ascertain the best proportions. When the mixture contained a less proportion of balsam than 35 per cent. the films were too brittle, and irregular in form. If it contained more than 70 per cent. of balsam the films did not readily harden, and were not formed without difficulty. A mixture of 55 per cent. of resin with 45 of balsam, which fused about 85°, gave good films, tough on cooling, but somewhat brittle. The mixture yielding the most satisfactory results contained 46 per cent. of resin and 54 of balsam. This mixture is sufficiently fused at 80° to be workable, but yields the best films at 93° to 95°. At 105° films can be obtained; and they are thinner than those formed from the more viscid fluid at 95°. At 110° films are still obtainable; and they frequently exhibit chromatic phenomena, but usually burst before hardening.

[The specimens exhibited to the Society are made with this mixture. They include a cubical frame of 2.5 centims. side, and a tetrahedral frame of 3.1 centims. side. Larger specimens have been obtained, however, though they generally show some imperfection of form. I have had a flat circular frame of 11 centims. diameter covered with a film of beautiful transparency. Brass wire appears better than iron for the frames.]

The films made with the mixture described are remarkably tough, and if preserved from rough handling appear to be of indefinite durability. A number of frames holding films have been hanging for over two months unprotected upon the wall of the laboratory of the writer, and are still intact. Brass wire of 0.33 millim. in diameter has been employed for the construction of the frames. When a thicker wire is used, the films become irregular from the longer retention of heat by the wire, and the consequent earlier cooling of the central portions of the films.

As with the soap-films, so with those of resinous matter, success depends largely upon the purity of material employed. Dust and oily matters must be scrupulously excluded; and the resin should be retained at a temperature near its boilingpoint for some time, to purify it of more volatile matter, before the balsam is mixed with it.

The most perfect films are obtained when the wire frames, after being dipped in the liquid, are removed to an air-bath at the temperature of about 80°, in which they are left, and the whole is allowed slowly to cool.

In proof of the toughness of the films, it may be mentioned that a recent flat film upon a circular frame of 4 centim. diameter of iron wire of 0.9 millim. gauge sustained, without breaking, the pressure of a cylindrical fifty-gramme weight, of 24 millims. diameter, placed upon its centre.

XXXIX. On Grove's Gas-Battery.

By HENRY FORSTER MORLEY, M.A., B.Sc.*

IT appears to me that the question as to the mode of action of the well-known gas-battery has not yet been definitely

settled.

1. The discoverer says, "The chemical or catalytic action can only be supposed to take place, with ordinary platina-foil, at the line or water-mark where the liquid, gas, and platina meet "t. Nevertheless he showed that water containing oxygen in one tube and hydrogen gas in the other tube gave a continuous current‡. As regards exp. 29 in the last-quoted excellent paper (viz. the experiment in which, hydrogen being in one tube and nitrogen in the other and no oxygen being dissolved in the liquid, hydrogen was found to appear in the nitrogen tube), as Mr. Grove does not say that there is a current, and as the presence of a current would contradict the conservation of energy, I am inclined to think that the effect is due to diffusion, and that it would occur whether the platinums were joined or not.

2. Mr. Justice Grove says that the phenomenon does not take place when the nitrogen is absent and its place filled by the liquid; and this is just what we should expect if the effect is due to diffusion. Mr. Grove thought it just possible that the hydrogen decomposed the water in its tube, combining with the oxygen, and that an equal amount of hydrogen was liberated in the other tube. Since the total amount of water is not changed, it is clear that such a decomposition could not be accompanied by a current.

3. Nevertheless Dr. Schönbein said that pure water containing no oxygen in one tube and an aqueous solution of hydrogen in the other gave a continuous current§. M. Gaugain makes the same assertion, but adds that he deprived his water of air by boiling. To boil water and then let it stand in the air is evidently not enough to deprive it of oxygen; hence these anomalous results may be due to the water not having been absolutely free from oxygen. Such a current, as before stated, would contradict conservation of energy: indeed it has been shown by Mr. Grove that water absolutely free from oxygen in one tube and hydrogen gas in the other tube produces no currentT.

*Communicated by the Physical Society.

† Phil. Mag. December 1842. See also Phil. Trans. 1843, p. 107. Phil. Trans. 1843, exp. 28 &c.

§ Phil. Mag. March 1843.

Comptes Rendus, February 25, 1867; Phil. Mag. June 1867. ¶ Phil. Trans. 1843, exp. 7 and elsewhere.

4. In one experiment Mr. Grove arranged his platinum plates, which I believe were platinized, in such a way as just to cut the surface of the liquid in the tubes: he got a strong current until the liquid rose above the platinum, when it became very weak. M. Gaugain says, and, I think, rightly, that this is due to the greater thickness of liquid through which the gas must now pass in order to get at the platinum-when the platinum is partly exposed the film along the line of junction being extremely thin.

