Text: Another somewhat modified instance of the same effect is afforded by a voltrametic cell containing two electrodes of platinum, which are connected with a Daniell cell the electromotive force of which is insufficient to decompose the electrolyte. Under this condition the ions carried to the electrodes cannot give off their electric charges. The whole apparatus behaves, as was first emphasized by Sir W. Thomson, like a condenser of enormous capacity. The quantity of electricity, indeed, collected in a condenser under the same electromotive force is inversely proportional to the distance of the plates. If this is diminished to 1/100th, the condenser takes in one hundred times as much electricity as before. Now, bringing the two surfaces of platinum and of the liquid into immediate contact, we reduce their interval to molecular distances. The capacity of such a condenser has been measured by Messrs, Varley, Kohlrausch, and Colley. I have myself made some determinations which show that oxygen absorbed in the fluid is of great influence on the apparent value. By removing all traces of gas, I have got a value a little smaller than that of Kohlrausch, which shows that if we divide equally the total value of polarization between two platinum plates of equal size, the distance between the two strata of positive and negative electricity- the one lying on the last molecules of the metal, the other on those of the fluid- ought to be 1/10,000,000th (Kohlrausch 1/15,000,000th) of a millimeter. We always come nearly to the same limit when we calculate the distances through which molecular forces are able to act, as already shown in several other instances by Sir W. Thomson. Owing to the enormous capacity of such an electrolytic condenser, the quantity of electricity which enters into it, if it charged even by a feeble electromotive force, is sufficiently great to be indicated easily by a galvanometer. What I now call charging the condenser, I have before called polarizing the metallic plate. Both, indeed, are the same process, because electric motion is always accompanied in the electrolytes by chemical decomposition. Observing the polarizing and depolarizing currents in a cell like that represented in Fig. 1, we can observe these phenomena with the most feeble electromotive forces of 1/1000 Daniell, and I found that down to this limit the quantity of electricity entering into the condenser was proportional to the electromotive force by which it was collected. By taking larger surfaces of platinum, I suppose it will be possible to reach a limit much lower than that. If any chemical force existed, besides that of the electric charges, which could bind all the pairs of opposite ions together and required any amount of work to be vanquished, an inferior limit ought to exist to such electromotive forces as are able to attract the ions to the electrodes and to charge these as condensers. No phenomenon indicating such a limit has as yet been discovered, and we must therefore conclude that no other force resists the motions of the ions through the interior of the liquid than the mutual attractions of their electric charges. These are able to prevent the atoms of the same kind which repel each other from collecting at one place, and atoms of the other kind attracted by the former from collecting at any other part of the fluid, as long as no external electric force favors such distribution. The electric attraction, therefore, is able to produce an equal distribution of the opposite constituent atoms throughout the liquid, so that all parts of it are neutralized electrically as well as chemically. On the other hand, as soon as an ion is to be separated from its electric charge, we find that the electric forces of the battery meet with a powerful resistance, the overpowering of which requires a good deal of work to be done, usually the ions, losing their electric charges, are at the same time separated from the liquid; some of them are evolved as gases, others are deposited as rigid strata on the surface of the electrodes, like galvanoplastic copper. But the union of two constituents having powerful affinity to form a chemical compound always produces, as you know very well, a great amount of heat, and heat is equivalent to work. Conversely, a decomposition of the compound substance requires work, because it restores the energy, for when the hydrogen is burned in the oxygen, they unite, form water, and develop a great amount of heat. In the water the two elements are contained, and their chemical attraction continues to work as before to keep them firmly united, but it can no longer produce any change, any positive action. We must reduce the combined elements into their first state-we must separate them, applying a force which is capable of vanquishing their affinity-before they are ready to renew their first activity. The amount of heat produced by the chemical combination is the equivalent of the work done to separate the compound and to restore hydrogen and oxygen uncombined. I have already given the value of this amount calculated as a weight raised against the force of gravity. Metals uniting with oxygen or halogens produce heat in the same way-some of them, like potassium, sodium, and zinc, even more than an equivalent quantity of hydrogen; less oxidizable metals, like copper, silver, and platinum, less. We find, therefore, that heat is generated when zinc drives copper out of its combination with the compound halogen of sulfuric acid, as is the case in a Daniell cell. If a galvanic current passes through any conductor, a metallic wire or an electrolytic fluid, it evolves heat. Dr. Joule was the first who proved experimentally that if no other work is done by the current, the total amount of heat evolved in a galvanic circuit during a certain time is exactly equal to that which ought to have been generated by the chemical actions which have been performed during that time. But this heat is not evolved at the surface of the electrodes where these chemical actions take place; it is evolved in all the parts of the circuit, proportionally to the galvanic resistance of each part. From this it is evident that the heat evolved is an immediate effect, no of the chemical action, but of the galvanic current and that the chemical work of the battery has been spent to produce only the electric action To keep up an electric current through an electric conductor, indeed, requires work to be done. New stores of positive electricity must be continually introduced at the positive end of the conductor, the repulsive force acting upon them having to be overcome; negative electricity, in the same way, into the negative end. This can be done by mere mechanical force, with an electric machine working by friction, or by electrostatic or by electromagnetic induction. In a galvanic current it is done by chemical force, but the work required remains the same.

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