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A small bell-jar which terminated in a corked orifice was inverted and partly filled with mercury, over which was poured a solution of mercuric chloride; a wire passed from the mercury through the cork to a plate of platinum which hung in the solution, without, however, touching the mercury. On pouring the mercuric chloride

upon

the mercury, the metallic surface was at once dimmed by a film of mercurous chloride, which increased in quantity; and in the course of an hour or two the insoluble chloride appeared also on the platinum plate, and in twenty-four hours it was sufficiently thick to permit of its removal. On repeating this experiment with gold instead of platinum, the same mercurous chloride was deposited; but at the same time the gold plate was amalgamated, showing that the reduction of the mercuric chloride had not stopped at the first stage, but had actually proceeded to the separation of the metal itself. A similar gold plate immersed in the same solution of corrosive sublimate, but not in connexion with the mercury, showed no trace of deposit or amalgamation; and it was ascertained that gold alone has no power of decomposing moistened mercurous chloride.

An experiment was made with a current ab extra. A cell of Grove's was found to decompose mercuric chloride with the formation of the mercurous compound at the negative platinum electrode, while chlorine was given off at the positive one.

This is in unison with what was found in the case of the copper salts.

That this action does not depend on the insolubility of the -ous chlorides was proved by the behaviour of the iron salts. Ordinary metallic iron is capable.of reducing the ferric to the ferrous salt at the common temperature; thus :

2 FeCl3 + Fe=3 FeCl. Platinum does not effect such a reduction; but when the iron is connected with platinum the change takes place more rapidly, and the reduced salt forms also on the negative metal. This, of course, does not render itself evident by any deposit, nor by any appreciable change of colour at first; but if the platinum plate be lifted out of the solution, and the liquid clinging to it be allowed to drain on to a paper moistened with some ferridcyanide of potassium, Turnbull's blue is the result. Or if a few drops of the ferridcyanide be mixed with the solution of the ferric chloride, on the junction of the iron and platinum the blue colour makes its appearance against each metal. Of course care was taken that the original salt contained no acid. The solution employed was one of 3.5 per cent. in strength. If

plates of magnesium and platinum be immersed in ferric chloride, metallic iron quickly makes its appearance on the pla

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With a weak external battery and platinum electrodes the salt was resolved into chlorine and ferrous chloride, but with a strong battery into chlorine and iron—thus affording another instance of the close analogy between the primary electrolysis produced in the cell itself, and the secondary electrolysis that may be produced by an external battery.

L. Researches in Acoustics.
By ALFRED M. MAYER*.–No. VII.

[Continued from p. 365.] Experiments on the Reflection of Sound from Flames and Heated

Gases. THE reading of the recent interesting research of Professor

Tyndall on “Experimental Demonstrations of the Stoppage of Sound by partial Reflections in a non-homogeneous Atmosphere (Proc. Roy. Soc. Jan. 1874; Nature,' Jan. 29, Feb. 5), and of the subsequent paper by Mr. Cottrell “On the Division of a Sound-Wave by a Layer of Flame or heated Gas into a reflected and a transmitted Wave" (Proc. Roy. Soc. Feb. 12, 1874), caused me to turn my attention to the experimental illustration of the reflection of sonorous vibrations from flames, heated gases, and from sheets of cold gases and vapours.

The following experiments are of easy execution, and show in a marked manner the reflecting-powers of sheets of flame and beated gas, and even serve to give approximate measures of these reflectingpowers.

Take two similar resonators and place the planes of their mouths at a right angle; then in this angle firmly fix the fork corresponding to the resonators, so that the broad face of one of its prungs faces the mouth of one resonator, while the space between the prongs faces the mouth of the other resonator. (See the figure.) By trial the two planes of the fork are placed at such distances from the resonators that

Communicated by the Author.

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complete interference of the vibrations issuing from their mouths is obtained, and the only sound that reaches the ear is the faint sound given by the fork's action on the air outside the angle included by the mouths of the resonators. If in these circumstances we close the mouth of either resonator with a piece of cardboard, the open resonator will strongly reinforce the sound of the fork. If we now also cover the mouth of the latter resonator with a piece of cardboard we shall again have silence. Also, if we substitute, for one of the pieces of cardboard, a slip of stout glazed note-paper, the same result is obtained. But if we replace the piece of note-paper by a similar piece of French tracing-paper, a faint sound issues from the resonator so covered, because the tracing-paper is sufficiently permeable to sonorous vibrations to permit the resonator to slightly reinforce the sound of the fork. This reinforcement becomes greater if we substitute for the tracing-paper a piece of tissue paper, such as is used in printed books to cover steel engravings; and a yet greater reinforcement is produced when we put in the place of the tissue paper a piece of the soft, loosely woven paper which is used by French instrument-makers for the inner wrapping of their packed wares. I thus obtained a graded series of substances, more and more permeable to sonorous vibrations.

I again obtained neutralization by interference, with the mouths of the resonators open, and then screened the mouth of one of them with a bat's-wing coal-gas flame. The vibrations issuing from the resonators were now no longer neutralized, but the vibrations from the uncovered resonator had a great ascendancy over the other, so that a strong sound issued from it. I now tried to destroy this superiority by screening its mouth successively with the graded series of paper screens.

