ELECTROLYSIS. A very slight acquaintance with the phenomena of conduction of electricity by different bodies shows us that conductors may be arranged in two very distinct classes. In one the passage of electricity produces no change in the chemical composition of the substance, unless indeed the electromotive force be so great that disruptive discharge occurs, or so large an amount of heat is generated that chemical effects ensue; the con- ductivity diminishes slowly as the temperature rises, and if the resistance of the rest of the circuit be small compared with that of the substance under consideration, an amount of heat is produced in the latter equivalent to the energy expended by the sources of electricity. To this class of conductors probably belong all solids, with the exception of hot glass, which conducts with decomposition at a temperature below the fusing point. The conductivity differs enormously in the different cases; those which con- duct most readily are the metals, alloys, the chemical elements generally, and some few metallic oxides and sul- phides (Faraday, Exp. Res, 440, ser. iv. ; Skey, Chem. News, xxiii.). Besides fused metals Faraday added one liquid, fused periodide of mercury, to the list, but subse- quently gave reasons for considering that it was misplaced (Exp. lies, 691, Ser. vii.). The other class of conductors presents a remarkable contrast to the one just described. In these the passage of electricity results in the chemical decomposition[1] of the substance of the conductor at the points where the electric current[2] enters and leaves the body; a rise of temperature produces in such bodies a very con- siderable increase in the conductivity, but the specific resist- ance of even the best conducting among them is always very great compared with that of the metals. (For details see article Electricity, p. 46 sqq.) Only part of the energy of the circuit is spent in heating the conductor, as a transformation of energy takes place in the chemical and molecular actions at the points where the current enters and leaves the conductor.
It is the behaviour of the second class of bodies under the influence of the electric current that we have now to discuss. The physical side of the subject has already been considered in the article Electricity; so we shall principally confine our attention to the phenomena of electrolysis which bear on the laws and principles of chemistry. Before going further it will be necessary to introduce the technical terms which have now become familiar, and, in order to be definite, we will consider somewhat closely a particular instance of electrochemical decomposition of the simplest type.
The cell in which the action takes place consists of a wide tube of hard glass, bent into a V-shape ; into this is introduced some silver chloride, which is kept fused during the experiment; into the liquid in one leg of the tube is dipped a platinum wire connected with the negative pole (zinc) of a battery[3] of 3 or 4 Grove’s cells, and into that in the other a piece of graphite or gas carbon connected with the positive pole of the same battery. We will suppose a galvanometer introduced into the circuit, and that the current strength as indicated thereby is, roughly speaking, constant, so that the quantity of electricity which passes can be measured roughly by the time occupied in passing. After the circuit has been closed a short time, bubbles of chlorine will begin to come ofl" from the carbon, while pure silver is deposited upon the platinum wire, but except at these points no alteration will take place at any part Q)“ the fluid. If the platinum wire with the attached silver be weighed at intervals, it will be found that the amount deposited after the current has become constant is propor- tional to the time, i.e., to the amount of electricity which has passed through the liquid. The same will be true of the chlorine if collected in the other leg of the tube, due allowance being made for the small bubbles retained by the carbon, &c. And the amount of chlorine will be chemically equivalent to the amount of silver; thus for every lllo’ grammes of silver on the platinum there will be 356 grammes of chlorine set free in the other leg of the tube. Moreover if the current be varied by varying the number of battery cells, it will be found that the amount of decom- position in a given time is proportional to the current, that is, again, to the quantity of electricity which traverses the substance.
Faraday, who was the first to define the laws which hold in electrochemical decomposition, introduced, for the sake of precision, a system of nomenclature which has since been generally employed. Wishing to regard the terminals corresponding, in any similar case, to the carbon and platinum in the above experiment merely as the “ doors ” by which the electricity enters and leaves the liquid, he denominated them electrodes, and, comparing the “ path ” of the current to those of the currents which may produce terrestrial magnetism, and hence to the course of the sun, he called the homologue of the carbon (where the current, so to speak, “rose,” or entered) the (mode, that of the platinum (where the current “set,” or left) the cat/(ode. The component parts, no matter how complex, into which the liquid was decomposed, corresponding to the Ag and C1 of the above, received the name of “ ions ”—that com- ponent which went down, with the current to the cathode, and there either was set free or combined with the cathode or the surrounding liquid, being the cation, and that which went up against the current, and appeared or promoted some chemical action at the anode, the anion. Moreover, the substance decomposed was called an electrolyte, and the process itself electrolysis. (Faraday, Exp. Res, 662 sell.)
The phenomena which occur at the electrodes when the ions there set free react upon the electrode or the surround- ing fluid, so that the resulting products of electrolysis are not the ions themselves, are called secondary actions.
The anion and the cation are frequently called the negative and positive ion resl‘mctively. Similarly the cathode and anode are termed the negative and positive electrodes; Daniell denoted them the platinode and the zincode, but these terms have fallen into disuse.
tion nearly all, if not all, are liquids. Faraday (Exp. Res, 433, 1340) apparently obtained some chemical decomposition in sulphuret of silver and a few other salts when solid, but this did not alter his Opinion that the mobility secured in the fluid state, either by fusion or by solution, was necessary to the phenorneua of electrolysis; and his view, which he supported by experiments on ice and other Solids that conduct when fused (Exp. Res, 380—397, 419—428), still obtains. Electrolytic action doubtless sometimes takes place in gases, but accurate investigation of the subject is
difficult on account of the extreme mobility of the particles
- ↑ We have not space here to discuss whether or not conduction in electrolytes is always attended with decomposition, although the question has engaged the attention of many writers on the subject. The reader who wishes for information upon the point may consult Faraday, Esp. Res. 966—9871}, ser. viii. ; Despretz, Compt. chd., t. xlii. p. 707; De la Rive, Archires, t. xxxii. p. 38; Logeman and Van Breda, Plu'l. Mtg. [4], viii. 465 ; Buff, Ann. d. Chem. u. I’lwrm., Bd. xeiv. s. 15; Foucault, Compt. Rand, t. xxxvii. p. 580; De la Rive, Ann. de Chimie, [3], t. xlvi. p. 41; Favre, Compt. Rend., lxxiii. p. 1463; Helmholtz, Berlin .llonatsbericht, 1873, Naehtrag zum J uliheft; and, for a summary of results, \Viedemann, Galen, Bd. i. §314—316, and Nachtrag, 36, § 334.
- ↑ The standard direction of the current is taken, as usual, to be from the copper through the wire to the zinc of au ordinary zinc- copper cell.
- ↑ It is not necessary to use a voltaic battery,—any source of elec- tricity serves,—bnt either a voltaic or a thermoelectric battery is usually employed, since these so conveniently supply a large quantity of electricity, with an electromotive forcc sufficient for the purpose.
