486 Mr. William CrooJces [June 11, 



nitric acid forms graphitic acid easy to recognise. Graphites are 

 of varying densities, from 2*0 to 3*0, and generally of crystalline 

 aspect. Graphite and diamond pass insensibly into one another. 

 Hard graphite and soft diamond are near the same specific gravity. 

 Tiie 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 

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

 Graphites are divided into "sprouting" and "non-sprouting." 

 When obtained by simple elevation of temperature 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 separate 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. I place a few grains in a test-tube and 

 heat it to about 170° C, when, as you see, it increases enormously in 

 bulk and fills the tube with a light form of amorphous carbon. 



The resistance of a graphite to oxidising agents is greater the 

 higher the temperature to which it has previously been exposed. 

 Graphites which are easily attacked by 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 

 characteristics of the diamond, showing you by the way a few 

 experiments that bear upon the subject. 



When heated in air or oxygen to a temperature varying from 

 760° to 875° C. according to its hardness, the diamond burns with 

 production of carbonic acid. It leaves an extremely light ash, some- 

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

 magnesia, silica, and titanium. In boart and carbonado the amount 

 of ash sometimes rises to 4 per cent., but in clear crystallised 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. 



Conclensed vapour of carbon 650 



Carbon from sugar, heated in an electrical furnace.. .. 660 



Artificial graphites, generally 660 



Graphite from ordinary cast iron 670 



Carbon from hlue ground, of an ochrey colour 6 



„ „ very hard and black .. .. 710 



Diamond, soft Brazilian 760 



„ hard Kimberley 780 



Boart from Brazil 790 



„ from Kimberley 790 



., very hard, impossible to cut . c . . 900 



At the risk of repeating an experiment shown so well at this 



