Page images

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.


[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 heated 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 prongs 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.

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 unyielding 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 a 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 Ut, 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 Ut. 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

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


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 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 Ut, tuningfork a piece of sheet caoutchouc, 1 inch in thickness, and about 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. 1871, 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 Ut, fork placed in front of its resonator, equalled about To 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 Too of a degree Fahr., expressed the amount by which the gas-flame would be heated if it absorbed all of the sonorous vibrations issuing from the Ut, resonator. But this is such a small fraction of the

entire heat in the flame, that it is far within the actual fluctuations in its temperature; and even if the flame were constant in temperature, this small increase could not be detected by any known thermometric method. We cannot, therefore, determine the amount of absorptive power of a flame or of a sheet of heated air for sonorous vibrations by experiments on their increased temperatures when sonorous vibrations impinge on these bodies.

Hoboken, April 10th, 1874.

LI. On some Phenomena connected with the Boiling of Liquids. By CHARLES TOMLINSON, F.R.S.*


HE Annales de Chimie et de Physique for March 1875 contain a memoir by Professor Gernez, entitled "Recherches sur l'Ebullition." This memoir, which occupies sixty-six pages, has for its main object the support of De Luc's theory of ebullition, which "attributes the production of bubbles of vapour to the formation of bubbles of air." Upwards of one third of the memoir is devoted to historical notices, in which the author goes over nearly the same ground as that previously trodden by myselft. Several pages of the history are occupied with an account of my researches‡; and they are referred to again and again in other parts of the memoir. Of course I have no right to complain if M. Gernez, in the pursuit of a definite object, should refer to such of my experiments as suit his purpose, and pass over others which he either cannot or does not choose to assimilate with the theory he is advocating. He attaches great importance to the authorities that he quotes, and insinuates some surprise that I should venture to differ from them. That I did not ignore them is evident from the time and care bestowed by me in the compilation of my "Historical Notes," in which the results, as obtained by a large number of distinguished observers, were brought into juxtaposition for the first time. I did and do desire to treat those results with all the respect that is due to established facts; but I claim the right to form an independent judgment as to the inferences to be drawn from them. I gather from the memoir that M. Gernez does not quarrel with my facts, but only with their interpretation, although he seems to regard it as a fault in me that I do not consider the theory * Communicated by the Author.

+ "Historical Notes on some Phenomena connected with the Boiling of Liquids," Phil. Mag. for March 1869.

"On the Action of Solid Nuclei in liberating Vapour from Boiling Liquids," Proceedings of the Royal Society, 1869, vol. xvii. p. 240. An abstract of this paper appeared in Les Mondes of the 12th of October and 2nd of November, 1871.

« PreviousContinue »