Electrolysis is that branch of the science of galvanism which treats of the laws and conditions of electro-chemical decomposition. As this decomposition is generally attended by electro-chemical combination, it is sometimes difficult to distinguish electro lysis from the more general subject of which embraces all chemical changes resulting in or from the galvanic current. In one case, however, the applica tion of the term is strictly correct—viz., where decompositions are effected by electrodes '(poles, see ANODE), which are not attacked by the elements of the electrolyte (the substance decomposed) discharged at- them. Throughout the article, there have been frequent allusions to electrochemical changes, but here we shall discuss more particu larly the laws of electro-chemical decomposition. No substance is decomposed by the current so long as it is in a solid or gaseous state, and it must first be brought to a liquid state, either by solution or fusion, before the current acts on it. The decomposition of water by platinum plates is always taken as the type of electrolytic action. In a very convenient apparatus for the purpose, a glass basin is made so as to admit a cork below, through which two wires pass having slips of platinum plate soldered to them above. Two glass tubes, open 'below, are hung over the plates, to hooks projecting from an upright support. The bowl is filled with acidulated water; and the tubes, after being filled with the same, are inverted, and hung with their lower ends inclosing the plates. When the wires projecting downwards from the cork are connected with the poles of the battery, hydrogen rises from the negative, and oxygen from the positive electrode, to fill each its separate tube. As the decomposition proceeds, twice as much hydrogen is liberated as oxygen. When the tubes are filled, they may be removed and examined. The oxygen thus obtained smells strongly of ozone. Hydrogen is here the type of the metals or other electropositive substances (cations), which, during electrolysis, are always disengaged at the negative electrode; and oxygen of the salt radicals, chlorine, iodine, sulphur, etc., which, being electronegative (anions), always appear at the posi tive pole. Moreover, the proportions of the volumes of the two gases being that of their chemical combining volumes, reminds us that, when a body is decomposed, its components are always separated in the proportions in which they were united, viz., those of their chemical equivalents. If the tubes of this apparatus were graduated, it would serve for a voltameter. If, instead of one such voltameter included in the circuit, we had several, we should find that, whatever amount of gas was liberated in one of these, the same amount would he liberated in all, and that independent of the size of the plates, and amount of acid in each. We learn, therefore, that the chemical power of the current is the same at every point of the circuit where it is manifested. If, instead
of two or three voltameters in the circuit, we bad one and two decomposing cells of the following description: a test tube, ]laving a platinum wire, on which the glass has been fused, passing through the bottom, is partially filled with stannous chloride. which is kept fused by the heat of a spirit-lamp. The platinum wire at the bottom of the tube forms one electrode, and one descending from the top forms the other, dipping below the fused chloride. If, then, this cell be included in the circuit along with the voltameter, and a similar cell containing fused plumbic chloride, so that the current enters the tubes by the upper electrodes, and leaves by the lower, the water, stannous chloride of tin,, and plumbic chloride, are decoMposed simultaneously by the current passing through each. In the voltameter, hydrogen and oxygen are disengaged; in the tubes, metallic tin is deposited at the lower electrode of the one, and lead at the other; whilst chlorine is liberated at the upper electrodes of both. If, now, the quantity of hydrogen, tin, and lead thus set free be weighed, the weights found correspond with their chemical formulae: Sn=118, Pb=207. From such experiments as these, we conclude that electrolytes are resolved under the action of the cu rent into anions and cations which appear at their respective eketrodee in the proportion of their atomic weight, or•multiples of their atomic weights. It is not only in cells exterior to the battery that this law holds, but in the cells of the battery itself. If the battery which effected the above decomposition consisted of six cells, for the equivalent atoms of hydrogen, tin, and lead separated without the battery, equivalent atoms of zinc in each cell would have been dissolved, and an equivalent disengagement of atoms of hydrogen at each of the copper plates, if the cells were one fluid. The above law holds not only for compounds whose elements enter into combination with their usual atomicity, but for those in which the elements, through the same, change th.?.ir atomic equivalents. Thus, if the same current pass through two decomposing cells, one con taining a solution of the cuprous chloride (CuC1), and the other of the cupric chloride (CnCl;), the same quantity of chlorine will be disengaged in both, but twice as much copper is deposited in the first as. in the second. Here the copper alone changes its atomicity, hence the in the amount of it in the consecutive cells, The accuracy of-the electrolytic law is somewhat compromised by the fact that liquids possess, to a certain extent, the power of conducting, physically, electricity without electrolytic action, so that all that passes in thiS way is chemically lost. Fortunately, the error thus introduced is very small, and can be therefore practically disregarded.