APPARATUS 83 



refractory oxides known, melt and ordinary charcoal is changed 

 into graphite. In such a furnace Moissan saturated molten iron 

 with carbon and cooled the mass rapidly, so as to produce 

 solidification on the outside. The still fluid portion within had 

 to cool under the great pressure which results from the tendency 

 to expansion during the solidification of such iron. A small 

 portion of the carbon crystallises under these circumstances in 

 the form of the diamond. In order to separate these small 

 crystals the iron has to be dissolved away by means of acids, 

 and the carbonaceous residue is again treated with an oxidising 

 mixture of sulphuric acid and nitre to remove the graphite, while 

 siliceous impurities are afterwards got rid of by use of hydro- 

 fluoric acid. In the end the small grains which remain are 

 examined under the microscope. The largest diamond obtained 

 by this process only measured half a millimetre in diameter, but 

 Moissan proved the identity of these tiny crystals with natural 

 diamond not only by reference to their crystalline form, but by 

 their density (about 3-3 to 3-5), their hardness being able to 

 scratch ruby, and by their combustibility in oxygen. 



Another product of the electric furnace is the substance 

 carborundum, a compound of carbon with the allied element 

 silicon. This is produced by heating together fine sand (silica) 

 with carbon and a little common salt. Carborundum has become 

 very valuable, on account of its hardness, as a material for 

 grinding and cutting metals in the engineering shop. 



The most valuable of all products from the electric furnace is, 

 however, calcium carbide. This again we owe to Moissan, who 

 studied for the first time systematically the carbides of all the 

 chief classes of metals. 



The production of calcium carbide, CaC 2 , is simple enough. 

 It results from the action of heat, in the electric furnace, on a 

 mixture of ground lime and coke. The carbon divides itself 

 between the oxygen of the lime and the calcium, so that carbonic 

 oxide gas escapes, while the carbide remains at the end of the 

 operation as a grey solid or sometimes in the form of black 

 crystals. 



For nearly twenty years calcium carbide has been manufac- 

 tured as a source of acetylene gas, which is now a familiar 

 illuminant, and, as already mentioned, is employed in con- 

 junction with oxygen for the production of a high temperature 

 blow-pipe flame. 



The gas is produced by allowing water to drip on the solid 



