Part 16 (1/2)

515. M. Grotthuss is inclined to believe that the elements of water, when about to separate at the poles, combine with the electricities, and so become gases. M. de la Rive's view is the exact reverse of this: whilst pa.s.sing through the fluid, they are, according to him, compounds with the electricities; when evolved at the poles, they are de-electrified.

516. I have sought amongst the various experiments quoted in support of these views, or connected with electro-chemical decompositions or electric currents, for any which might be considered as sustaining the theory of two electricities rather than that of one, but have not been able to perceive a single fact which could be brought forward for such a purpose: or, admitting the hypothesis of two electricities, much less have I been able to perceive the slightest grounds for believing that one electricity in a current can be more powerful than the other, or that it can be present without the other, or that one can be varied or in the slightest degree affected, without a corresponding variation in the other[A]. If, upon the supposition of two electricities, a current of one can be obtained without the other, or the current of one be exalted or diminished more than the other, we might surely expect some variation either of the chemical or magnetical effects, or of both; but no such variations have been observed.

If a current be so directed that it may act chemically in one part of its course, and magnetically in another, the two actions are always found to take place together. A current has not, to my knowledge, been produced which could act chemically and not magnetically, nor any which can act on the magnet, and not _at the same time_ chemically[B].

[A] See now in relation to this subject, 1627-1645.--_Dec. 1838._

[B] Thermo-electric currents are of course no exception, because when they fail to act chemically they also fail to be currents.

517. _Judging from facts only_, there is not as yet the slightest reason for considering the influence which is present in what we call the electric current,--whether in metals or fused bodies or humid conductors, or even in air, flame, and rarefied elastic media,--as a compound or complicated influence. It has never been resolved into simpler or elementary influences, and may perhaps best be conceived of as _an axis of power having contrary forces, exactly equal in amount, in contrary directions_.

518. Pa.s.sing to the consideration of electro-chemical decomposition, it appears to me that the effect is produced by an _internal corpuscular action_, exerted according to the direction of the electric current, and that it is due to a force either _super to_, or _giving direction to the ordinary chemical affinity_ of the bodies present. The body under decomposition may be considered as a ma.s.s of acting particles, all those which are included in the course of the electric current contributing to the final effect; and it is because the ordinary chemical affinity is relieved, weakened, or partly neutralized by the influence of the electric current in one direction parallel to the course of the latter, and strengthened or added to in the opposite direction, that the combining particles have a tendency to pa.s.s in opposite courses.

519. In this view the effect is considered as _essentially dependent_ upon the _mutual chemical affinity_ of the particles of opposite kinds.

Particles _aa_, fig. 53, could not be transferred or travel from one pole N towards the other P, unless they found particles of the opposite kind _bb_, ready to pa.s.s in the contrary direction: for it is by virtue of their increased affinity for those particles, combined with their diminished affinity for such as are behind them in their course, that they are urged forward: and when any one particle _a_, fig. 54, arrives at the pole, it is excluded or set free, because the particle _b_ of the opposite kind, with which it was the moment before in combination, has, under the superinducing influence of the current, a greater attraction for the particle _a'_, which is before it in its course, than for the particle _a_, towards which its affinity has been weakened.

520. As far as regards any single compound particle, the case may be considered as a.n.a.logous to one of ordinary decomposition, for in fig. 54, _a_ may be conceived to be expelled from the compound _ab_ by the superior attraction of _a'_ for _b_, that superior attraction belonging to it in consequence of the relative position of _a'b_ and _a_ to the direction of the axis of electric power (517.) superinduced by the current. But as all the compound particles in the course of the current, except those actually in contact with the poles, act conjointly, and consist of elementary particles, which, whilst they are in one direction expelling, are in the other being expelled, the case becomes more complicated, but not more difficult of comprehension.

521. It is not here a.s.sumed that the acting particles must be in a right line between the poles. The lines of action which may be supposed to represent the electric currents pa.s.sing through a decomposing liquid, have in many experiments very irregular forms; and even in the simplest case of two wires or points immersed as poles in a drop or larger single portion of fluid, these lines must diverge rapidly from the poles; and the direction in which the chemical affinity between particles is most powerfully modified (519. 520.) will vary with the direction of these lines, according constantly with them. But even in reference to these lines or currents, it is not supposed that the particles which mutually affect each other must of necessity be parallel to them, but only that they shall accord generally with their direction. Two particles, placed in a line perpendicular to the electric current pa.s.sing in any particular place, are not supposed to have their ordinary chemical relations towards each other affected; but as the line joining them is inclined one way to the current their mutual affinity is increased; as it is inclined in the other direction it is diminished; and the effect is a maximum, when that line is parallel to the current[A].

