and the mixture then fused. The 

 necessary oxygen is supplied by the decomposition of the saltpetre, and the diamond powder 

 is very quickly burnt up. Diamond powder is also easily oxidised when heated at 180° to 

 230° C. with a mixture of potassium chromate and sulphuric acid. Diamond resists the 

 . action of such powerful chemical reagents as caustic potash, hydrofluoric acid, concentrated 

 sulphuric acid, aqua regia (a mixture of hydrochloric and nitric acids), a mixture of sodium 

 .chlorate and nitric acid, iodic acid, and other energetic solvents. Few other substances resist 



