ARTIFICIAL DIAMOND
from sulphur, silicon, phosphorus, etc. and to pack it in a carbon crucible with pure charcoal from sugar. The crucible is then put into the body of the electric furnace and a powerful arc formed close above it between carbon poles, utilising a current of 700 amperes at 40 volts pressure (Fig. 20). The iron rapidly melts and saturates itself with carbon. After a few minutes' heating to a temperature above 40000 C. a temperature at which the iron melts like wax and volatilises in clouds the current is stopped and the dazzling fiery crucible is plunged beneath the surface of cold water, where it is held till it sinks below a red heat. As is well known, iron increases in volume at the moment of passing from the liquid to the solid state. The sudden cooling solidifies the outer layer of iron and holds the inner molten mass in a tight grip. The expansion of the inner liquid on solidifying produces an enormous pressure, and under the stress of this pressure the dissolved carbon separ117
