234 



N'A TURE 



[July 23, 1874 



it begins to sing again ; by lowering it quickly the note is mucli 

 strengthened, as it is also by turning on an unlit gas-jet under 

 it, and especially by swinging the tube round horizontally, the 

 lower end foremost througli the air, which increases the draught 

 and the strength ol the note most considerably. The note is 

 silenced when the tube is held at rest inverted, or horizontally, 

 but it begins again as soon as the tube is restored to its erect 

 position. A closely twisted coil of thin platinum wire was 

 compressed in the tube in the place of the wire gauze, and was 

 made red-hot over the llunsen-flame, which was then extin- 

 guished, and the gas .igain turned on immediately, causing the 

 platinum wire to continue to glow by catalytic action. As long 

 as its red-heat continued, tlie musical sound of the tube also con- 

 tinued to be produced. A glass tube 2 ft. lonj by ijin. in 

 diameter, stopped near one end with platinum wire-gauze, to 

 the centre of which a small piece of spongy platinum is fastened, 

 performs in this w.ay over an unlighted gas-jet, when started 

 by preparatorily heating the platinum gauze, for any length of 

 time. Although unable to do so over ordinary coal-gas, yet it 

 is very probable that over hydrogen (as a heat below redness is 

 sufhcient to maintain the sound) a tube tlius fitted with pieces of 

 platinum sponge laid upon wire-gauze would st-Art and continue 

 to sound by itself. 



When a glass bulb is blown at the end of a glass tube it fre- 

 quently happens that in cooling it emits a very clear and distinct 

 humming sound. The note has appeared to me considerably 

 graver than what would be expected from air vibrations in the 

 small bulbs in which it occasionally occurs ; thus in blowing the 

 small candle-bomb, shown of its proper size on the left hand 

 in the figure, a very loud note, of apparently about middle C 

 pitch, or even lower, accompanied its cooling. In drying the 

 glass bulb of a broken VVollaston's cryophorus, shown with its 

 bent tube on tlie right in the sketch, by warming it very gently 

 over a gas-llanie to expel some adhering moisture, I was startled 

 on removing it from tlie flan-.e to hear the same humming note, 

 although the bulb was scarcely hotter than could be touched 

 with the hand, resembling in pitch (although its softness may 

 have had a misleading effect upon the estimate) one of the lowest 

 bass notes of an organ. Being familiar with the depth of lone 

 obtainable with Ilelmholtz's spherical resonators, I am led to 

 suppose that the combination of a bulb with a tube may have a 

 much lower fundamental note than either of those cavities would 

 have alone. But the acting source of the note requires also to 

 be considered, and if, as appears evident, low beats and resultant 

 tonei cannot Ije reinforced without strengthening their primaries, 

 the deep pitch of bulb-emitted notes may possibly arise from the 

 nature of the air impulses by which they are produced. These 

 appear to be of the same kind as the air-oscillations in the hot- 

 gauze harmonicon. As the energy of the sound-waves cannot 

 be produced without a corresponding motive cause, in the latter 

 it is the ascending current of the rarefied, and in the former the 

 in-draughi of the contracting air, both produced by the dissipa- 

 tion or appropriation of a certain store of heat. The cold air 

 entering the hot Ijulb or ascending through the heated wire 

 meshes, expands in doing so, recoils upon Uself, and throws the 

 air column of which it forms a part into vibrations, which con- 

 tinue as long as the flow of air and heat together continue to 

 support the motion. The rapid succession of explosions of the 

 gas-flame harmonicon are, in fact, reproduced ; the expansive 

 force of the small puffs or explosions that produce the sound 

 bemg merely derived from a limited stock of sensible heat, in- 

 stead of from a constant supply of heat of combustion. Con- 

 sidering the volume and duration of the sound long after the 

 wire-gauze has ceased to be visibly red-hot, the energy of the 

 effect produced by the heated gauze seems to be out ol all pro- 

 poition to its magnitude; but the effects of the mechanical 

 transformation of heat are, as is well known, always sufficiently 

 startling, and sometimes even prodigious when the conditions 

 under which it takes place are at all favourable to the process of 

 the transformation. 



I was not aware, when writing this description, that musical 

 sounds produced by heating glass bulbs had been examined so 

 long ago as the beginning of this century, as described in Prof. 

