tact with the yet undecomposed deutoxide of hydrogen, the needle advanced to above 60. The same pair, immersed in common acid, would have deflected the needle no further than 29° or 30°. 11. The effect of the presence of oxygen at the negative element is well observed by making it, in water, the positive electrode of a voltaic battery. By this means oxygen is deposited on its surface, and is there ready to produce an extraordinary intensity. This deposit of oxygen is in fact the cause of the action of Ritter's secondary piles. 12. The following was also a very convenient method of showing the increase of intensity arising from the presence of oxygen. Some dilute sulphuric acid was agitated with chlorine until the former had taken up as much of the gas as it could. By pouring a solution of sulphate of oxide of silver into the liquid, I now precipitated chloride of silver, leaving sulphuric acid and oxygen in solution. When a pair, consisting of platinized silver and iron, was placed in the acid thus prepared, the galvanometer was permanently deflected 50°, and by agitating the platinized silver the needle advanced as far as 60°. When a piece of amalgamated zinc was substituted for the iron the permanent deflection of the needle was 65°, and by agitating the negative element as before, the needle advanced to 70°. Had the same pairs been immersed in a simple solution of sulphuric acid, the permanent deflections would have been no greater than 30° and 63°. 13. Similar results were obtained with the solution of chlorine, as might have been anticipated from its strong affinity for hydrogen. 14. From the above experiments, we see that the agitation of the negative element is productive of an increase of intensity, simply because it is thereby brought into contact with bodies capable of combining with the hydrogen, which would otherwise have been evolved from it. When those bodies are present in considerable quantities, as in (10), (12), and (13), the intensity of the current is great, even though the pair be left quiet, because then the negative plate can collect them readily upon its surface. Again, by causing the current to encounter great resistance, the effects of agitation which we have noticed are proportionally increased, because then the number of particles required for neutralizing the hydrogen is less. Hence it is that when I have used a resistance of 500 or 600 yards of thin wire, I have frequently found the deviation of the needle (even when the pair was left quiet in a common solution of sulphuric acid) considerably greater than was due to that resistance. This is also the probable reason why De la Rive in one instance* found the intensity of the copper-zinc pile the same, whether charged with water or nitric acid. 15. In the course of the preceding experiments I was forcibly struck with the very great intensity of the pairs at the moment of * Ann. de Chimie, 1836, part i, p. 179. their immersion, compared with that which they were able to maintain permanently. It appeared to me that the theories which had been put forth to explain the first effect of immersion, though seemingly plausible with regard to the zinc battery, were not at all equal to account for the same phenomenon as existing to a far greater extent when iron is used as a positive element. 16. A rod of iron and a small plate of platinized silver were immersed in a dilute solution of sulphuric acid. On connecting them with the galvanometer (5), the needle was permanently deflected 2910. After a few preliminary trials to ascertain the proper point, I caused the needle to be maintained by a glass weight at 55°, beyond which it was free to travel. I then exposed the platinized silver to the air during one minute of time. On re-immersing it the needle sprang as far as 60°, and then immediately recoiled to its resting place at 55°, thus indicating a transitory current of about 5710. 17. On exposing the platinized silver for 5" only, the transitory current, ascertained in a manner similar to that just mentioned, was 41°. 18. Greater effects were obtained by washing and drying the platinized silver before it was immersed. In this way the needle, adjusted at 62, would spring as far as 66°, indicating a transitory current of about 64°. Having now removed the glass weight, the needle took up a permanent position at 2910, as at the beginning of the experiments. 19. When, instead of the platinized silver, the positive element (iron) was exposed to the air, whether simply or in conjunction with washing and drying, no appreciable increase of intensity was occasioned by its immersion. And although, on a repetition of the experiment, I sometimes observed slight effects, I conceive that they were owing to the power which the negative element seems to possess of collecting upon its surface the air held in solution by the circumambient liquid. 20. With an arrangement of platinized silver and amalgamated zinc, I obtained results of a similar though less striking character. The galvanometer indicated a permanent deflection of 629, and after washing and drying the platinized silver, I had a transitory deviation of 72°. The iminersion of the amalgamated zinc, after washing and drying, produced no effect. 