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As the cell in an atmosphere of carbonic-acid gas showed considerable action, in fact nearly half as much as that in the air, each cell was short-circuited for 23 hours, with the expectation that any oxygen in the closed vessel would be used up; and, indeed, the most prominent crystals of silver in the cell in carbonic-acid gas became reddened, while a cuprous deposit extended over the whole of the crystals in the other cell. When, however, the short wires were removed and the galvanometer interposed, the cell in the air gave a deflection of 136, practically the same as before, but that in carbonic-acid gas, instead of showing a great decrease, rose to 80. It was then found that the vessel containing the latter slowly admitted air; so the contents were swept out by a fresh stream of carbonic-acid gas, and it was made properly air-tight. After connexion by a short wire for 3 days the galvanometer indicated a deflection of 20, that of the cell in the air being 110, temperature 10° C. As this showed a very great reduction of the chemical action, carbonic-acid gas was again passed through the vessel for an hour or two; and after a connexion of two more days the indication of the galvanometer was only 3, while the other cell gave 115, the temperature being now 10°-5 C. The action, therefore, was at last reduced almost to nothing; and the original fault in the experiment brought out, perhaps more clearly than would otherwise be seen, how eagerly the solution will absorb even minute quantities of oxygen from the surrounding gas.
An important point to determine was the best strength of the copper nitrate solution. Six per cent. was generally preferred, for two reasons:-first, it gives about the maximum of effect-a solution four times as strong gives less than half the deflection, and a solution only a quarter as strong gives only two thirds; secondly, a stronger solution than this 6 per cent. is apt to produce a deposit, not of pure cuprous oxide, but of a subnitrate, which was supposed to clog up the silver crystals to a greater extent.
Another point investigated was the best proportion between the areas of the metallic surfaces. Experiments were made with vertical plates, in which the silver was kept at a uniform size and the copper was diminished by covering it more and more with varnish; and another set was made in which the copper remained the same, while the silver plate was reduced.
The results may be thus exhibited :
The increase of the copper surface, therefore, has comparatively little effect.
The increase, therefore, of the silver or negative metal causes an almost proportionate increase in the chemical action. doubtless, arises from the necessity of oxygen, and explains the value of the large surface exposed by the silver crystals in the tray.
The effect of heat on the action of this cell was examined; it increases the action greatly: thus an arrangement which gave a deflection of 40 at 20° C. gave one of 250 at 50° C.; and the augmentation was observed to be much more rapid in the higher than in the lower portions of this range of temperature.
If the formula given above for the reaction be a true one, it follows that every atom of copper deposited on the silver in the state of suboxide must be compensated by an atom of copper dissolved from the copper plate. This was proved quantitatively. In a cell that had been in action for a week the loss of the copper plate was 0-391 grm., while the suboxide deposited on the silver was found to be equivalent to 0.398 grm. of metallic copper. This deposit of suboxide, though it soon forms apparently a complete Phil. Mag. S. 4. Vol. 47. No. 309. Jan. 1874.
covering to the silver, does not greatly diminish the action; it is probably porous, besides being itself a conductor of electricity. In some cases we have found it deposited in crystals sufficiently large to be seen by the naked eye, and which are shown by a magnifyingglass to be regular octahedra.
The internal resistance of this battery, as might be expected, is small.
The electrolytic power of the current was examined. One cell, the plates of which were about two inches in diameter, was found sufficient to decompose such metallic salts as the nitrates of copper, silver, or lead, copper sulphate or stannous chloride, in aqueous solution, when platinum was used for the negative electrode, and for the positive the same metal as existed in the salt experimented on. Six cells were sufficient to decompose dilute sulphuric acid slowly and dilute hydrocloric acid pretty quickly, copper electrodes being employed.
The theoretical interest of this battery lies mainly in the fact that it differs essentially from every other galvanic arrangement, inasmuch as the binary compound in solution is incapable of being decomposed either by the positive metal alone or by the two metals in conjunction; it cannot serve, in fact, as the liquid element of the circuit without the presence of another body ready to combine with one of its constituents when set free.
Grove's gas-battery is essentially different from ours if the oxygen and hydrogen condensed on the platinum plates play the part of the two metals; but it closely resembles ours if hydrogen acts the part of the positive and platinum that of the negative metal; the dilute sulphuric acid, a hydrogen compound, will then be decomposed on account of the simultaneous presence of the oxygen, which can combine with the liberated hydrogen. Viewed in this manner Grove's gas-battery is only a special case of the general reaction which we have described in our previous paper; and the formula will be::
mPt+H2O+H, SO ̧+(n − 2)H.