[A] In reference to this subject see now electrolytic induction and discharge, Series XII. -- viii. 1343-1351, &c.--_Dec. 1838._

522. That the actions, of whatever kind they may be, take place frequently in oblique directions is evident from the circ.u.mstance of those particles being included which in numerous cases are not in a line between the poles.

Thus, when wires are used as poles in a gla.s.s of solution, the decompositions and recompositions occur to the right or left of the direct line between the poles, and indeed in every part to which the currents extend, as is proved by many experiments, and must therefore often occur between particles obliquely placed as respects the current itself; and when a metallic vessel containing the solution is made one pole, whilst a mere point or wire is used for the other, the decompositions and recompositions must frequently be still more oblique to the course of the currents.

523. The theory which I have ventured to put forth (almost) requires an admission, that in a compound body capable of electro-chemical decomposition the elementary particles have a mutual relation to, and influence upon each other, extending beyond those with which they are immediately combined. Thus in water, a particle of hydrogen in combination with oxygen is considered as not altogether indifferent to other particles of oxygen, although they are combined with other particles of hydrogen; but to have an affinity or attraction towards them, which, though it does not at all approach in force, under ordinary circ.u.mstances, to that by which it is combined with its own particle, can, under the electric influence, exerted in a definite direction, be made even to surpa.s.s it. This general relation of particles already in combination to other particles with which they are not combined, is sufficiently distinct in numerous results of a purely chemical character; especially in those where partial decompositions only take place, and in Berthollet's experiments on the effects of quant.i.ty upon affinity: and it probably has a direct relation to, and connexion with, attraction of aggregation, both in solids and fluids. It is a remarkable circ.u.mstance, that in gases and vapours, where the attraction of aggregation ceases, there likewise the decomposing powers of electricity apparently cease, and there also the chemical action of quant.i.ty is no longer evident. It seems not unlikely, that the inability to suffer decomposition in these cases may be dependent upon the absence of that mutual attractive relation of the particles which is the cause of aggregation.

524. I hope I have now distinctly stated, although in general terms, the view I entertain of the cause of electro-chemical decomposition, _as far as that cause can at present be traced and understood_. I conceive the effects to arise from forces which are _internal_, relative to the matter under decomposition--and _not external_, as they might be considered, if directly dependent upon the poles. I suppose that the effects are due to a modification, by the electric current, of the chemical affinity of the particles through or by which that current is pa.s.sing, giving them the power of acting more forcibly in one direction than in another, and consequently making them travel by a series of successive decompositions and recompositions in opposite directions, and finally causing their expulsion or exclusion at the boundaries of the body under decomposition, in the direction of the current, _and that_ in larger or smaller quant.i.ties, according as the current is more or less powerful (377.). I think, therefore, it would be more philosophical, and more directly expressive of the facts, to speak of such a body, in relation to the current pa.s.sing through it, rather than to the poles, as they are usually called, in contact with it; and say that whilst under decomposition, oxygen, chlorine, iodine, acids, &c., are rendered at its negative extremity, and combustibles, metals, alkalies, bases, &c., at its positive extremity (467.), I do not believe that a substance can be transferred in the electric current beyond the point where it ceases to find particles with which it can combine; and I may refer to the experiments made in air (465.) and in water (495.), already quoted, for facts ill.u.s.trating these views in the first instance; to which I will now add others.

525. In order to show the dependence of the decomposition and transfer of elements upon the chemical affinity of the substances present, experiments were made upon sulphuric acid in the following manner. Dilute sulphuric acid was prepared: its specific gravity was 1.0212. A solution of sulphate of soda was also prepared, of such strength that a measure of it contained exactly as much sulphuric acid as an equal measure of the diluted acid just referred to. A solution of pure soda, and another of pure ammonia, were likewise prepared, of such strengths that a measure of either should be exactly neutralized by a measure of the prepared sulphuric acid.