5. M. Gaugain made a cell in which the platinum plates were movable, and determined, by the method of opposition, the electromotive force when the plates were partly exposed; he then lowered them until they were wholly immersed, and determined the electromotive force immediately. In this experiment the current was only allowed to flow for a few seconds. He found that the two determinations were the same, and concluded that the action of the battery depends entirely upon dissolved gas. It is, however, open to any one to assert that the platinums, when lowered, retained minute bubbles of gas on their surface, and that thus there were still many points of contact of liquid, gas, and platinum.

6. M. Gaugain, following Dr. Schönbein, asserts that "the oxygen serves simply to depolarize the positive wire," and "that its function is that of sulphate of copper in Daniell's cell"-in other words, that, were it not for the opposition current developed by the freshly-deposited hydrogen, the current could be kept up indefinitely without the presence of oxygen. As I have before stated, I cannot conceive this state of things.

I. In order to show that some, at all events, of the current in the gas-battery is due to dissolved gases, I made the following experiments in the laboratory of Professor Carey Foster:A gas-couple with wholly submerged non-platinized platinum plates was charged by electrolysis and short-circuited for a week, after which the lengths of the columns of oxygen and hydrogen were read off by means of a telescope on different days, the couple being all the while short-circuited. A similar couple, from which the platinum plates were removed after it had been charged, was similarly treated.

The barometer-reading was, of course, corrected for expansion, for the column of liquid below the gas, and for aqueous tension, the slight effect of sulphuric acid on the aqueous tension being neglected. A correction was applied for the curved ends of the tubes, and the corrected lengths reduced to 0° C. 760 millims.

Phil. Mag. S. 5. Vol. 5. No. 31. April 1878.

T

Practically the volume of the gas in these tubes was not altered by diffusion.

For the tubes which contained the platinum plates the lengths

The second line is calculated from the first by least squares, on the assumption of a uniform decrease of hydrogen. The greatest error is millim.; and 1.7 millim. has disappeared. The oxygen seems to have been supplied by the air.

II. On December 11 I joined the plates through a galvanometer of 6917 ohms resistance. The connexion through the galvanometer was made without previously breaking the circuit; yet a current was instantly shown; after 19 hours the deflection was 20 divisions. By comparison with a Daniell's cell whose electromotive force I assumed to be 1.1, I found that a deflection of 1 division indicated a current of '00000000056 electromagnetic unit.

If we assume that the current in the short circuit is the same as that passing through the galvanometer, an assumption which later experiments show to be not far from the truth, we shall find that 8 cubic millims. of hydrogen per week would be required to keep up this current. Now 32 cubic millims. have actually disappeared per week. The difference may be partly due to the inaccuracy of the assumption just made, and partly to the fact that some of the hydrogen combines with oxygen that has found its way from the air into the hydrogentube, the local currents thus produced not contributing to the main current.

III. An experiment similar to I., in which, however, the gases were prepared chemically, and in which there was also a gas-couple whose plates were not joined by a wire. The lengths of gas, in millimetres, corrected as before, were:For the couple with joined plates

In this case a good deal of gas seems to have been lost by diffusion.

The ratio of hydrogen lost to oxygen lost in the three cases is 4.8, 2.1, and 18 respectively. If we assume that 1.8 is the ratio of the gases lost through diffusion, and that the loss of oxygen in the first two cases is due solely to this cause, we shall find that 1.79 and 11 cubic centim. of hydrogen respectively still remain to be accounted for in the two cases. attribute this loss to local currents in the second case, and partly to these but chiefly to the main current in the first case, most of the necessary oxygen being supplied by the air to the liquid.

I

IV. If the hydrogen in a gas-couple with submerged plates be warmed by the hand, the current is increased; and if it be cooled the current is diminished: indeed it is very sensitive to changes of temperature, and of pressure also; and hence it is hardly possible to determine its strength with much accuracy. The further any horizontal layer of liquid in the hydrogen-tube is from the gas, the less hydrogen does it contain. Any expansion of the gas from heat or decrease of pressure brings a more saturated solution into contact with the immersed plate and the current increases, whereas contraction produces the opposite effect.

V. When a cell has been recently charged by electrolysis the current is at first very strong; but it soon falls off, and at last remains of nearly constant strength. This is because the water was at first saturated with the gas, but this gas being used up by the current takes some time to be restored by solution at the surface, and when equilibrium is attained the liquid round the plate will contain less dissolved gas the further it is from the surface. M. Gaugain attributed the falling-off in the strength of the current to the deposition of hydrogen on the positive plate; there is no need, however, for any such supposition. I employed a battery in which the plates were wholly immersed; and the final current varied with the depth of the top of the plate in the hydrogen-tube from the surface, and with the resistance in circuit, as the following Table shows: