When the products of an electrolysis are stable and can be removed from the cell in the form in which they were deposited, there is no reac tion taking place except the electrolytic decom position itself and this is said to be a primary reaction. In many cases, however, the products of electrolysis undergo further reaction and appear in some other form than that in which they were originally deposited. In this case the reaction is said to be secondary. These sec ondary reactions may be divided into two classes, depending on whether the products of decomposition react on the material of the electrodes, or whether they react on the electrolyte. If desired, each of these classes can be still further subdivided into two classes, the first as to whether the action is on the cathode or on the anode, and the second as to whether the action is on the catholyte or anolyte. For example, if a solution of sodium sulphate were electrolyzed the primary reaction would result in the deposition of metallic sodium on the cathode and of the SO. radical on the anode. The sodium would then react with the water in the electrolyte with the for mation of NaOH and hydrogen, while the SO, would react with the water, forming 112SO4 and oxygen. On the other hand, if a solution of NaCI were electrolyzed with a mercury cathode and a silver anode the sodium set free at the cathode would react on it with the formation of an amalgam, and the chlorine set free at the anode would combine with it with the forma tion of AgC1. Since the electrolysis of the water of an aqueous solution results in the formation of hydrogen at the cathode and of oxygen at the anode, we can have in the solu tion as the result of secondary reaction either a reducing or an oxidizing action by using condi tions which favor the absorption in the solu tion of whichever is desired. A large electrode and a low current density favor the absorption of the gas in the nascent condition as fast as formed, while a small electrode and a h:gh cur rent density tend to cause the throwing off of the gas as free bubbles almost as fast as formed and with only a limited opportunity for absorption. Low current density at the cathodf
and high at the anode will then give a strong reducing action, while high density at the cathode and law at the anode will give strong oxidizing action. For further discussion of the phenomena accompanying the passage of a cur rent of electricity through a solution, see the article on ELECTROLYSIS.
In electrothermal applications it is the heating action of the current that is sought rather than its chemical action at the electrodes during electrolysis. An apparatus for the utilization of the heating action of the cur rent for carrying on a high-temperature re action is known as an electric furnace. In case the combined action of the high tempera ture and the chemical action of the current are both used, the apparatus is called an electro lytic furnace. In the former, as a matter of convenience and economy in handling the cur rent, alternating current is usually used; in the latter, since electrolysis is sought, direct current is a necessity. See ELECTRIC FURNACES and ELECTROCHEMICAL INDUSTRIES.
Many of our present day commercial tions require temperatures higher than are at tainable from the combustion of a fuel, and for operations of this kind electric heating is a necessity. In many other cases it has been found more • economical to substitute electric heating for combustion heating. The particu lar economy in electric heating is due to the fact that the heat is generated within the charge being heated and does not have to be forced through the refractory wall of the container which, on account of its low conductivity, im poses a heavy loss in efficiency. It is also sible to secure certain electrochemical by the passage of a high tension electric charge through gases. The two chief reactions of this kind are the conversion of oxygen, into ozone, O., and the oxidation of atmos pheric nitrogen to nitric acid. (See ELF..c-rRo