Electricity is employed in chemical industry in two ways. In the first place, it may be used to produce very high temperatures required for the reduction of some metallic ores, for melting highly-refractory substances, and for making steel. It is, however, rather with the second method, called electrolysis, that we are here mainly concerned. [Illustration: Fig. 15. THE ELECTROLYSIS OF SALT SOLUTION] Solutions of acids, bases, and salts, and in some cases the fused substances themselves, conduct the electric current; but at the same time they suffer decomposition. This method of decomposing a substance is known as _electrolysis_, or a breaking up by the agency of electricity. The apparatus required in a very simple case is shown in Fig. 15. It merely consists of some suitable vessel to contain the liquid; two plates—one to lead the current into the solution, the other to lead it away again—and wires to connect the plates to the poles of a battery, storage-cell, or dynamo. Each plate is called an _electrode_, and distinguished as positive or negative according as it is joined to the positive or negative pole of the current generator. By convention, electricity is supposed to “flow” from the positive pole of the battery to the positive electrode or _anode_, and then through the solution to the negative electrode or _cathode_, and so back to the negative pole of the generator, thus completing the circuit external to the battery. When acids, alkalis, and salts are dissolved in water, there is strong evidence to show that they break up to a greater or less extent into at least two parts called _ions_. These are atoms, or groups of atoms, which have either acquired or lost one or more _electrons_.[5] They move about quite independently of one another and in any direction until the electrodes are placed in the liquid. Then they are constrained to move in two opposing streams—those which have acquired electrons all move towards the negative electrode, and those which have lost electrons towards the other. At the electrodes themselves, the former give up and the latter take up electrons, and become atoms again. Let us now consider a concrete example. Common salt is composed of atoms of sodium and atoms of chlorine paired. When a small quantity of this substance is dissolved in a large quantity of water, the pairing no longer obtains. The chlorine atoms move away independently accompanied by an extra satellite or electron, and the sodium atoms move away also but with their electron strength one below par. When the current is introduced into the liquid, the sodium ions travel towards the cathode and chlorine ions towards the anode, and when they reach the goal, sodium ions gain one electron and chlorine ions lose one, and both become atoms again. Chlorine atoms combine in pairs forming molecules and escape from the solution in the greenish yellow cloud that we call chlorine gas. The