The Electrolytic Furnace.—When a direct current of sufficient volume is caused to pass through a molten salt not only will the salt be maintained in fusion by the heat developed by its own resistance, but it will be (electrolyzed,* that is to say, it will be decomposed in such manner that one of its component parts, which may be a metal, will tend to accumulate at one electrode, while another component, which was before in combination with the metal, will ap pear at the other electrode. The electrode at which the current is assumed to enter the molten bath, and at which the negative or non metallic portion of the compound appears, is called the while the other electrode, which receives the positive or metallic element and at which the current is considered to leave the bath, is designated the Thus if common salt, sodium chloride, be fused and traversed by a direct current, the negative ele ment chlorine will appear at the anode and the metal sodium at the cathode. If the substance of either electrode be such that the element liberated in contact with it can combine with or dissolve in it, the corresponding compound or solution will be formed: If, for instance, the fused sodium chlor:de be electrolyzed with an anode of carbon and a cathode of molten lead, chlorine will be evolved at the anode and escape from the bath, while sodium, dissolving in the lead, will yield an alloy from which the metal sodium, or its hydroxide caustic soda, may be obtained. In electrolytic furnaces also it is essential carefully to regulate the temperature, not only because an unnecessary high tempera ture means a waste of energy, but because of losses arising through volatilization of the bath and the recombination of the separated prod ucts of the electrolysis. So important is this last factor that in certain cases, as, for instance, in the separation of metallic sodium from molten caustic soda, it is absolutely essential to the success of the process that the temperature be maintained within limits of a few degrees only.
The Incandescent term is commonly applied to those furnaces wherein the heat is developed by the passage of the current through a body which initially at least is solid.
Such body may comprise a rod or core of car bon or corbonaceous mixture; a granular bed or core consisting of fragments of coke, retort carbon or graphite; the charge itself, often admixed with a quantity of carbon sufficient for its reduction; the furnace product when this is conductive and possesses a volatilizing point sufficiently high to permit the necessary tem perature to be attained; or a pyroelectrolyte, that is to say, an oxide or mixture of oxides which is normally non-conductive or substan tially so, but which while remaining unfused becomes capable at temperatures considerably above the normal of carrying the current. Each of these resistance materials possesses its ad vantages for particular lines of work, but all have in common the advantage of permitting accurate and ready adjustment of the tempera ture by varying the amount of current passing. These incandescent furnaces have, therefore, the widest applicability, and in case the resist ance material used is carbon the maximum tem perature attainable is probably not inferior to that of the terminals of the electric arc. The above defined types are not always sharply dis tinct, but under certain conditions the operation proceeds under two or perhaps all three of the methods. Thus if the resistance consists of fragments of carbon, the current may traverse the interspaces in the form of minute arcs; and if this fragmentary carbon be commingled with a suitable ore or compound there may be present also an electrolytic effect; the primary fusion of an electrolyte is often accomplished by means of a resistance rod connecting the electrodes, or this fusion may be accomplished by the arc. Furthermore a given furnace struc ture is often capable of either mode of opera tion according to the character of the charge and the adjustment of the electrodes with refer ence thereto. The most important as well as the most characteristic applications of the elec tric furnace have relation to certain elements and compounds which cannot beproduced directly, if at all, by other means. See ELECTRO