« ZurückWeiter »
dilute sulphuric acid with a piece of iron, the needle would deviate 634° for some time, and then gradually decline until it took up a permanent situation at 292 By experimenting in the same way with amalgamated zinc as a positive element, I had a transitory deAlection of 76°, and a permanent defection of 63o.
22 Very trilling 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 defect 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 bigher 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 riuz 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 270, 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. Afier it had been exposed during a quarter of an hour, I re-immersed it, when the needle sprang from 27° to 30o and back, indicating a transitory deflection of about 283. 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 27, 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 ihe 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, złoth 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 Dæbereiner, 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 = c – h; in the latter, d = 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 63.1 = 00.034 Q, and 2910 = 04.0072 Q, of which the difference is 0-.0268 Q, when iron is the positive element. We have also 76° 09.056 Q, and 639 = 00.027 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 :: 1:127.
* 37. Afinity of potassium for oxygen.—Twenty grains of potassium were combined with about ten ounces of mercury. The amalgam was poured into a wooden cup, into the bottom of which a copper wire connected with the galvanometer (5) had been let. At about half an inch above the surface of the amalgam I secured a piece of platinum, also in connexion with the galvanometer. On pouring dilute sulphuric acid into the cup the needle was deflected 74° (= 00:05 Q) during three successive minutes, but the local action of the amalgam was so vigourous that at the end of this interval of time most of the potassium was dissolved, and the needle declined very fast. On treating 20 grains of zinc in precisely the same manner, I had a deviation of 499 (= 02.0152 Q). Hence
k - h = 0.05, and
- h = 0.0152, whence
348 h. But from (35), z =
1.93, and h 1; therefore k, the affinity of potassium, = 4:06.
38. It is necessary, however, to pay attention to the circumstances under which the experiments were made, in order to obtain correct ideas concerning the above iutensities of affinity. The increase of the intensity of the voltaic apparatus by heat is by no means great; and as all the experiments were conducted at common temperatures, no regard need be paid to it. But then the intensities of affinity were obtained by comparing currents which had been produced under peculiar circumstances with regard to the condition of the elements of the galvanic arrangements ; in one case the hydrogen was evolved in a gaseous state ; whilst in the other, the hydrogen, by combining with free and condensed oygen, did not escape. Now we shall see from the following experiments that electric intensity is expended in the act of converting a body into the gaseous state.
39. I took ten glass jars (see Fig. 3), made them perfectly clean and dry*, and placed them in series on a non-conducting substance. Into these I poured a quantity of dilute sulphuric acid, taking care not to wet the glass within an inch of the top of each. Pairs of platinized silver and amalgamated zinc were placed in the jars, and connexions, furnished with the mercury cups 1, 2, 3, &c., were established between them seriatim. A decomposing cell, d, furnished with platinum wires, was connected on one hand with the battery, and on the other with the galvanometer (5). Lastly, I provided a copper wire, w, by means of which connexion could be conveniently made between the galvanometer g, and any of the mercury cups, 1, 2, 3, &c.
40. Into d I poured a small quantity of dilute sulphuric acid. Then, by placing the wire w in each of the mercury cups, beginning
* It is necessary to be very careful in insulating the apparatus, in order to obtain the maximum intensity of a battery. The divided porcelain trough has frequently great conducting powers (particularly when the glaze has been partially destroyed), which render it unfitfor accurate experiments.
at 10 and ending at 10, I observed the deviations of the galvanometer contained in the following table.
41. Now if we divide the straight line, AB, Fig. 4, into ten equal parts, representing pairs on Mr. Smee's plan, and if at each division we erect straight lines, perpendicular to À B and proportional to the comparative quantities of electricity just given, the principles of electric action demand that the line drawn through the extremities of those perpendiculars should be straight. It is in fact so nearly a straight line, that its slight discrepancies therefrom may be properly referred to unavoidable errors of experiment. Produce the straight line C D so as to meet A B in X, and the straight line A X, equal to 2.8, will indicate the number of pairs necessary to decompose water.
42. Fig. 5 represents an experiment of the same kind, with a solution of sulphate of oxide of zinc in the decomposing cell. Oxide of zinc was decomposed, the oxygen being evolved at the positive, and the zinc being reduced at the negative electrode. The intensity necessary to decompose oxide of zinc is equal to that of 3.7 of Mr. Smee's pairs.
43. With sulphate of protoxide of iron I did not at first succeed, on account of the formation of peroxide at the positive electrode. However, by placing the negative electrode among some crystals of the salt, pouring water thereon, and suspending the positive electrode in the water, I obviated that difficulty, and obtained the results which are projected in Fig. 6, and which indicates 3.3 pairs as the intensity necessary to decompose protoxide of iron.
44. Now from (41) and (42) we have (using the same letters as before) 2:8 pairs h, and 3.7 pairs = z; whence 2.8z = 3.7 h, andz=1.32 h; or, in other words, the intensity required to separate oxide of zinc into metal and gas is to the intensity required to separate water into its gaseous elements as 1:32: 1. But from (35), the