It is therefore perfectly possible that, since the contact of the metals with the air is of such importance, it is there the action really lies, and that the ring is a chemical pair plunged in air instead of water. It is well known, indeed, that such a ring oxidizes in air as it does in water, only more slowly. Faraday mentions the following in proof of the chemical theory of the galvanic pair: Let (Fig. 2) A and B be two glass vessels containing phuret of potassium. Two platinum plates, P and P', are put into the vessel A, and an iron plate F, with a platinum plate P" in B. To the platinum plate P' a platinum wire p, and to the iron plate F an iron wire f, are attached. From P and P", wires proceed to the galvanometer G. The sulphuret of potassium is, for a liquid, a good conductor of electricity, but is chemically inactive when associated with platinum and iron iu a circuit. When the wires p and f are joined, if an clectro-motive force were developed at their surface of contact, all the conditions sary for a circuit being present, a current would be generated, which would deflect the needle of the galvanometer. This last, however, gives not the slightest evidence of a current. If zinc be interposed at the junction of p and f. the galvanometer is equally unaffected; but if a piece of paper moistened with sulphuric acid be placed between the ends of these wires, a decided deflection- ensues, and the iron becomes the positive element of a platinum-iron pair. We have thus elusive evidence that the simple contact of the iron and the platinum is unattended by electro-motive force, and that this is developed only by the chemical action upon the iron of an interposed liquid. Again. into one of the vessels just referred to, let two plates, one of copper, the other of silver, be placed, and let communication be lished between them and galvanometer. The needle at first deflects briskly in a direction which shows that copper is the positive element of the pair, it then gradually returns to its first position, and. again, deflects in the opposite direction; showing that the silver is now illonptis•tive element. MVO(' some time it returns, Mid again deflects in the original direction, and goes on thus changing. If the plates be examined during these changes, it is observed that sulphuret of copper is formed when the copper is positive, and sulphuret of silver when the silver is positive; the alternate action being attributable to the relative condition of the plates when coated with their sulphurets. The electromotive force of a silver copper pair is thus shown to be not invariable in direction as the contact theorists maintain; but to change its direction with the seat of chemical action.
Chentkal conditions (f the Galvanic Pair.—Wc have hitherto supposed that, in the. gahinic pair, the zinc alone had affinity for the oxygen of the water, but chemistry teaches us that copper likewise has the same . affinity, though to a less degree. Hence we must conclude that there originates at the copper an electromotive force acting con trary to that of the zinc; and that the electro-motive force of the pair is the difference of these opposing forces. Were we to take two similar plates of zinc, instead of one of Zinc And the other of copper, we should thus have two equal forces tending to propel two equal currents ill opposite directions. ,In this case the two forces would equilibrate each other, and electrical and chemical inaction would be the consequence, a conclusion quite in keeping with experiment. It therefore becomes necessary to couple the zinc with a metal such as copper, less oxidable that' itself. In keeping with this theory, is found that if the zinc be coupled with a metal less oxidable still ,thau copper, the resultant electro-motive force is increased. ,A pair consisting of zinc and silver gives an electricity of higher tension, and consequently a more powerfnl current than one of zinc and copper, and one of zinc and platinum a stronger current still: silver being less oxidable than copper, and platinum less than silver. As zinc forms the principal ele
meat of expense in maintaining the current, a platinum-zine pair is more economical than either of the other two just named, because, for the same quantity of zinc dissolved; it gives the best electrical result. The greater, thou, the disparity in oxidability, or in lia bility to be affected by the exciting liquid of the metals of the pair, the greater is its power.
In the galvanic cell we have found that not only the metals, but likewise the ele ments of the liquid, act as if they assumed opposite electricities. The zinc is positive with reference to the copper, and the hydrogen stands in the same relation to the oxy gen. In the " eleetro-eheiniml order of the elements" (q.v.), the elements are approxi mately arranged according to the part they would play if associated in a galvanic pair. beginning with potassium, the most electro-positive,* and ending with oxygen, the most electro-negative; each being positive to the one succeeding, and negative to the one pre ceding it. 'Chemically speaking, electro-positive has much the same meaning as oxid able. We may here repeat the more common elements in the same order: Potassium, sodium, magnesium, zinc, iron, lead, copper, silver, platinum, hydrogen, carbon, chlo rine. sulphur, oxygen. If it were proposed to ascertain from this .list the action of a platinum-iron pair hinnersed in a solution of hydrochloric acid (IICI), we should proceed to argue thus: Iron, preceding platinum, is positive in relation to it. Chlorine succeeds hydrogen. and is relatively negative. Chlorine, the negative element of the squid, would accordingly- be discharged at the electropositive iron, and the ferrous chloride (Feel) would be formed. The electro-positive hydrogen would be disengaged at the electro-negative platinum. The interpolar current, consequently, proceeds from the platinum to the iron. If, however, no chemical affinity existed between iron and chlo rine, no electricity would he generated, as chemical is essential to galvanic action. From such a list alone we cannot predict the result of any supposed combination. The metals themselves; as we have already seen, frequently change their relative positions, accord ing to the action of the liquid in which they are put, so that the order given is by no means absolute. The electro-negative plate remains in presence of the electro positive totally unaffected, and more so than if it were placed by itself in the exciting liquid. Hydrochloric acid, for instance, readily attacks iron; but if a piece of zinc he put into the liquid, and be made to touch it, the iron will remain untouched until the zinc has been first dissolved. Wherever, therefore, iron is exposed to corrosive action, it may be protected from it by coupling it with zinc. This accounts, in some degree, for the durability of iron coated with zinc, or, as it is called, "galvanized iron" (q.v.). In the same way zinc protects copper from corrosive action. On the other hand, zinc corrodes more readily in presence of these metals, and hence the necessity fOr using zinc nails for zinc roofs instead of iron or copper nails. When pure zinc is put into dilute sul phuric acid, almost no change is visible. whilst ordinary commercial zinc is rapidly dis solved by it. This arises, in all probability, from different portions of the latter stand ing in different chemical relations, arising from the heterogeneous structure introduced . by extraneous substances. Galvanic pairs are thus established within the metal. andthe metal dissolves in consequence. In a designed galvanic pair, local circuits would thus be formed at different parts of the zinc. plate, which, besides occasioning a useless waste of the metal, would lessen the strength of the main circuit, were it not found that amal gamated zinc possesses the properties of the pure metal.