The loose, soft paper was not equal to it; nor was the tissue paper ; but the tracing-paper just equalled the effect of the gas-flame in guarding the mouth of the resonator from the entrance of sonorous vibrations. On lowering the gas-flame, so that its top luminous border was just below the mouth of the resonator, and therefore only a sheet of heated air ascended across the latter, the balance of the tissue paper against the hot gases and vapour remained unimpaired. Thus it appears that the reflecting-power of a sheet of coal-gas flame or of a sheet of the heated carbonic acid and vapour of water just above it, exactly equals, in the above described circumstances, the reflecting power of tracingpaper.

I have also found that the passage of a sheet of cold coal-gas across the mouth of the resonator was sufficient to destroy the balance of the interference, and caused a faint sound to issue from the other resonater ; a similar effect, and nearly equal in intensity, was obtained with a sheet of cold carbonic acid gas; while cold dry hydrogen closed the mouth of the resonator more effectively than either of the above gases, but was far inferior in this shielding action to the sheet of heated gases above the bat's-wing gas-flame. We should not place too much confidence in measures of the reflecting-power of surfaces made by the method just described, and which I have used merely to give approximations of the reflecting-powers of the above named gaseous sheets; for the substance which closes the mouth of the resonator may allow a considerable portion of the sonorous vibrations to enter the latter, and yet the resonator may not be able to reinforce the sound by reason of its being thus thrown out of tune with the fork by an anyielding surface closing its aperture. Thus, a sheet of thick note-paper prevents resonance as effectively as a thick piece of Bristol-board, or a plate of metal; yet we know well that these substances have very different powers to reflect sonorous vibrations. As a flat coal-gas flame equals piece of tracing-paper in deflecting sonorous vibrations, it follows that we can substitute the former for the latter in all experiments where the presence of the paper produces, by its reflecting-power, an alteration in intensity or in pitch. Thus, if we vibrate a fork before the mouth of a resonator while the nipple of the latter is open, we obtain a far inferior reinforcement to what takes place when the nipple is closed. Now the nipple can be partly closed with a gas-flame or a sheet of heated air. Thus, alternately closing and opening the nipple of an Uta resonator with the flame of a Bunsen burner, gives excellent results*. The reflecting-power of a bat’s-wing flame is also well shown by successively closing and opening the mouth of any resonant box of forks in the octave Ut, to Utg. Also, if the plug be taken out of the ends of closed organ-pipes and these pipes be placed horizontally, the reflecting effect of the flame is heard when the latter is passed forward and backward across the open ends of the pipes while the ear is placed in the axes of the pipes. The simplest method, however, is to sound the fork (either continuously by electro-magnetism, or by a bow) in front of its resonator, and successively to close and open the mouth of the latter with a flame or sheets of heated gas, or of cold vapours or gases. The contemplation of these experiments naturally calls up the question, Is the action of the flame due entirely to reflection ? may it not also absorb part of the sonorous vibrations, as in the analogous phenomena of the reflection of light? If the intensity of the sonorous vibrations which have traversed the flame equals the intensity of the vibrations which impinged on the flame minus the intensity of those which were reflected from the flame, then there is no absorption of these vibrations by the flame; but if this equality does not exist, then there is absorption in the flame; and this means that the flame is heated by the sonorous vibrations—which enter the flame as sonorous vibrations, but issue from the flame as heat vibrations. It thus, at first, appears that the absorption of the sonorous vibrations might be detected by their production of an increase in the temperature of the flame, just as sonorous vibrations are absorbed by caoutchouc and reappear as heat in this substance.

* In all of the experirnents described in this paper care was taken that no heated air or gases entered the resonators and thereby put them ont of tune.

In the following manner I have recently made experiments in the direction of determining the equivalent of a given sonorous aerial vibration in fraction of a Joule's unit of 772 foot-pounds. I stretched between the prongs of an Utg tuningfork a piece of sheet caoutchouc, to inch in thickness, and about 1 inch broad. The effect of this rubber on the vibrating fork is rapidly to extinguish its vibrations, while the rubber itself is heated; and if a fork be vibrated continuously by one and the same force when the rubber is stretched on it, and then when it is taken off, the aerial vibrations produced by the fork are far more intense in the latter circumstances than in the former. By a method described by me in Feb. 187), I measured the relative intensities of the aerial vibrations in these two conditions of vibration. The sheet of caoutchouc was enclosed in a compound thermo-battery, and the fork vibrated during a known interval; the rubber was heated by the vibrations which would have appeared as sonorous vibrations if the rubber had been removed from the fork. The amount of heat given to the caoutchouc was accurately determined by the deflections of a Thomson reflecting-galvanometer connected with the thermo-battery; and by knowing the interval during which the fork vibrated, the amount of heat given to the caoutchouc during this interval, and the equivalent of the heated rubber in water, I calculated the intensity of the sonorous vibrations in terms of a thermal unit, from which I at once obtained the value of the sonorous aerial vibrations when the fork was not heating the rubber—in other words, when it vibrated freely. I thus found that the sonorous aerial vibrations produced during ten seconds by an Utg fork placed in front of its resonator, equalled about Toodoo of a Joule's unit; that is, they can be expressed by the work done in lifting 54 grains one foot high. This quantity of heat, which is equal to the heating of one pound of water todooo of a degree Fabr., expressed the amount by which the gas-flame would be heated if it absorbed all of the sonorous vibrations issuing from the Utg resonator. But this is such a small fraction of the

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