Part 24 (1/2)

803. This view of the result was strengthened by a third experiment, where the s.p.a.ce between the electrodes was increased to one third of an inch; for now the interfering effects were much diminished, and the number of the lead came out 89.04; and it was fully confirmed by the results obtained in the cases of _transfer_ to be immediately described (818.).

The experiments on iodide of lead therefore offer no exception to the _general law_ under consideration, but on the contrary may, from general considerations, be admitted as included in it.

804. _Protiodide of tin._--This substance, when fused (402.), conducts and is decomposed by the electric current, tin is evolved at the _anode_, and periodide of tin as a secondary result (779. 790.) at the _cathode_. The temperature required for its fusion is too high to allow of the production of any results fit for weighing.

805. _Iodide of pota.s.sium_ was subjected to electrolytic action in a tube, like that in fig. 68. (789.). The negative electrode was a globule of lead, and I hoped in this way to retain the pota.s.sium, and obtain results that could be weighed and compared with the volta-electrometer indication; but the difficulties dependent upon the high temperature required, the action upon the gla.s.s, the fusibility of the platina induced by the presence of the lead, and other circ.u.mstances, prevented me from procuring such results. The iodide was decomposed with the evolution of iodine at the _anode_, and of pota.s.sium at the _cathode_, as in former cases.

806. In some of these experiments several substances were placed in succession, and decomposed simultaneously by the same electric current: thus, protochloride of tin, chloride of lead, and water, were thus acted on at once. It is needless to say that the results were comparable, the tin, lead, chlorine, oxygen, and hydrogen evolved being _definite in quant.i.ty_ and electro-chemical equivalents to each other.

807. Let us turn to another kind of proof of the _definite chemical action of electricity_. If any circ.u.mstances could be supposed to exert an influence over the quant.i.ty of the matters evolved during electrolytic action, one would expect them to be present when electrodes of different substances, and possessing very different chemical affinities for such matters, were used. Platina has no power in dilute sulphuric acid of combining with the oxygen at the _anode_, though the latter be evolved in the nascent state against it. Copper, on the other hand, immediately unites with the oxygen, as the electric current sets it free from the hydrogen; and zinc is not only able to combine with it, but can, without any help from the electricity, abstract it directly from the water, at the same time setting torrents of hydrogen free. Yet in cases where these three substances were used as the positive electrodes in three similar portions of the same dilute sulphuric acid, specific gravity 1.336, precisely the same quant.i.ty of water was decomposed by the electric current, and precisely the same quant.i.ty of hydrogen set free at the _cathodes_ of the three solutions.

808. The experiment was made thus. Portions of the dilute sulphuric acid were put into three basins. Three volta-electrometer tubes, of the form figg. 60. 62. were filled with the same acid, and one inverted in each basin (707.). A zinc plate, connected with the positive end of a voltaic battery, was dipped into the first basin, forming the positive electrode there, the hydrogen, which was abundantly evolved from it by the direct action of the acid, being allowed to escape. A copper plate, which dipped into the acid of the second basin, was connected with the negative electrode of the _first_ basin; and a platina plate, which dipped into the acid of the third basin, was connected with the negative electrode of the _second_ basin. The negative electrode of the third basin was connected with a volta-electrometer (711.), and that with the negative end of the voltaic battery.

809. Immediately that the circuit was complete, the _electro-chemical action_ commenced in all the vessels. The hydrogen still rose in, apparently, undiminished quant.i.ties from the positive zinc electrode in the first basin. No oxygen was evolved at the positive copper electrode in the second basin, but a sulphate of copper was formed there; whilst in the third basin the positive platina electrode evolved pure oxygen gas, and was itself unaffected. But in _all_ the basins the hydrogen liberated at the _negative_ platina electrodes was the _same in quant.i.ty_, and the same with the volume of hydrogen evolved in the volta-electrometer, showing that in all the vessels the current had decomposed an equal quant.i.ty of water. In this trying case, therefore, the _chemical action of electricity_ proved to be _perfectly definite_.

810. A similar experiment was made with muriatic acid diluted with its bulk of water. The three positive electrodes were zinc, silver, and platina; the first being able to separate and combine with the chlorine _without_ the aid of the current; the second combining with the chlorine only after the current had set it free; and the third rejecting almost the whole of it.

