A small space is left at the top, as shown.  After the plug has been inserted and the joint properly luted, the electric circuit is closed and the current allowed to pass through the retort, traversing its entire length through the body of mixed ore and carbon.  The carbon constituents of the mass become incandescent, generating a very high degree of heat, and being in direct contact with the ore, the latter is rapidly and effectually reduced and distilled.  The heat evolved reduces the ore and distills the zinc, and the zinc fumes are condensed in the condensing chamber, precisely as in the present method of zinc making, with this important exception that, aside from the reaction produced by heating carbon in the presence of zinc oxide, the electric current, in passing through the zinc oxide, has a decomposing and disintegrating action upon it, not unlike the effect produced by an electric current in a solution.  This action accelerates the reduction, and promotes economy in the process.

Another form of furnace is illustrated by Fig.  III., which is a perspective view of a furnace adapted for the reduction of ores and salts of non-volatile metals and similar chemical compounds.  Figs.  IV. and V. are longitudinal and transverse sections, respectively, through the same, illustrating the manner of packing and charging the furnace.

The walls and floors L L’, of the furnace are made of fire bricks, and do not necessarily have to be very thick or strong, the heat to which they are subjected not being excessive.  The carbon plates are smaller than the cross section of the box, as shown, and the spaces between them and the end walls are packed with fine charcoal.

The furnace is covered with a removable slab of fireclay, N, which is provided with one or more vents, n, for the escaping gases.

The space between the carbon plates constitutes the working part of the furnace.  This is lined on the bottom and sides with a packing of fine charcoal, O, or such other material as is both a poor conductor of heat and electricity—­as, for example, in some cases, silica or pulverized corundum or well-burned lime—­and the charge, P, of ore and broken, granular, or pulverized carbon occupies the center of the box, extending between the carbon plates.  A layer of granular charcoal, O’, also covers the charge on top.  The protection afforded by the charcoal jacket, as regards the heat, is so complete, that with the covering-slab removed, the hand can be held within a few inches of the exposed charcoal jacket; but with the top covering of charcoal also removed and the core exposed, the hand cannot be held within several feet.  The charcoal packing behind the carbon plates is required to confine the heat and to protect them from combustion.

With this furnace, aluminum can be reduced directly from its ores; and chemical compounds from corundum, cryolite, clay, etc., and silicon, boron, calcium, manganese, magnesium, and other metals are in like manner obtained from their ores and compounds.  The reduction of ores according to this process can be practiced, if circumstances require it, without any built furnace.