 Tyndall's work on Sound, by the late G. I)e la Kive, who ob- 

 tained them by boiling water in thermometer bulbs. The vapour 

 in its passage along the tube is condensed, and by the collapse 

 that accompanies its contraction throws the air column in the 

 tube into vibration ; this action is thus exactly the opposite of 

 what occurs when fresh-blown strongly heated glass bulbs are 

 allowed to cool, the expansion, instead of the contraction, then 



giving the necessary impulse. I am also disposed, since reading 

 Prof. Tyndall's description and explanation, to ascribe the low 

 note of the warmed cryophorous bulbs to the escape of aqueous 

 vapour from it in the manner of De la Rive's experiment, rather 

 than to the influx of cold air into the bulb to which I attributed 

 it at first. 



It is well known that at a nodal point of a vibrating air- 

 column there is no oscillation, but alternate expansion and con- 

 traction of the air, while in the middle point of a vibrating seg- 

 ment the opposite is the case. Neither of these places is ac- 

 cordingly a suitable one for the combined air-pressures and os- 

 cillations to take place, which in a sounding flame or at a heated 

 diaphragm can never occur separately or independently of 

 each other, the strength of each little puff or explosion depend- 

 ing at once upon the direction and amount of the contributing 

 oscdlation ; the position of the heating cause must accordingly 

 be between the ventral and the nodal points. It is the .same 

 with the air- currents that excite the vibrations of a flute, railway 

 whistle, common bird-call, or organ-pipe ; the oscillations and 

 throws of pressure of the air at the embouchures are not only 

 simultaneous, but they must also be so related to each other that 

 on inward oscillation accompanies increase of pressure, since a 

 part of the blast is then thrown into the air-column and com- 

 presses it. r'rom an easy law connecting together the changes 

 of pressure with the motion of the air at any point of a sta- 

 tionary air-wave, it appears that in these instruments, exactly 

 as in the hot-bulb, or in the hot-gauze and gas-flame harmonicon, 

 the ventral point (as far as a true one exists) is not at the 

 embouchures of the wind-instruments, nor at the heating and 

 cooling points of the several kinds of heat-sirens orharmonicons, 

 but outside of them in .such a position as to place the exciting 

 air-puffs between the nodal and the ventral point. Prof. Tyndall 

 has truly pointed out in his sound lectures that whenever 

 stationary undulations are kept up against friction, as when a 

 stretched string is kept in uniform vibration by the hand, the nodal 

 points are not absolutely stationary points, but present a little 

 oscillation. It is equally true that the string does not remain 

 accurately parallel to itself where it ought to show true ventral 

 points, and accordingly resists a hand applied thereto keep up its 

 oscillations with a certain force ; but this resistance is weak, 

 and it acts through a wide excursion, while near the nodal 

 points the necessary efforts of the hand are greater and exerted 

 through very small disphcements. In intermediate positions the 

 nearer the string is held to a nodal point, and the smaller its 

 excursions, the stronger must be the jerks given to it by the hand 

 to keep up its oscillations. In air-instruments (including the 

 harmonicon and flute) the jerks of the hand correspond to the 

 explosive force of the small admitted puffs of air, depending in 

 heat-harmonicons on the intensity of the heat or combustion, 

 and also on the quantity of the matter burned or heated in the 

 successive puffs ; and in wind-instruments no doubt principally 

 on the pressure and ]ierhaps to some extent also on the quantity 

 of the admitted blasts. According to the position of the em- 

 bouchure (including a flame-jet or a heated gauze under the ex- 

 pression) in the vibrating segment of a wave of resonance, its 

 beneficial action in maintaining the air-wave will be evoked or 

 suspended in obedience to the particular conditions that exist in 

 the air-wave at that point ; the only absolute requirement for its 

 activity being that entanglement of a fresh supply of blast must 

 coincide with a moment of rising pressure at that point of the 

 air-wave. This is easily accomplished in wind-instruments, the 

 large excursions of the air at the embouchures ensuring a plenti- 

 ful introduction of the entering wind-puffs at the proper time ; 

 the action in this case is quite free from complication, as without 

 considering the small gains of pressure periodically given by the 

 blast as it flows inwards, and a small suction that it exercises (to 

 which I believe that Mr. Hermann Smith is the first to draw 

 attention in his excellent communication on this subject in 

 N.tTUKE, vol. X. p. 161), as it retrea's, nothing prevents the to- 

 and-fro displacements at the mouth of an organ-pipe from so 

 deflecting the current of the air-blast inwards and outw.ards as 

 constantly to apply its useful energy to the best effect. Inward 

 motion of air towards a node is accompanied by rising and 

 outward motion by falling pressure, and as the losses of both ol 

 these kinds of energy are properly renewed by the blast in 

 entering or retreating, the resonance of the wind-instrument is 

 kept up. The friction and loss jf energy in high harmonics is 

 probably much greater than in graver notes, and, the air-excur- 

 sions being also smaller, it is perhaps on this account that a 

 stronger blast or a nicer direction of it by the mouth-piece 