21. The maximum effects of immersion were produced in the following manner. A plate of silver was rubbed with a little nitric acid, and then exposed to a red heat, by means of which the film of nitrate of oxide of silver was decomposed and metallic silver reduced. When the plate prepared in this way was associated in By this process all the oxygen is not driven off, but a considerable quan tity remains adhering to the silver so tenaciously that it is not entirely removed by making the plate quite bright with glass paper. The oxygen thus deposited (it can hardly, I think, be considered as chemically combined with the silver) is the cause of the great intensity of the current immediately after immersion. By simply heating the silver to rodness the same general effects can be produced, though not to the same extent. W dilute sulphuric acid with a piece of iron, the needle would deviate 631 for some time, and then gradually decline until it took up a permanent situation at 291. By experimenting in the same way with amalgamated zinc as a positive element, I had a transitory deflection of 76°, and a permanent deflection of 63°. 22 Very trifling transitory effects were obtained by the immersion of iron, when that metal was associated with amalgamated zinc. But this might have been anticipated, because the transitory current is owing to the presence of oxygen on the negative plate; and it is obvious that the hydrogen evolved by the local action of the iron, would, whilst in a nascent state, combine with that oxygen, and thus prevent a great part of it from exercising any influence upon the intensity of the current. 23. An experiment was also made with an arrangement of copper, amalgamated zinc, and dilute sulphuric acid. It was able to deflect the needle 51° pretty permanently. On washing and drying the copper, and experimenting as in (16), I observed a transitory deflection of 72°. This experiment deserves attention, because it shows that the transitory current occasioned by the copper is the same as that exhibited by platinized silver when experimented with in the same way (20). I take it as an argument, that when copper is in its best state it forms with amalgamated zinc a battery as intense as the platinized silver. 24. That the transitory currents which we are discussing are not occasioned by the diffusion of the salt formed about the positive element during the cessation of voltaic action, is obvious from the fact that (when the proper precautions are observed) they are not produced by the agitation (8), or by the immersion (19 and 20) of the metal about which the salt is formed. And if anything can render this more evident, it is the fact that the immersion of the copper plate of a Daniell's battery causes the needle to advance little or no higher than its permanent situation, as might have been anticipated from the theory which refers the transitory effects to chemical combination at the negative plate, on account of the slight affinity of copper for oxygen. The following experiments are also decisive of this question. 25. A glass jar, a, Fig. 1, containing some dilute sulphuric acid, was placed upon the plate p p of an air-pump. A small rod of iron, i, was immersed in the liquid, and connected by means of the pumpplate to the galvanometer (5). An open receiver, r, was now placed over the jar, and the ground brass plate b, with its stuffing-box and and sliding rod (the latter having the small piece of platinized silver, s, affixed to its extremity), was placed on the top of the receiver. A copper wire, fastened to the ring of the sliding rod, connnected the platinized silver with the galvanometer. 26. The sliding rod was now moved until the platinized silver in connexion with it was immersed in the acidulated water. Then the pump was worked until a very excellent vacuum was obtained, and so tight was every part of the apparatus that it could be left alone for half an hour without the admission of any appreciable quantity of air. The galvanometer indicated a permanent deflection* of 279. I now placed a piece of glass so as to prevent the needle from going lower than 27°, and by means of the sliding rod I removed the platinized silver entirely out of the acid. After it had been exposed during a quarter of an hour, I re-immersed it, when the needle sprang from 27 to 30° and back, indicating a transitory deflection of about 28. Although the defect of immersion exhibited by this experiment is extremely small, it appeared to be almost entirely occasioned by the repose of the electric condition of the iron, for when, instead of entirely withdrawing the platinized silver, its extremity was just allowed to touch the liqutd, the transitory deflection was only 2710, after an exposure during a quarter of an hour. 27. On admitting a quantity of air into the receiver sufficient to counterbalance the pressure of one inch of mercury, the effects of immersion were considerable after a very short exposure of the platinized silver. In a quarter of an hour it collected upon its surface sufficient oxygen to cause the needle to spring from 27° to 78°, whether it had or had not remained in contact with the liquid during its exposure. 