The three negative electrodes were, as before, platina plates fixed within gla.s.s tubes. In this experiment, as in the former, the quant.i.ty of hydrogen evolved at the _cathodes_ was the same for all, and the same as the hydrogen evolved in the volta-electrometer. I have already given my reasons for believing that in these experiments it is the muriatic acid which is directly decomposed by the electricity (764.); and the results prove that the quant.i.ties so decomposed are _perfectly definite_ and proportionate to the quant.i.ty of electricity which has pa.s.sed.

811. In this experiment the chloride of silver formed in the second basin r.e.t.a.r.ded the pa.s.sage of the current of electricity, by virtue of the law of conduction before described (394.), so that it had to be cleaned off four or five times during the course of the experiment; but this caused no difference between the results of that vessel and the others.

812. Charcoal was used as the positive electrode in both sulphuric and muriatic acids (808. 810.); but this change produced no variation of the results. A zinc positive electrode, in sulphate of soda or solution of common salt, gave the same constancy of operation.

813. Experiments of a similar kind were then made with bodies altogether in a different state, i.e. with _fused_ chlorides, iodides, &c. I have already described an experiment with fused chloride of silver, in which the electrodes were of metallic silver, the one rendered negative becoming increased and lengthened by the addition of metal, whilst the other was dissolved and eaten away by its abstraction. This experiment was repeated, two weighed pieces of silver wire being used as the electrodes, and a volta-electrometer included in the circuit. Great care was taken to withdraw the negative electrodes so regularly and steadily that the crystals of reduced silver should not form a _metallic_ communication beneath the surface of the fused chloride. On concluding the experiment the positive electrode was re-weighed, and its loss ascertained. The mixture of chloride of silver, and metal, withdrawn in successive portions at the negative electrode, was digested in solution of ammonia, to remove the chloride, and the metallic silver remaining also weighed: it was the reduction at the _cathode_, and exactly equalled the solution at the _anode_; and each portion was as nearly as possible the equivalent to the water decomposed in the volta-electrometer.

814. The infusible condition of the silver at the temperature used, and the length and ramifying character of its crystals, render the above experiment difficult to perform, and uncertain in its results. I therefore wrought with chloride of lead, using a green-gla.s.s tube, formed as in fig. 72. A weighed platina wire was fused into the bottom of a small tube, as before described (789.). The tube was then bent to an angle, at about half an inch distance from the closed end; and the part between the angle and the extremity being softened, was forced upward, as in the figure, so as to form a bridge, or rather separation, producing two little depressions or basins _a, b_, within the tube. This arrangement was suspended by a platina wire, as before, so that the heat of a spirit-lamp could be applied to it, such inclination being given to it as would allow all air to escape during the fusion of the chloride of lead. A positive electrode was then provided, by bending up the end of a platina wire into a knot, and fusing about twenty grains of metallic lead on to it, in a small closed tube of gla.s.s, which was afterwards broken away. Being so furnished, the wire with its lead was weighed, and the weight recorded.

815. Chloride of lead was now introduced into the tube, and carefully fused. The leaded electrode was also introduced; after which the metal, at its extremity, soon melted. In this state of things the tube was filled up to _c_ with melted chloride of lead; the end of the electrode to be rendered negative was in the basin _b_, and the electrode of melted lead was retained in the basin _a_, and, by connexion with the proper conducting wire of a voltaic battery, was rendered positive. A volta-electrometer was included in the circuit.

816. Immediately upon the completion of the communication with the voltaic battery, the current pa.s.sed, and decomposition proceeded. No chlorine was evolved at the positive electrode; but as the fused chloride was transparent, a b.u.t.ton of alloy could be observed gradually forming and increasing in size at _b_, whilst the lead at _a_ could also be seen gradually to diminish. After a time, the experiment was stopped; the tube allowed to cool, and broken open; the wires, with their b.u.t.tons, cleaned and weighed; and their change in weight compared with the indication of the volta-electrometer.

