Part 29 (1/2)
If sulpho-nitric acid had been used in the exciting vessel, both the nitre and the chloride of lead would have suffered decomposition like the water (906.).
983. The results thus obtained of conduction without decomposition, and the necessity of a certain electrolytic intensity for the separation of the _ions_ of different electrolytes, are immediately connected with the experiments and results given in -- 10. of the Fourth Series of these Researches (418. 423. 444. 419.). But it will require a more exact knowledge of the nature of intensity, both as regards the first origin of the electric current, and also the manner in which it may be reduced, or lowered by the intervention of longer or shorter portions of bad conductors, whether decomposable or not, before their relation can be minutely and fully understood.
984. In the case of water, the experiments I have as yet made, appear to show, that, when the electric current is reduced in intensity below the point required for decomposition, then the degree of conduction is the same whether sulphuric acid, or any other of the many bodies which can affect its transferring power as an electrolyte, are present or not. Or, in other words, that the necessary electrolytic intensity for water is the same whether it be pure, or rendered a better conductor by the addition of these substances; and that for currents of less intensity than this, the water, whether pure or acidulated, has equal conducting power. An apparatus, fig.
84, was arranged with dilute sulphuric acid in the vessel A, and pure distilled water in the vessel B. By the decomposition at _c_, it appeared as if water was a _better_ conductor than dilute sulphuric acid for a current of such low intensity as to cause no decomposition. I am inclined, however, to attribute this apparent superiority of water to variations in that peculiar condition of the platina electrodes which is referred to further on in this Series (1040.), and which is a.s.sumed, as far as I can judge, to a greater degree in dilute sulphuric acid than in pure water. The power therefore, of acids, alkalies, salts, and other bodies in solution, to increase conducting power, appears to hold good only in those cases where the electrolyte subject to the current suffers decomposition, and loses all influence when the current transmitted has too low an intensity to affect chemical change. It is probable that the ordinary conducting power of an electrolyte in the solid state (419.) is the same as that which it possesses in the fluid state for currents, the tension of which is beneath the due electrolytic intensity.
985. Currents of electricity, produced by less than eight or ten series of voltaic elements, can be reduced to that intensity at which water can conduct them without suffering decomposition, by causing them to pa.s.s through three or four vessels in which water shall be successively interposed between platina surfaces. The principles of interference upon which this effect depends, will be described hereafter (1009. 1018.), but the effect may be useful in obtaining currents of standard intensity, and is probably applicable to batteries of any number of pairs of plates.
986. As there appears every reason to expect that all electrolytes will be found subject to the law which requires an electric current of a certain intensity for their decomposition, but that they will differ from each other in the degree of intensity required, it will be desirable hereafter to arrange them in a table, in the order of their electrolytic intensities.
Investigations on this point must, however, be very much extended, and include many more bodies than have been here mentioned before such a table can be constructed. It will be especially needful in such experiments, to describe the nature of the electrodes used, or, if possible, to select such as, like platina or plumbago in certain cases, shall have no power of a.s.sisting the separation of the _ions_ to be evolved (913).
987. Of the two modes in which bodies can transmit the electric forces, namely, that which is so characteristically exhibited by the metals, and usually called conduction, and that in which it is accompanied by decomposition, the first appears common to all bodies, although it occurs with almost infinite degrees of difference; the second is at present distinctive of the electrolytes. It is, however, just possible that it may hereafter be extended to the metals; for their power of conducting without decomposition may, perhaps justly, be ascribed to their requiring a very high electrolytic intensity for their decomposition.
987-1/2. The establishment of the principle that a certain electrolytic intensity is necessary before decomposition can be effected, is of great importance to all those considerations which arise regarding the probable effects of weak currents, such for instance as those produced by natural thermo-electricity, or natural voltaic arrangements in the earth. For to produce an effect of decomposition or of combination, a current must not only exist, but have a certain intensity before it can overcome the quiescent affinities opposed to it, otherwise it will be conducted, producing no permanent chemical effects. On the other hand, the principles are also now evident by which an opposing action can be so weakened by the juxtaposition of bodies not having quite affinity enough to cause direct action between them (913.), that a very weak current shall be able to raise the sum of actions sufficiently high, and cause chemical changes to occur.
988. In concluding this division _on the intensity necessary for electrolyzation_, I cannot resist pointing out the following remarkable conclusion in relation to intensity generally. It would appear that when a voltaic current is produced, having a certain intensity, dependent upon the strength of the chemical affinities by which that current is excited (916.), it can decompose a particular electrolyte without relation to the quant.i.ty of electricity pa.s.sed, the _intensity_ deciding whether the electrolyte shall give way or not. If that conclusion be confirmed, then we may arrange circ.u.mstances so that the _same quant.i.ty_ of electricity may pa.s.s in the _same time_, in at the _same surface_, into the _same decomposing body in the same state_, and yet, differing in intensity, will _decompose in one case and in the other not_:--for taking a source of too low an intensity to decompose, and ascertaining the quant.i.ty pa.s.sed in a given time, it is easy to take another source having a sufficient intensity, and reducing the quant.i.ty of electricity from it by the intervention of bad conductors to the same proportion as the former current, and then all the conditions will be fulfilled which are required to produce the result described.
-- iii. _On a.s.sociated Voltaic Circles, or the Voltaic Battery._
989. Pa.s.sing from the consideration of single circles (875. &c.) to their a.s.sociation in the voltaic battery, it is a very evident consequence, that if matters are so arranged that two sets of affinities, in place of being opposed to each other as in figg. 73. 76. (880. 891.), are made to act in conformity, then, instead of either interfering with the other, it will rather a.s.sist it. This is simply the case of two voltaic pairs of metals arranged so as to form one circuit. In such arrangements the activity of the whole is known to be increased, and when ten, or a hundred, or any larger number of such alternations are placed in conformable a.s.sociation with each other, the power of the whole becomes proportionally exalted, and we obtain that magnificent instrument of philosophic research, the _voltaic battery_.
990. But it is evident from the principles of definite action already laid down, that the _quant.i.ty_ of electricity in the current cannot be increased with the increase of the _quant.i.ty of metal_ oxidized and dissolved at each new place of chemical action. A single pair of zinc and platina plates throws as much electricity into the form of a current, by the oxidation of 32.5 grains of the zinc (868.) as would be circulated by the same alteration of a thousand times that quant.i.ty, or nearly five pounds of metal oxidized at the surface of the zinc plates of a thousand pairs placed in regular battery order. For it is evident, that the electricity which pa.s.ses across the acid from the zinc to the platina in the first cell, and which has been a.s.sociated with, or even evolved by, the decomposition of a definite portion of water in that cell, cannot pa.s.s from the zinc to the platina across the acid in the second cell, without the decomposition of the same quant.i.ty of water there, and the oxidation of the same quant.i.ty of zinc by it (924. 949.). The same result recurs in every other cell; the electro-chemical equivalent of water must be decomposed in each, before the current can pa.s.s through it; for the quant.i.ty of electricity pa.s.sed and the quant.i.ty of electrolyte decomposed, _must_ be the equivalents of each other. The action in each cell, therefore, is not to increase the quant.i.ty set in motion in any one cell, but to aid in urging forward that quant.i.ty, the pa.s.sing of which is consistent with the oxidation of its own zinc; and in this way it exalts that peculiar property of the current which we endeavour to express by the term _intensity_, without increasing the _quant.i.ty_ beyond that which is proportionate to the quant.i.ty of zinc oxidized in any single cell of the series.
991. To prove this, I arranged ten pairs of amalgamated zinc and platina plates with dilute sulphuric acid in the form of a battery. On completing the circuit, all the pairs acted and evolved gas at the surfaces of the platina. This was collected and found to be alike in quant.i.ty for each plate; and the quant.i.ty of hydrogen evolved at any one platina plate was in the same proportion to the quant.i.ty of metal dissolved from any one zinc plate, as was given in the experiment with a single pair (864. &c.). It was therefore certain, that, just as much electricity and no more had pa.s.sed through the series of ten pair of plates as had pa.s.sed through, or would have been put into motion by, any single pair, notwithstanding that ten times the quant.i.ty of zinc had been consumed.
992. This truth has been proved also long ago in another way, by the action of the evolved current on a magnetic needle; the deflecting power of one pair of plates in a battery being equal to the deflecting power of the whole, provided the wires used be sufficiently large to carry the current of the single pair freely; but the _cause_ of this equality of action could not be understood whilst the definite action and evolution of electricity (783. 869.) remained unknown.
993. The superior decomposing power of a battery over a single pair of plates is rendered evident in two ways. Electrolytes held together by an affinity so strong as to resist the action of the current from a single pair, yield up their elements to the current excited by many pairs; and that body which is decomposed by the action of one or of few pairs of metals, &c., is resolved into its _ions_ the more readily as it is acted upon by electricity urged forward by many alternations.
994. Both these effects are, I think, easily understood. Whatever _intensity_ may be, (and that must of course depend upon the nature of electricity, whether it consist of a fluid or fluids, or of vibrations of an ether, or any other kind or condition of matter,) there seems to be no difficulty in comprehending that the _degree_ of intensity at which a current of electricity is evolved by a first voltaic element, shall be increased when that current is subjected to the action of a second voltaic element, acting in conformity and possessing equal powers with the first: and as the decompositions are merely opposed actions, but exactly of the same kind as those which generate the current (917.), it seems to be a natural consequence, that the affinity which can resist the force of a single decomposing action may be unable to oppose the energies of many decomposing actions, operating conjointly, as in the voltaic battery.
995. That a body which can give way to a current of feeble intensity, should give way more freely to one of stronger force, and yet involve no contradiction to the law of definite electrolytic action, is perfectly consistent. All the facts and also the theory I have ventured to put forth, tend to show that the act of decomposition opposes a certain force to the pa.s.sage of the electric current; and, that this obstruction should be overcome more or less readily, in proportion to the greater or less intensity of the decomposing current, is in perfect consistency with all our notions of the electric agent.
996. I have elsewhere (947.) distinguished the chemical action of zinc and dilute sulphuric acid into two portions; that which, acting effectually on the zinc, evolves hydrogen at once upon its surface, and that which, producing an arrangement of the chemical forces throughout the electrolyte present, (in this case water,) tends to take oxygen from it, but cannot do so unless the electric current consequent thereon can have free pa.s.sage, and the hydrogen be delivered elsewhere than against the zinc. The electric current depends altogether upon the second of these; but when the current can pa.s.s, by favouring the electrolytic action it tends to diminish the former and increase the latter portion.
997. It is evident, therefore, that when ordinary zinc is used in a voltaic arrangement, there is an enormous waste of that power which it is the object to throw into the form of an electric current; a consequence which is put in its strongest point of view when it is considered that three ounces and a half of zinc, properly oxidized, can circulate enough electricity to decompose nearly one ounce of water, and cause the evolution of about 2100 cubic inches of hydrogen gas. This loss of power not only takes place during the time the electrodes of the battery are in communication, being then proportionate to the quant.i.ty of hydrogen evolved against the surface of any one of the zinc plates, but includes also _all_ the chemical action which goes on when the extremities of the pile are not in communication.
998. This loss is far greater with ordinary zinc than with the pure metal, as M. De la Rive has shown[A]. The cause is, that when ordinary zinc is acted upon by dilute sulphuric acid, portions of copper, lead, cadmium, or other metals which it may contain, are set free upon its surface; and these, being in contact with the zinc, form small but very active voltaic circles, which cause great destruction of the zinc and evolution of hydrogen, apparently upon the zinc surface, but really upon the surface of these incidental metals. In the same proportion as they serve to discharge or convey the electricity back to the zinc, do they diminish its power of producing an electric current which shall extend to a greater distance across the acid, and be discharged only through the copper or platina plate which is a.s.sociated with it for the purpose of forming a voltaic apparatus.
[A] Quarterly Journal of Science, 1831, p. 388; or Bibliotheque Universelle, 1830, p. 391.
999. All these evils are removed by the employment of an amalgam of zinc in the manner recommended by Mr. Kemp[A], or the use of the amalgamated zinc plates of Mr. Sturgeon (863.), who has himself suggested and objected to their application in galvanic batteries; for he says, ”Were it not on account of the brittleness and other inconveniences occasioned by the incorporation of the mercury with the zinc, amalgamation of the zinc surfaces in galvanic batteries would become an important improvement; for the metal would last much longer, and remain bright for a considerable time, even for several successive hours; essential considerations in the employment of this apparatus[B].”
[A] Jameson's Edinburgh Journal, October 1828.
[B] Recent Experimental Researches, p. 42, &c. Mr. Sturgeon is of course unaware of the definite production of electricity by chemical action, and is in fact quoting the experiment as the strongest argument _against_ the chemical theory of galvanism.
1000. Zinc so prepared, even though impure, does not sensibly decompose the water of dilute sulphuric acid, but still has such affinity for the oxygen, that the moment a metal which, like copper or platina, has little or no affinity, touches it in the acid, action ensues, and a powerful and abundant electric current is produced. It is probable that the mercury acts by bringing the surface, in consequence of its fluidity, into one uniform condition, and preventing those differences in character between one spot and another which are necessary for the formation of the minute voltaic circuits referred to (998.). If any difference does exist at the first moment, with regard to the proportion of zinc and mercury, at one spot on the _surface_, as compared with another, that spot having the least mercury is first acted on, and, by solution of the zinc, is soon placed in the same condition as the other parts, and the whole plate rendered superficially uniform. One part cannot, therefore, act as a discharger to another; and hence _all_ the chemical power upon the water at its surface is in that equable condition (949.), which, though it tends to produce an electric current through the liquid to another plate of metal which can act as a discharger (950.), presents no irregularities by which any one part, having weaker affinities for oxygen, can act as a discharger to another. Two excellent and important consequences follow upon this state of the metal.
The first is, that _the full equivalent_ of electricity is obtained for the oxidation of a certain quant.i.ty of zinc; the second, that a battery constructed with the zinc so prepared, and charged with dilute sulphuric acid, is active only whilst the electrodes are connected, and ceases to act or be acted upon by the acid the instant the communication is broken.
1001. I have had a small battery of ten pairs of plates thus constructed, and am convinced that arrangements of this kind will be very important, especially in the development and ill.u.s.tration of the philosophical principles of the instrument. The metals I have used are amalgamated zinc and platina, connected together by being soldered to platina wires, the whole apparatus having the form of the couronne des ta.s.ses. The liquid used was dilute sulphuric acid of sp. gr. 1.25. No action took place upon the metals except when the electrodes were in communication, and then the action upon the zinc was only in proportion to the decomposition in the experimental cell; for when the current was r.e.t.a.r.ded there, it was r.e.t.a.r.ded also in the battery, and no waste of the powers of the metal was incurred.