hyper-oxide attracts from the nearest molecule of water its hydrogen, and thus causes electrolysis throughout the whole liquid. To cover a platinum plate with hyper-oxide of lead, it is connected with the positive pole of a battery of several pairs, whose negative pole is connected with a similar platinum plate. The two plates are now immersed in a solution of nitrate of lead, when the positive plate is at once covered with a layer of super-oxide of the metal. The current which a polarized platinum plate yields with a clean one, is, of course, transient; it disappears with the electro-motive coating of the plate, and this is removed necessarily in consequence of the formation of the current. For example, let us consider a positive platinum plate polarized by hydrogen; this being combined with a clean platinum plate, a current arises which passes from the coated to the clean plate; thus, at the coated plate, in consequence of the current, oxygen will escape, and combine with the hydrogen which appears there. In like manner, the strata of chlorine, hyper-oxide of lead, &c., with which the platinum plate has been negatively polarized, gradually disappear, the chlorine or oxygen of the super-oxide combining with the hydrogen escaping at this plate. Since platinum plates polarized by hyper-oxide are more strongly electro-negative than clean plates, by combining plates of zinc and platinum covered with hyper-oxide of lead, exceedingly powerful galvanic batteries can be constructed. The practical application of such batteries is as yet opposed by the fact that the stratum of super-oxide, the production of which is somewhat troublesome, very soon disappears. Wheatstone has given us a measurement of the electro-motive force of the hyper-oxide battery in the memoir already cited (Pog. Ann. LXII, 522.) He found for the electro-motive force of 30 1. Zinc amalgam, sulphate of copper, copper... 470 313 626 423 924 1081 451 68 1065 9. Potassa amalgam, dilute sulphuric acid, hyperoxide of lead .... 98 1535 10. Zinc amalgam, dilute sulphuric acid, hyper oxide of manganese............. 11. Potassa amalgam, dilute sulphuric acid, hyper oxide of manganese The first column of figures contains the values of the electro-motive forces measured by revolutions of Wheatstone's rheostat; the last column gives the values reduced to chemical measure, assuming that the electro-motive force of the first combination is equal to that of Daniell's battery. We see here how much greater an electro-motive force the combination of amalgamated zinc with hyper-oxide of lead indicates, than amalgamated zinc and platinum, even if care is taken, as in No. 3, to prevent galvanic polarization from taking place at the negative metal. The combination No. 3 is one of zinc and platinum corresponding to Daniell's battery. Metallic platinum will be separated from the solution of chloride and deposited upon the platinum plate by the current, thus hindering galvanic polarization, as in Daniell's battery by the deposition of copper. We can thus consider the numerical value of No. 3 above, namely, 626, as the measure of the electrical difference between amalgamated zinc and platinum. Comparing the electro-motive force of No. 3 with that of Grove's battery, we find a considerable difference, since the former is only 626, the latter 777, or according to my measurements 829, (section 18.) I think I can conclude from this difference that the nitric acid in Grove's, as well as Bunsen's battery, not only prevents polarization by the removal of oxygen, but that it acts as an electro-motor, also in the manner of the hyper-oxide. A circumstance which renders this view still more probable is this-that the electro-motive force of a combination of hyper-oxide of manganese with zinc, (No. 10,) is not sensibly greater than that of Grove and Bunsen's battery. The above table also shows how considerably the electro-motive force can be augmented by replacing the electro-positive amalgam of zinc, by the still more electro-positive amalgam of potassium; the expense of the latter amalgam, however, renders its practical application in such batteries impossible. $43. Grove's gas battery.-Grove's battery can be understood from Fig. 29, which represents a single element. A varnished metallic cover is fastened air-tight on the glass jar a. This cover has three openings; the glass tubes b and c pass air-tight through two of them. The third is somewhat larger and can be closed by a stopper. Each of the tubes is 30 centimetres long and 1.8 centimetre in diameter. At the upper end of each tube a platinum wire is fused into the glass, having at the top a cup for mercury, and to the other end of the wire a platinized platinum plate is soldered, which extends nearly to the lower end of the tube. The following is the process for charging such an element: Fill the vessel a with water, through the opening d; close d and then invert the whole apparatus; in this way the tubes b and c are filled with water. After restoring the element to an upright position, pass through the opening d the connecting tube of the gas apparatus. One of the tubes is filled in this way with hydrogen, and the other with oxygen to about the entire length. Fig. 30 represents a wooden trough intended to hold four such elements; it is exhibited on a scale one-fourth of that of Fig. 29. The elements being in position, the small mercury cups are connected by copper wires; into the last cup to the left a wire passes from the binding screw r, and into the last cup to the right, one from the binding screw 8. The poles u and v are fixed in the two binding screws. Fig. 31. This form of the gas battery is almost exactly the same as that which Grove describes as the most convenient, in the appendix to a memoir: "On the voltaic gas battery, its application to eudiometry." (Phil. Trans. 1843, Pt II, page 51; Pogg. Ann. im Ergänzungband II, 1848.) The arrangement, however, described in the memoir, admits of the removal of the tubes for the purpose of examining the gases. For this purpose the tubes b and c must not be cemented into the cover of the vessel a, but they must be inserted through corks so that they can be removed and replaced at pleasure. Fig. 31, represents the arrangement indicated by Grove in the above cited paper; a a is a glass vessel like a Woulfe's bottle; the middle opening is closed by a glass stopper; the glass tubes are adapted to the other openings by ground collars. § 44. Theory of the gas battery.