The practical interest of our arrangement lies in the fact that it is an approximation towards a constant air-battery. Should it ever come into use elsewhere than on the lecture-table, it will probably be in the form of a combination of zinc and copper, with an aërated solution of zinc chloride; for that arrangement has an electromotive force six times that of the arrangement we have more particularly studied, and about three quarters that of a Daniell's cell. The numbers representing the difference of potential between the two metals, which were actually obtained by means of an electrometer belonging to Sir William Thomson, were :—
Chloride of zinc is preferred to the sulphate, as it offers less internal resistance, and a solution of 20 per cent. is recommended as about the best conductor*. A single cell of this description is capable of decomposing dilute sulphuric or hydrochloric acid when copper electrodes are employed. The two metals might be arranged as in a Daniell's battery; the zinc would of course require no amalgamation; and the whole might be left for weeks or months without any attention. The oxide of zinc produced generally falls to the bottom of the vessel, and may be separated whenever it is thought desirable.
The power is thus obtained at a minimum of expense; for the oxygen which combines with the zinc costs nothing. Such a battery would appear to be specially adapted to cases where the galvanic current has to be frequently broken, as in telegraphy; for at each period of rest it renews its strength by the absorption or diffusion of more oxygen from the air.
May 1, 1873.-William Spottiswoode, M.A., Treasurer and
Vice-President, in the Chair.
The following communication was read :
"On the Condensation of a Mixture of Air and Steam upon Cold Surfaces." By Osborne Reynolds, M.A., Fellow of Queen's College, Cambridge.
1. The object of this investigation is to ascertain how far the presence of a small quantity of air affects the power of a cold surface to condense steam. A priori it seemed probable that it might retard condensation very much; for when pure steam comes up to a cold surface and is condensed, it leaves an empty space which is immediately filled with fresh steam; so that the passage of the steam up to the cold surface is unobstructed, and if the surface could carry off the heat fast enough, then the rate of condensation would be unlimited. If, however, the steam is mixed with air, then, as the mixture comes into contact with the cold surface, the steam will be condensed and the air will be left between the fresh steam and the cold surface; so that after condensation has commenced that surface will be protected by a stratum of air, and fresh steam will have either to displace this or pass through it before it in turn can be condensed.
2. This question, besides its philosophical interest, has important practical bearings on the steam-engine.
First. If the quantity of air mixed with the steam affects the rate at which it condenses, then the ratio which the pressure of air bears to the pressure of steam in a condenser will materially affect * On the authority of Mr. Herbert MacLeod.
its efficiency: this is particularly important with reference to the surface-condenser.
Second. If air prevents the condensation of steam, then by sending air into the boiler of a high-pressure engine, the condensation at the surface of the cylinder will be prevented, which, if allowed to occur, becomes a source of great waste; for when the steam comes into a cold cylinder it condenses, heating the cylinder and leaving water, which will again be evaporated as soon as the steam escapes; and this, in evaporating, will cool the cylinder. By preventing this, the mixing of air with the steam would effect the same object as the steam-jacket, only in a more efficient manner; for the heat communicated to the steam in the cylinder from the jacket is not nearly so effective as that which is communicated from the boiler, in consequence of the steam in the cylinder being at a lower temperature than that in the boiler.
3. The experiments for this investigation were, by the kind permission of Dr. Roscoe, carried out by Mr. Pasley, a student in the Chemical Laboratory of the Owens College; and I beg to tender him my best thanks.
4. In making these experiments two objects were particularly kept in view :
First. To ascertain if there is a great difference in the rate of condensation of pure steam and a mixture of steam and air-to ascertain in fact whether pure steam condenses at an unlimited speed.
Second. To ascertain if (and according to what law) the effect of air on the condensation increases as the proportion of air to steam increases.
5. Of these two undertakings the first is much the most difficult. The rate of condensation of pure steam is so great that it is practically impossible to measure it; and to institute a comparison between this and the condensation of a mixture of steam and air is like comparing the infinite with the finite. It is practically impossible to keep any surface cold when an unlimited supply of pure steam is condensed upon it, so that under such circumstances the quantity of pure steam condensed is limited by the power of the surface to carry off the heat. The best method of obtaining a quantitative result seems to be by introducing sufficient cold water into a flask of steam to condense it all, and ascertain whether this condensation is effected suddenly or slowly.
6. The presence of hot water in the flask with the steam very much assists in ascertaining the rapidity of condensation. When there is no hot water in the flask, the condensation by the injected water is only a question of time; the gauge will come to the same point whether the condensation is quick or slow, the only difference being in the speed at which it will rise-a difference not easy to appreciate, especially when the motion is quick. But if hot water is present, then as the steam in the flask is condensed it is replaced by fresh steam from the water, and the interval between the condensation and the consequent ebullition is the only time