296 DEVELOPMENT OF HEAT. chap.
When the current has begun to pass through, the heat
developed is sufficient to keep the whole mass molten. As
the aluminium is separated, alumina (clay or bauxite) or some ^
other appropriate material is introduced through the openings
H and Hi, The metal formed is allowed to flow into the
receiver U from time to time, through the hole S, which can
be closed by the rod T,
It was soon found that the aluminium formed in this way was contaminated by particles of carbon from the cathode C. In order to prevent such contamination, iron or copper is added, and this collects at the bottom, E, of the crucible. In this way valuable aluminium alloys can be obtained. It was afterwards found that pure aluminium could be obtained by making the melt more mobile in various ways, as by the addition of lithium fluoride or potassium fluoride [HaU (P)], or by keeping only the central part of the salt fused, so that a solid, non-conducting crust remains on the walls of the crucible, except at the very lowest points. When the latter device is adopted, a special hollowed-out copper cathode is set in the bottom of the crucible, and this is kept cool by the circulation of water, so as to prevent it from fusing (Borchers). The possibility of concentrating the heat in a small part of the mass^ and thus avoiding contamination from the walls of the crucible, which become coated with a solid crust of the electrolyte, is one of the most important advantages which electrical heating possesses over the ordinary method ; this advantageous property of the electric furnace has been particularly called attention to by Borchers {10), who has made much use of it.
In order to avoid the inconvenient preliminary heating of the material, a small quantity of the mixture is fused in the crucible C, by placing the anode A in contact with the bottom of the crucible, and fresh electrolyte is then added until the whole is full. This introduction of material is occasionally regulated by an apparatus similar to that used in arc lights. When the resistance between the electrodes diminishes the current strength increases, and in
�� �
