Experimental Researches in Electricity Part 25

846. The equivalent numbers do not profess to be exact, and are taken almost entirely from the chemical results of other philosophers in whom I could repose more confidence, as to these points, than in myself.

847. TABLE OF IONS.

_Anions_.

Oxygen 8 Chlorine 35.5 Iodine 126 Bromine 78.3 Fluorine 18.7 Cyanogen 26 Sulphuric acid 40 Selenic acid 64 Nitric acid 54 Chloric acid 75.5 Phosphoric acid 35.7 Carbonic acid 22 Boracic acid 24 Acetic acid 51 Tartaric acid 66 Citric acid 58 Oxalic acid 36 Sulphur (?) 16 Selenium (?) Salpho-cyanogen

_Cations_.

Hydrogen 1 Pota.s.sium 39.2 Sodium 23.3 Lithium 10 Barium 68.7 Strontium 43.8 Calcium 20.5 Magnesium 12.7 Manganese 27.7 Zinc 32.5 Tin 57.9 Lead 103.5 Iron 28 Copper 31.6 Cadmium 55.8 Cerium 46 Cobalt 29.5 Nickel 29.5 Antimony 61.67 Bis.m.u.th 71 Mercury 200 Silver 108 Platina 98.6?

Gold (?)

Ammonia 17 Pota.s.sa 47.2 Soda 31.3 Lithia 18 Baryta 76.7 Strontia 51.8 Lime 28.5 Magnesia 20.7 Alumina. (?) Protoxides generally.

Quinia 171.6 Cinchona 160 Morphia 290 Vegeto-alkalies generally.

848. This Table might be further arrange into groups of such substances as either act with, or replace, each other. Thus, for instance, acids and bases act in relation to each other; but they do not act in a.s.sociation with oxygen, hydrogen, or elementary substances. There is indeed little or no doubt that, when the electrical relations of the particles of matter come to be closely examined, this division must be made. The simple substances, with cyanogen, sulpho-cyanogen, and one or two other compound bodies, will probably form the first group; and the acids and bases, with such a.n.a.logous compounds as may prove to be _ions_, the second group.

Whether these will include all _ions_, or whether a third cla.s.s of more complicated results will be required, must be decided by future experiments.

849. It is _probable_ that all our present elementary bodies are _ions_, but that is not as yet certain. There are some, such as carbon, phosphorus, nitrogen, silicon, boron, alumium, the right of which to the t.i.tle of _ion_ it is desirable to decide as soon as possible. There are also many compound bodies, and amongst them alumina and silica, which it is desirable to cla.s.s immediately by unexceptionable experiments. It is also _possible_, that all combinable bodies, compound as well as simple, may enter into the cla.s.s of _ions_; but at present it does not seem to me probable. Still the experimental evidence I have is so small in proportion to what must gradually acc.u.mulate around, and bear upon, this point, that I am afraid to give a strong opinion upon it.

850. I think I cannot deceive myself in considering the doctrine of definite electro-chemical action as of the utmost importance. It touches by its facts more directly and closely than any former fact, or set of facts, have done, upon the beautiful idea, that ordinary chemical affinity is a mere consequence of the electrical attractions of the particles of different kinds of matter; and it will probably lead us to the means by which we may enlighten that which is at present so obscure, and either fully demonstrate the truth of the idea, or develope that which ought to replace it.

851. A very valuable use of electro-chemical equivalents will be to decide, in cases of doubt, what is the true chemical equivalent, or definite proportional, or atomic number of a body; for I have such conviction that the power which governs electro-decomposition and ordinary chemical attractions is the same; and such confidence in the overruling influence of those natural laws which render the former definite, as to feel no hesitation in believing that the latter must submit to them also. Such being the case, I can have, no doubt that, a.s.suming hydrogen as 1, and dismissing small fractions for the simplicity of expression, the equivalent number or atomic weight of oxygen is 8, of chlorine 36, of bromine 78.4, of lead 103.5, of tin 59, &c., notwithstanding that a very high authority doubles several of these numbers.

-- 13. _On the absolute quant.i.ty of Electricity a.s.sociated with the particles or atoms of Matter._

852. The theory of definite electrolytical or electro-chemical action appears to me to touch immediately upon the _absolute quant.i.ty_ of electricity or electric power belonging to different bodies. It is impossible, perhaps, to speak on this point without committing oneself beyond what present facts will sustain; and yet it is equally impossible, and perhaps would be impolitic, not to reason upon the subject. Although we know nothing of what an atom is, yet we cannot resist forming some idea of a small particle, which represents it to the mind; and though we are in equal, if not greater, ignorance of electricity, so as to be unable to say whether it is a particular matter or matters, or mere motion of ordinary matter, or some third kind of power or agent, yet there is an immensity of facts which justify us in believing that the atoms of matter are in some way endowed or a.s.sociated with electrical powers, to which they owe their most striking qualities, and amongst them their mutual chemical affinity.

As soon as we perceive, through the teaching of Dalton, that chemical powers are, however varied the circ.u.mstances in which they are exerted, definite for each body, we learn to estimate the relative degree of force which resides in such bodies: and when upon that knowledge comes the fact, that the electricity, which we appear to be capable of loosening from its habitation for a while, and conveying from place to place, _whilst it retains its chemical force_, can be measured out, and being so measured is found to be _as definite in its action_ as any of _those portions_ which, remaining a.s.sociated with the particles of matter, give them their _chemical relation_; we seem to have found the link which connects the proportion of that we have evolved to the proportion of that belonging to the particles in their natural state.

853. Now it is wonderful to observe how small a quant.i.ty of a compound body is decomposed by a certain portion of electricity. Let us, for instance, consider this and a few other points in relation to water. _One grain_ of water, acidulated to facilitate conduction, will require an electric current to be continued for three minutes and three quarters of time to effect its decomposition, which current must be powerful enough to retain a platina wire 1/104 of an inch in thickness[A], red-hot, in the air during the whole time; and if interrupted anywhere by charcoal points, will produce a very brilliant and constant star of light. If attention be paid to the instantaneous discharge of electricity of tension, as ill.u.s.trated in the beautiful experiments of Mr. Wheatstone[B], and to what I have said elsewhere on the relation of common and voltaic electricity (371. 375.), it will not be too much to say that this necessary quant.i.ty of electricity is equal to a very powerful flash of lightning. Yet we have it under perfect command; can evolve, direct, and employ it at pleasure; and when it has performed its full work of electrolyzation, it has only separated the elements of _a single grain of water_.

[A] I have not stated the length of wire used, because I find by experiment, as would be expected in theory, that it is indifferent.

The same quant.i.ty of electricity which, pa.s.sed in a given time, can heat an inch of platina wire of a certain diameter red-hot, can also heat a hundred, a thousand, or any length of the same wire to the same degree, provided the cooling circ.u.mstances are the same for every part in all cases. This I have proved by the volta-electrometer. I found that whether half an inch or eight inches were retained at one constant temperature of dull redness, equal quant.i.ties of water were decomposed in equal times. When the half-inch was used, only the centre portion of wire was ignited. A fine wire may even be used as a rough but ready regulator of a voltaic current; for if it be made part of the circuit, and the larger wires communicating with it be s.h.i.+fted nearer to or further apart, so as to keep the portion of wire in the circuit sensibly at the same temperature, the current pa.s.sing through it will be nearly uniform.

[B] Literary Gazette, 1833, March 1 and 8. Philosophical Magazine, 1833, p. 201. L'Inst.i.tut, 1833, p.261.

854. On the other hand, the relation between the conduction of the electricity and the decomposition of the water is so close, that one cannot take place without the other. If the water is altered only in that small degree which consists in its having the solid instead of the fluid state, the conduction is stopped, and the decomposition is stopped with it.

Whether the conduction be considered as depending upon the decomposition, or not (443. 703.), still the relation of the two functions is equally intimate and inseparable.

855. Considering this close and twofold relation, namely, that without decomposition transmission of electricity does not occur; and, that for a given definite quant.i.ty of electricity pa.s.sed, an equally definite and constant quant.i.ty of water or other matter is decomposed; considering also that the agent, which is electricity, is simply employed in overcoming electrical powers in the body subjected to its action; it seems a probable, and almost a natural consequence, that the quant.i.ty which pa.s.ses is the _equivalent_ of, and therefore equal to, that of the particles separated; i.e. that if the electrical power which holds the elements of a grain of water in combination, or which makes a grain of oxygen and hydrogen in the right proportions unite into water when they are made to combine, could be thrown into the condition of _a current_, it would exactly equal the current required for the separation of that grain of water into its elements again.

856. This view of the subject gives an almost overwhelming idea of the extraordinary quant.i.ty or degree of electric power which naturally belongs to the particles of matter; but it is not inconsistent in the slightest degree with the facts which can be brought to bear on this point. To ill.u.s.trate this I must say a few words on the voltaic pile[A].

[A] By the term voltaic pile, I mean such apparatus or arrangement of metals as up to this time have been called so, and which contain water, brine, acids, or other aqueous solutions or decomposable substances (476.), between their plates. Other kinds of electric apparatus may be hereafter invented, and I hope to construct some not belonging to the cla.s.s of instruments discovered by Volta.

857. Intending hereafter to apply the results given in this and the preceding series of Researches to a close investigation of the source of electricity in the voltaic instrument, I have refrained from forming any decided opinion on the subject; and without at all meaning to dismiss metallic contact, or the contact of dissimilar substances, being conductors, but not metallic, as if they had nothing to do with the origin of the current,

I still am fully of opinion with Davy, that it is at least continued by chemical action, and that the supply const.i.tuting the current is almost entirely from that source.

858. Those bodies which, being interposed between the metals of the voltaic pile, render it active, _are all of them electrolytes_ (476.); and it cannot but press upon the attention of every one engaged in considering this subject, that in those bodies (so essential to the pile) decomposition and the transmission of a current are so intimately connected, that one cannot happen without the other. This I have shown abundantly in water, and numerous other cases (402. 476.). If, then, a voltaic trough have its extremities connected by a body capable of being decomposed, as water, we shall have a continuous current through the apparatus; and whilst it remains in this state we may look at the part where the acid is acting upon the plates, and that where the current is acting upon the water, as the reciprocals of each other. In both parts we have the two conditions _inseparable in such bodies as these_, namely, the pa.s.sing of a current, and decomposition; and this is as true of the cells in the battery as of the water cell; for no voltaic battery has as yet been constructed in which the chemical action is only that of combination: _decomposition is always included_, and is, I believe, an essential chemical part.

859. But the difference in the two parts of the connected battery, that is, the decomposition or experimental cell, and the acting cells, is simply this. In the former we urge the current through, but it, apparently of necessity, is accompanied by decomposition: in the latter we cause decompositions by ordinary chemical actions, (which are, however, themselves electrical,) and, as a consequence, have the electrical current; and as the decomposition dependent upon the current is definite in the former case, so is the current a.s.sociated with the decomposition also definite in the latter (862. &c.).

860. Let us apply this in support of what I have surmised respecting the enormous electric power of each particle or atom of matter (856.). I showed in a former series of these Researches on the relation by measure of common and voltaic electricity, that two wires, one of platina and one of zinc, each one-eighteenth of an inch in diameter, placed five-sixteenths of an inch apart, and immersed to the depth of five-eighths of an inch in acid, consisting of one drop of oil of vitriol and four ounces of distilled water at a temperature of about 60 Fahr., and connected at the other extremities by a copper wire eighteen feet long, and one-eighteenth of an inch in thickness, yielded as much electricity in little more than three seconds of time as a Leyden battery charged by thirty turns of a very large and powerful plate electric machine in full action (371.). This quant.i.ty, though sufficient if pa.s.sed at once through the head of a rat or cat to have killed it, as by a flash of lightning, was evolved by the mutual action of so small a portion of the zinc wire and water in contact with it, that the loss of weight sustained by either would be inappreciable by our most delicate instruments; and as to the water which could be decomposed by that current, it must have been insensible in quant.i.ty, for no trace of hydrogen appeared upon the surface of the platina during those three seconds.

861. What an enormous quant.i.ty of electricity, therefore, is required for the decomposition of a single grain of water! We have already seen that it must be in quant.i.ty sufficient to sustain a platina wire 1/104 of an inch in thickness, red-hot, in contact with the air, for three minutes and three quarters (853.), a quant.i.ty which is almost infinitely greater than that which could be evolved by the little standard voltaic arrangement to which I have just referred (860. 871.). I have endeavoured to make a comparison by the loss of weight of such a wire in a given time in such an acid, according to a principle and experiment to be almost immediately described (862.); but the proportion is so high that I am almost afraid to mention it. It would appear that 800,000 such charges of the Leyden battery as I have referred to above, would be necessary to supply electricity sufficient to decompose a single grain of water; or, if I am right, to equal the quant.i.ty of electricity which is naturally a.s.sociated with the elements of that grain of water, endowing them with their mutual chemical affinity.

862. In further proof of this high electric condition of the particles of matter, and the _ident.i.ty as to quant.i.ty of that belonging to them with that necessary for their separation_, I will describe an experiment of great simplicity but extreme beauty, when viewed in relation to the evolution of an electric current and its decomposing powers.

863. A dilute sulphuric acid, made by adding about one part by measure of oil of vitriol to thirty parts of water, will act energetically upon a piece of zinc plate in its ordinary and simple state: but, as Mr. Sturgeon has shown[A], not at all, or scarcely so, if the surface of the metal has in the first instance been amalgamated; yet the amalgamated zinc will act powerfully with platina as an electromotor, hydrogen being evolved on the surface of the latter metal, as the zinc is oxidized and dissolved. The amalgamation is best effected by sprinkling a few drops of mercury upon the surface of the zinc, the latter being moistened with the dilute acid, and rubbing with the fingers or two so as to extend the liquid metal over the whole of the surface. Any mercury in excess, forming liquid drops upon the zinc, should be wiped off[B].

[A] Recent Experimental Researches, &c., 1830, p.74, &c.

[B] The experiment may be made with pure zinc, which, as chemists well know, is but slightly acted upon by dilute sulphuric acid in comparison with ordinary zinc, which during the action is subject to an infinity of voltaic actions. See De la Rive on this subject, Bibliotheque Universelle, 1830, p.391.

864. Two plates of zinc thus amalgamated were dried and accurately weighed; one, which we will call A, weighed 163.1 grains; the other, to be called B, weighed 148.3 grains. They were about five inches long, and 0.4 of an inch wide. An earthenware pneumatic trough was filled with dilute sulphuric acid, of the strength just described (863.), and a gas jar, also filled with the acid, inverted in it[A]. A plate of platina of nearly the same length, but about three times as wide as the zinc plates, was put up into this jar. The zinc plate A was also introduced into the jar, and brought in contact with the platina, and at the same moment the plate B was put into the acid of the trough, but out of contact with other metallic matter.

[A] The acid was left during a night with a small piece of unamalgamated zinc in it, for the purpose of evolving such air as might be inclined to separate, and bringing the whole into a constant state.

865. Strong action immediately occurred in the jar upon the contact of the zinc and platina plates. Hydrogen gas rose from the platina, and was collected in the jar, but no hydrogen or other gas rose from _either_ zinc plate. In about ten or twelve minutes, sufficient hydrogen having been collected, the experiment was stopped; during its progress a few small bubbles had appeared upon plate B, but none upon plate A. The plates were washed in distilled water, dried, and reweighed. Plate B weighed 148.3 grains, as before, having lost nothing by the direct chemical action of the acid. Plate A weighed 154.65 grains, 8.45 grains of it having been oxidized and dissolved during the experiment.

866. The hydrogen gas was next transferred to a water-trough and measured; it amounted to 12.5 cubic inches, the temperature being 52, and the barometer 29.2 inches. This quant.i.ty, corrected for temperature, pressure, and moisture, becomes 12.15453 cubic inches of dry hydrogen at mean temperature and pressure; which, increased by one half for the oxygen that must have gone to the _anode_, i.e. to the zinc, gives 18.232 cubic inches as the quant.i.ty of oxygen and hydrogen evolved from the water decomposed by the electric current. According to the estimate of the weight of the mixed gas before adopted (791.), this volume is equal to 2.3535544 grains, which therefore is the weight of water decomposed; and this quant.i.ty is to 8.45, the quant.i.ty of zinc oxidized, as 9 is to 32.31. Now taking 9 as the equivalent number of water, the number 32.5 is given as the equivalent number of zinc; a coincidence sufficiently near to show, what indeed could not but happen, that for an equivalent of zinc oxidized an equivalent of water must be decomposed[A].

[A] The experiment was repeated several times with the same results.

867. But let us observe _how_ the water is decomposed. It is electrolyzed, i.e. is decomposed voltaically, and not in the ordinary manner (as to appearance) of chemical decompositions; for the oxygen appears at the _anode_ and the hydrogen at the _cathode_ of the body under decomposition, and these were in many parts of the experiment above an inch asunder.

Again, the ordinary chemical affinity was not enough under the circ.u.mstances to effect the decomposition of the water, as was abundantly proved by the inaction on plate B; the voltaic current was essential. And to prevent any idea that the chemical affinity was almost sufficient to decompose the water, and that a smaller current of electricity might, under the circ.u.mstances, cause the hydrogen to pa.s.s to the _cathode_, I need only refer to the results which I have given (807. 813.) to shew that the chemical action at the electrodes has not the slightest influence over the _quant.i.ties_ of water or other substances decomposed between them, but that they are entirely dependent upon the quant.i.ty of electricity which pa.s.ses.

868. What, then, follows as a necessary consequence of the whole experiment? Why, this: that the chemical action upon 32.31 parts, or one equivalent of zinc, in this simple voltaic circle, was able to evolve such quant.i.ty of electricity in the form of a current, as, pa.s.sing through water, should decompose 9 parts, or one equivalent of that substance: and considering the definite relations of electricity as developed in the preceding parts of the present paper, the results prove that the quant.i.ty of electricity which, being naturally a.s.sociated with the particles of matter, gives them their combining power, is able, when thrown into a current, to separate those particles from their state of combination; or, in other words, that _the electricity which decomposes, and that which is evolved by the decomposition of a certain quant.i.ty of matter, are alike._

869. The harmony which this theory of the definite evolution and the equivalent definite action of electricity introduces into the a.s.sociated theories of definite proportions and electrochemical affinity, is very great. According to it, the equivalent weights of bodies are simply those quant.i.ties of them which contain equal quant.i.ties of electricity, or have naturally equal electric powers; it being the ELECTRICITY which _determines_ the equivalent number, _because_ it determines the combining force. Or, if we adopt the atomic theory or phraseology, then the atoms of bodies which are equivalents to each other in their ordinary chemical action, have equal quant.i.ties of electricity naturally a.s.sociated with them. But I must confess I am jealous of the term _atom_; for though it is very easy to talk of atoms, it is very difficult to form a clear idea of their nature, especially when compound bodies are under consideration.

870. I cannot refrain from recalling here the beautiful idea put forth, I believe, by Berzelius (703.) in his development of his views of the electro-chemical theory of affinity, that the heat and light evolved during cases of powerful combination are the consequence of the electric discharge which is at the moment taking place. The idea is in perfect accordance with the view I have taken of the _quant.i.ty_ of electricity a.s.sociated with the particles of matter.

871. In this exposition of the law of the definite action of electricity, and its corresponding definite proportion in the particles of bodies, I do not pretend to have brought, as yet, every case of chemical or electro-chemical action under its dominion. There are numerous considerations of a theoretical nature, especially respecting the compound particles of matter and the resulting electrical forces which they ought to possess, which I hope will gradually receive their development; and there are numerous experimental cases, as, for instance, those of compounds formed by weak affinities, the simultaneous decomposition of water and salts, &c., which still require investigation. But whatever the results on these and numerous other points may be, I do not believe that the facts which I have advanced, or even the general laws deduced from them, will suffer any serious change; and they are of sufficient importance to justify their publication, though much may yet remain imperfect or undone. Indeed, it is the great beauty of our science, CHEMISTRY, that advancement in it, whether in a degree great or small, instead of exhausting the subjects of research, opens the doors to further and more abundant knowledge, overflowing with beauty and utility, to those who will be at the easy personal pains of undertaking its experimental investigation.

872. The definite production of electricity (868.) in a.s.sociation with its definite action proves, I think, that the current of electricity in the voltaic pile: is sustained by chemical decomposition, or rather by chemical action, and not by contact only. But here, as elsewhere (857.), I beg to reserve my opinion as to the real action of contact, not having yet been able to make up my mind as to whether it is an exciting cause of the current, or merely necessary to allow of the conduction of electricity, otherwise generated, from one metal to the other.

873. But admitting that chemical action is the source of electricity, what an infinitely small fraction of that which is active do we obtain and employ in our voltaic batteries! Zinc and platina wires, one-eighteenth of an inch in diameter and about half an inch long, dipped into dilute sulphuric acid, so weak that it is not sensibly sour to the tongue, or scarcely to our most delicate test-papers, will evolve more electricity in one-twentieth of a minute (860.) than any man would willingly allow to pa.s.s through his body at once. The chemical action of a grain of water upon four grains of zinc can evolve electricity equal in quant.i.ty to that of a powerful thunder-storm (868. 861.). Nor is it merely true that the quant.i.ty is active; it can be directed and made to perform its full equivalent duty (867. &c.). Is there not, then, great reason to hope and believe that, by a closer _experimental_ investigation of the principles which govern the development and action of this subtile agent, we shall be able to increase the power of our batteries, or invent new instruments which shall a thousandfold surpa.s.s in energy those which we at present possess?

874. Here for a while I must leave the consideration of the _definite chemical action of electricity_. But before I dismiss this series of experimental Researches, I would call to mind that, in a former series, I showed the current of electricity was also _definite in its magnetic action_ (216. 366. 367. 376. 377.); and, though this result was not pursued to any extent, I have no doubt that the success which has attended the development of the chemical effects is not more than would accompany an investigation of the magnetic phenomena.

_Royal Inst.i.tution, December 31st, 1833._