817. In this experiment the positive electrode had lost just as much lead as the negative one had gained (795.), and the loss and gain were very nearly the equivalents of the water decomposed in the volta-electrometer, giving for lead the number 101.5. It is therefore evident, in this instance, that causing a _strong affinity_, or _no affinity_, for the substance evolved at the _anode_, to be active during the experiment (807.), produces no variation in the definite action of the electric current.

818. A similar experiment was then made with iodide of lead, and in this manner all confusion from the formation of a periodide avoided (803.). No iodine was evolved during the whole action, and finally the loss of lead at the _anode_ was the same as the gain at the _cathode_, the equivalent number, by comparison with the result in the volta-electrometer, being 103.5.

819. Then protochloride of tin was subjected to the electric current in the same manner, using of course, a tin positive electrode. No b.i.+.c.hloride of tin was now formed (779. 790.). On examining the two electrodes, the positive had lost precisely as much as the negative had gained; and by comparison with the volta-electrometer, the number for tin came out 59.

820. It is quite necessary in these and similar experiments to examine the interior of the bulbs of alloy at the ends of the conducting wires; for occasionally, and especially with those which have been positive, they are cavernous, and contain portions of the chloride or iodide used, which must be removed before the final weight is ascertained. This is more usually the case with lead than tin.

821. All these facts combine into, I think, an irresistible ma.s.s of evidence, proving the truth of the important proposition which I at first laid down, namely, _that the chemical power of a current of electricity is in direct proportion to the absolute quant.i.ty of electricity which pa.s.ses_ (377. 783.). They prove, too, that this is not merely true with one substance, as water, but generally with all electrolytic bodies; and, further, that the results obtained with any _one substance_ do not merely agree amongst themselves, but also with those obtained from _other substances_, the whole combining together into _one series of definite electro-chemical actions_ (505.). I do not mean to say that no exceptions will appear: perhaps some may arise, especially amongst substances existing only by weak affinity; but I do not expect that any will seriously disturb the result announced. If, in the well-considered, well-examined, and, I may surely say, well-ascertained doctrines of the definite nature of ordinary chemical affinity, such exceptions occur, as they do in abundance, yet, without being allowed to disturb our minds as to the general conclusion, they ought also to be allowed if they should present themselves at this, the opening of a new view of electro-chemical action; not being held up as obstructions to those who may be engaged in rendering that view more and more perfect, but laid aside for a while, in hopes that their perfect and consistent explanation will ultimately appear.

822. The doctrine of _definite electro-chemical action_ just laid down, and, I believe, established, leads to some new views of the relations and cla.s.sifications of bodies a.s.sociated with or subject to this action. Some of these I shall proceed to consider.

823. In the first place, compound bodies may be separated into two great cla.s.ses, namely, those which are decomposable by the electric current, and those which are not: of the latter, some are conductors, others non-conductors, of voltaic electricity[A]. The former do not depend for their decomposability upon the nature of their elements only; for, of the same two elements, bodies may be formed, of which one shall belong to one cla.s.s and another to the other cla.s.s; but probably on the proportions also (697.). It is further remarkable, that with very few, if any, exceptions (414. 691.), these decomposable bodies are exactly those governed by the remarkable law of conduction I have before described (694.); for that law does not extend to the many compound fusible substances that are excluded from this cla.s.s. I propose to call bodies of this, the decomposable cla.s.s, _Electrolytes_ (664.).

[A] I mean here by voltaic electricity, merely electricity from a most abundant source, but having very small intensity.

824. Then, again, the substances into which these divide, under the influence of the electric current, form an exceedingly important general cla.s.s. They are combining bodies; are directly a.s.sociated with the fundamental parts of the doctrine of chemical affinity; and have each a definite proportion, in which they are always evolved during electrolytic action. I have proposed to call these bodies generally _ions_, or particularly _anions_ and _cations_, according as they appear at the _anode_ or _cathode_ (665.); and the numbers representing the proportions in which they are evolved _electro-chemical equivalents_. Thus hydrogen, oxygen, chlorine, iodine, lead, tin are _ions_; the three former are _anions_, the two metals are _cations_, and 1, 8, 3, 125, 104, 58, are their _electro-chemical equivalents_ nearly.

825. A summary of certain points already ascertained respecting _electrolytes, ions_, and _electro-chemical equivalents_, may be given in the following general form of propositions, without, I hope, including any serious error.