28. When, instead of the vacuum, I used an atmosphere of hydrogen, the exposure of the platinized silver for any length of time did not render the current more intense at the moment of immersion than it remained permanently. And even when the hydrogen was diluted with one quarter of its bulk of atmospheric air the transitory effects did not appear, on account, no doubt, of the uniont of the oxygen with the hydrogen as fast as the former, or both, collected upon the plate. On using a mixture of equal bulks of hydrogen and air, the transitory effects were very small, even after the platinized silver had been exposed for ten minutes. 29. I made several experiments with carbonic acid, but the transitory currents did not entirely disappear as was anticipated. The gas, though prepared carefully and in different ways, could not be obtained perfectly pure, and when exposed to an alkaline solution, th of it would remain uncondensed. In order therefore to remove any free oxygen which the gas might contain, I exposed it during two days and two nights to the action of a stick of phosphorus. After this, immersion caused no, or at most, very trivial transitory effects; but on admitting only one per cent. of oxygen they became very considerable,-a striking example of the power possessed by metals of collecting and condensing oxygen upon their surfaces. I do not bring forward this experiment as a proof of the No change in the permanent deflection of the needle was occasioned by the removal of atmospheric pressure. The phenomenon of Doebereiner, so fully investigated by Faraday, to whose paper, published in the Phil. Trans. for 1834, I refer the reader for some valuable observations on the power possessed by metals of condensing gases upon their surfaces. entire non-action of carbonic acid, because the phosphorous was found to have decomposed it partially. 30. All these phenomena are easily understood, if, with the great body of philosophers, we keep in view the intimate relation which subsists between chemical affinity and the electric current. For let p, Fig.2, represent a plate of platinum; z, a plate of zinc, or other electro-positive metal; and e, one of a series of atoms of water extending from p to z. The intensity of the current along the wire w, is proportional to the affinity of oxygen for the positive metal, minus the affinity of oxygen for hydrogen. But if p be covered with a film of oxygen, the current will be entirely proportional to the affinity of the positive metal for oxygen. In the former case, c = h; in the latter, c′ = z. 31. Considering these equations, it is obvious why, as I have observed (15), the transitory currents are better exhibited with iron than with zinc as a positive element; for in proportion to the smallness of z, provided it remain greater than h, will the difference between c and c' be more manifest. If c' c be the same for both iron and zinc, we shall have a proof of the accuracy of these principles, 32. Thus from (21), turning the deflections of the needle into quantities of electricity, we have 6310°-034 Q, and 2910 0.0072 Q, of which the difference is 0.0268 Q, when iron is the positive element. We have also 76° =0°.056 Q, and 63° = 0o·027 Q, of which the difference is 0°-029 Q, when zinc is the positive element. I consider these differences as nearly equal as could have been expected from the nature of the experiments. 33. I might now proceed to consider in detail several phenomena (such as the very rapid corrosion of metals when they are exposed to the joint action of air and moisture, &c.) which are occasioned by the great intensity of galvanic action, in consequence of the mixture of oxygen with the liquid. But I hasten to fulfil my principal design. Intensities of the Affinities which unite bodies with Oxygen. 34. In order to ascertain the intensities of galvanic arrangements we may either use a galvanometer furnished with a short and thick, wire, or with a long and thin wire (within certain limits (14)). In the former case, the calculations must be conducted on the principles of Ohm; in the latter it is only necessary to take care that the resistances of the pairs under comparison are pretty nearly equal, in order that the deviations of the needle may be depended upon in calculating the intensity of the current. I have adopted the latter plan on account of the superior facilities which it presents. 35. Affinity of zinc for oxygen.—From (32) we have the intensity of the action of zinc = 0.056 Q; and the intensity required for the electrolysis of water = 0° 020 Q. Hence 29: 56: : 1, the affinity of hydrogen, : 1.93, that of zinc for oxygen. 36. Affinity of iron for oxygen, likewise obtained from (32) is 1°.27, for 268: 340:: I: 127. |