the other two furnaces mentioned, has been an important piece of apparatus in the laboratory. In this connection we need only recall Moissan's comprehensive investigations, in which he has succeeded for the first time in producing several metals and carbides in a pure state.
Production of Calcium Carbide. — Within the last few yeara calcium carbide, used in the preparation of acetylene, has obtained an ever-increasing economic importance. The calcium carbide industry has, no doubt, a great future before it, especially in countries where water-power is easy to obtain. parts of lime and 36 parts of coal to a temperature of about 2000°. The reaction takes place according to the equation —
CaO -J- 30 = CaCa (Calcium carbide) -|- CO.
Instead of lime, an equivalent quantity of limestone (CaCOa) may be used, since at the high temperature this is dissociated into lime and carbon dioxide. If an insufficiency of coal be taken, metallic calcium is formed, and this, dis- solving in the carbide, gives rise to certain difficulties. On the other hand, an excess of coal contaminates the carbide and hinders its proper fusion. Furthermore, the lime used should be almost free from sulphates and phosphates, other- wise sulphides and phosphides are formed, which render the acetylene prepared from the carbide impure, and must be removed. The presence of magnesia in the lime also interferes with the fusion of the carbide. If the furnace used is first coated with coal, this is partially attacked, and 10 per cent, less coal is introduced into the charge; the charge is put into the furnace in the form of small lumps of coal and lime about the size of a hazel-nut.
Calcium carbide is comparatively easy to prepare, and a number of types of furnace for its production have been invented, amongst which is the resistance furnace of Borchers mentioned above. The furnace constructed by Eathenau (If)) consists of a containing vessel, UU (Fig. 56), provided
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