-Schönbein has set forth his views on this subject in two memoirs in Poggendorff's Annalen; the first in volume LVIII, page 361; the second in volume LXXIV, page 241, His view is, "that the hydrogen, in the above described arrangement, with reference to the generation of the current, plays a primary, and the oxygen only a secondary or depolarizing part. The hydrogen alone is certainly able to generate a current of polarization, as Schönbein's experiments (in section 39) prove. A platinum plate, immersed but a short time in an atmosphere of hydrogen, gives, in combination with a clean platinum plate, a current, even if the liquid in which they are immersed contain no free oxygen. Therefore, it is clear that a Grove's battery must yield a current if one half the tube is entirely filled with acidified water, while the other half contains hydrogen, even if all free oxygen has been previously expelled from the liquid, and the entrance of atmospheric air is prevented. This current will soon cease, because, in consequence of it, the hydrogen disappears from the platinum plate not previously in contact with gas, and, therefore, the difference which caused the formation of the current disappears. If the current is to continue, then the hydrogen escaping at the other side, in consequence of the current, must be removed, and this is, according to Schönbein's view, the function of the oxygen in the gas battery. Schönbein, therefore, holds the opinion, that oxygen does not act in the gas battery as an electro-motor, but only as a depolarizer. He sustains this opinion by the observation, the credibility of which is unjustly disputed by the editor of the "Jahresbericht von Liebig und Kopp," that pure oxygen is unable to polarize a plate in the same manner as hydrogen does. The numerical values before given for the polarization of platinum plates in different gases, renders it possible to state the question in precise terms. The entire polarization in a voltameter is at a maximum about 1200; one-half of this polarization is due to the plate coated with hydrogen, the other half to the positive platinum plate coated with oxygen containing ozone. Now the question is: Is the electro-motive force of an element of the Grove gas battery equal to 1200; or is it, according to Schönbein's view, only 600? Although a platinum plate coated with pure oxygen, combined with another in acidified water, generates no current, yet there is here always an electrical difference, even though it should not be sufficient to bring about decomposition in the intermediate stratum of water; hence it is probable, that the electro-motive force of a Grove gas element, charged with hydrogen and pure oxygen, is greater than 600, if it does not attain the value 1200. At the first glance, nothing appears easier than to decide this question by measuring directly the electro-motive force of the gas battery; but a closer examination shows that such a measurement is utterly impossible. The platinum plates of the gas pile are not entirely coated with gas, but only partially. Therefore, we have here a similar case to that in which one of a pair of platinum plates is partially covered with zinc. By applying the different methods for determining the electro-motive force of the current, which here traverses the wire connecting the platinum plates, we shall certainly not obtain the true value of the electrical difference between zinc and platinum, (wholly disregarding the polarization which appears at the clean platinum plate). On account of the partial coating of the platinum plate with gas, lateral currents are formed, so that the current, which traverses the closing wire, is only a part of the effect produced by the electrical opposition in the battery; hence, also, in part, the exceedingly feeble force of the current in the gas pile. $45. Effects of the gas pile.-Grove obtained the following effects with a gas battery of 50 elements: 1. A shock which could be felt by five persons joining hands. 2. In a moderately sensitive galvanometer, the current produced a constant deflection of 60°. 3. Considerable divergence of a gold leaf electroscope. 4. Between charcoal points a spark visible in full day-light. 5. Electrolytic decomposition of iodide of potassium and acidified water. To produce a sensible decomposition of water, from cells of the above described construction, four elements are sufficient. A single cell decomposes iodide of potassium. A circuit of ten elements of this kind with dilute sulphuric acid of the spec. grav. 1.2, and filled alternately with hydrogen and oxygen, was closed with an interposed voltameter and left standing 36 hours. At the end of this time 2.1 cubic inches of detonating gas had been developed; in each of the hydrogen tubes 1.5 cubic inches had disappeared; in each of the oxygen tubes 0.7 cubic inch; thus, together, 2.2 cubic inches of gas had disappeared. The difference (2.2 to 2.1) is due to a small absorption of the oxygen by the water. If a sensible current is to be produced, the platinised platinum plates must not be wholly immersed beneath the surface of the water, but they must extend partly out of the liquid into the atmosphere of gas. A battery, whose tubes were charged alternately with hydrogen and dilute nitric acid, gave a current, and three pairs were sufficient to decompose water in an interposed voltameter. The gas pile yields a very powerful current if chlorine is substituted for oxygen. A chlorine and hydrogen battery of two elements is sufficient to decompose water between platinum plates. Carbonic oxide gas acts in the gas pile like hydrogen. Other gases-for example, nitrogen-are absolutely without effect. For instance, place a mixture of nitrogen and oxygen in one tube, and hydrogen in the other; after closing the circuit all the oxygen is gradually but completely absorbed, while the nitrogen remains the same. Grove's proposition to apply the gas pile in eudiometrical experiments is based upon this. In a second memoir, which may be found in Poggendorff's Annalen, (2to Ergänzungsbande, seite 407,) Grove describes the following remarkable experiment. One of the tubes of the gas pile was charged with oxygen; in the other a weighed piece of phosphorus was placed by means of a small glass Fig. 32 cup fastened to a glass rod, as represented in Fig. 32, and then the tube was partially filled with nitrogen. The apparatus indicated a current by an interposed galvanometer. After being closed four months, during which time the galvanometer constantly indicated a current, the water had increased in the oxygen tube one cubic inch, but not at all in the nitrogen tube; the piece of phosphorus, on the other hand, had become 0.4 grain lighter. This result is easily explained; the vapor of phosphorus was diffused in the atmosphere of nitrogen, and this acted exactly like hydrogen in the ordinary gas battery. |