526. Four gla.s.s cups were then arranged, as in fig. 55; seventeen measures of the free sulphuric acid (525.) were put into each of the vessels _a_ and _b_, and seventeen measures of the solution of sulphate of soda into each of the vessels A and B. Asbestus, which had been well-washed in acid, acted upon by the voltaic pile, well-washed in water, and dried by pressure, was used to connect _a_ with _b_ and A with B, the portions being as equal as they could be made in quant.i.ty, and cut as short as was consistent with their performing the part of effectual communications, _b_ and A were connected by two platina plates or poles soldered to the extremities of one wire, and the cups _a_ and B were by similar platina plates connected with a voltaic battery of forty pairs of plates four inches square, that in _a_ being connected with the negative, and that in B with the positive pole.

The battery, which was not powerfully charged, was retained in communication above half an hour. In this manner it was certain that the same electric current had pa.s.sed through _a b_ and A B, and that in each instance the same quant.i.ty and strength of acid had been submitted to its action, but in one case merely dissolved in water, and in the other dissolved and also combined with an alkali.

527. On breaking the connexion with the battery, the portions of asbestus were lifted out, and the drops hanging at the ends allowed to fall each into its respective vessel. The acids in _a_ and _b_ were then first compared, for which purpose two evaporating dishes were balanced, and the acid from _a_ put into one, and that from _b_ into the other; but as one was a little heavier than the other, a small drop was transferred from the heavier to the lighter, and the two rendered equal in weight. Being neutralized by the addition of the soda solution (525.), that from _a_, or the negative vessel, required 15 parts of the soda solution, and that from _b_, or the positive vessel, required 16.3 parts. That the sum of these is not 34 parts is princ.i.p.ally due to the acid removed with the asbestus; but taking the mean of 15.65 parts, it would appear that a twenty-fourth part of the acid originally in the vessel _a_ had pa.s.sed, through the influence of the electric current, from _a_ into _b_.

528. In comparing the difference of acid in A and B, the necessary equality of weight was considered as of no consequence, because the solution was at first neutral, and would not, therefore, affect the test liquids, and all the evolved acid would be in B, and the free alkali in A. The solution in A required 3.2 measures of the prepared acid (525.) to neutralize it, and the solution in B required also 3.2 measures of the soda solution (525.) to neutralize it. As the asbestus must have removed a little acid and alkali from the gla.s.ses, these quant.i.ties are by so much too small; and therefore it would appear that about a tenth of the acid originally in the vessel A had been transferred into B during the continuance of the electric action.

529. In another similar experiment, whilst a thirty-fifth part of the acid pa.s.sed from _a_ to _b_; in the free acid vessels, between a tenth and an eleventh pa.s.sed from A to B in the combined acid vessels. Other experiments of the same kind gave similar results.

530. The variation of electro-chemical decomposition, the transfer of elements and their acc.u.mulation at the poles, according as the substance submitted to action consists of particles opposed more or less in their chemical affinity, together with the consequent influence of the latter circ.u.mstances, are sufficiently obvious in these cases, where sulphuric acid is acted upon in the _same quant.i.ty_ by the _same_ electric current, but in one case opposed to the comparatively weak affinity of water for it, and in the other to the stronger one of soda. In the latter case the quant.i.ty transferred is from two and a half to three times what it is in the former; and it appears therefore very evident that the transfer is greatly dependent upon the mutual action of the particles of the decomposing bodies[A].

[A] See the note to (675.),--_Dec. 1838._

531. In some of the experiments the acid from the vessels _a_ and _b_ was neutralized by ammonia, then evaporated to dryness, heated to redness, and the residue examined for sulphates. In these cases more sulphate was always obtained from _a_ than from _b_; showing that it had been impossible to exclude saline bases (derived from the asbestus, the gla.s.s, or perhaps impurities originally in the acid,) and that they had helped in transferring the acid into _b_. But the quant.i.ty was small, and the acid was princ.i.p.ally transferred by relation to the water present.

532. I endeavoured to arrange certain experiments by which saline solutions should be decomposed against surfaces of water; and at first worked with the electric machine upon a piece of bibulous paper, or asbestus moistened in the solution, and in contact at its two extremities with pointed pieces of paper moistened in pure water, which served to carry the electric current to and from the solution in the middle piece. But I found numerous interfering difficulties. Thus, the water and solutions in the pieces of paper could not be prevented from mingling at the point where they touched.