220 DIAMONDS. 



acid forms graphitic acid easy to recognize. Graphites are of varying 

 densities, from 2 to 3, and generally of crystalline aspect. Graphite 

 and diamond pass insensibly into one another. Hard graphite and soft 

 diamond are near the same specific gravity. The difference appears to 

 be one of pressure at the time of formation. 



Some forms of graphite exhibit a remarkable property, by which it is 

 possible to ascertain approximately the temperature at which graphites 

 were formed, or to which they have subsequently been exposed. Graph 

 ites are divided into " sprouting" and "nonsprouting." When obtained 

 by simple elevation of temiDernture in the arc or the electric furnace 

 they do not sprout; but when they are formed by dissolving carbon in 

 a metal at a high temperature, and then allowing the graphite to sepa- 

 rate out on cooling, the sprouting variety is formed. One of the best 

 varieties is that which can be separated from platinum in ebullition in 

 a carbon crucible. The phenomenon of sprouting is easily shown. 

 Place a few grains in a test tube and heat it to about 170° C, when it 

 increases enormously in bulk and fills the tube with a light form of 

 amorphous carbon. 



The resistance of graphite to oxidizing agents is greater the higher 

 the temperature to which it has previously been exposed. Graphites 

 which are easily attacked bj'^ a mixture of fuming nitric acid and 

 potassium chlorate are rendered more resistant by strong heat in the 

 electric furnace. 



I will now briefly survey the chief chemical and physical character 

 istics of the diamond, showing you, by the way, a few experiments 

 that bear upon the subject. 



COMBUSTION OF THE DIAMOND. 



When heated in air or oxygen to a temperature varying from 760° 

 to 875° 0., according to its hardness, the diamond burns with produc 

 tion of carbonic acid. It leaves an extremely light ash, sometimes 

 retaining the shape of the crystal, consisting of iron, lime, magnesia, 

 silica, and titanium. In bort and carbonado the amount of ash some- 

 times vises to 4 per cent, but in clear crystallized diamonds it is seldom 

 higher than 0.05 per cent. By far the largest constituent of the ash is 

 iron. 



The following table shows the temperatures of combustion in oxygen 



of different kinds of carbon: 



°c. 



Condensed vapor of carbon 650 



Carbon from sugar, heated in an electrical furnace 660 



Artificial graphites, generally 660 



Graphite from ordinary cast iron 670 



Carbon from blue ground, of an ochery color 690 



Carbon from blue ground, very hard and black 710 



Diamond, soft Brazilian 760 



Diamond, hard Kimberley 780 



Bort from Brazil 790 



Bort from Kimberley 790 



Bort, very hard, impossible to cut 900 



