[Illustration: FIG. 91.--Grotthuss’ theory of electrolysis. Grotthuss in (1806), announced his theory that the molecules in an electrolyte have their individual electro-positive and electro-negative atoms charged positively and negatively respectively. In an ordinary liquid, for instance in water, the molecules are arranged indifferently, like row 1, with their positive and negative ends pointing in all directions. When the charged plates A and B connected to the + and - poles of a battery are inserted in the water, the molecules under the action of the laws of electrostatic action turn as shown in row 2, so that all the hydrogen or shaded ends (+) are turned towards the (-) plate B and all the oxygen or unshaded ends (-) towards the (+) plate A. All along the row the electrical forces are supposed to tear the molecules asunder, depositing H on B and O on A. The atoms in the middle of the liquid, however, recombine, for the hydrogen atoms in their journey towards B meet the oxygen atoms travelling in the opposite direction, and we get the state of affairs represented in row 3. The next step is to rotate once more the atoms into the positions shown in row 2, and so on. In this way the theory accounts for the products only appearing at the electrodes and not in the body of the liquid.] If, however, the anode be of copper instead of platinum, no sulphuric acid will be formed, neither will oxygen be given off at the anode. As copper is deposited at the cathode, an equal quantity will be dissolved at the anode, so that the original constitution of the liquid is maintained. The atoms separated from each other by the electric current were called _ions_ by Faraday; those going to the anode being _anions_, and those going to the cathode being _kathions_. Anions are generally regarded as _electro-negative_, because they move as if attracted to the positive electrode, while kathions are regarded as _electro-positive_. In order to explain the transfer of electricity and the transfer of matter through the electrolyte, Grotthuss put forward the hypothesis that when two metal plates at different potentials are placed in a cell, the effect produced in the liquid is that the molecules of the liquid arrange themselves in innumerable chains, as shown in fig. 91, in which every molecule has its atoms pointing in a certain direction, the electro-positive atom being attracted towards the cathode and the electro-negative towards the anode. An interchange then takes place all along the line, the free atoms appearing at the electrodes, and every atom discharging a minute charge of electricity upon the electrode at which it is liberated. =Electro-chemical Series.=--This is an arrangement of the metals in a series in such a manner that the most electro-positive is at one end and the most electro-negative at the other. The order of the metals varies with the electrolyte in which the metals are tested.