320 



NATURE 



\7tily 18, 1878 



of the advantage of a bulb. It is obvious that it che oJ>en end 

 of the tube were heated, the effect of the transfer of heat would 

 be even more unfavourable than in the case of a temperature 

 uniform throughout. 



The sounds emitted by a jet of hydrogen, burning in an open 

 tube, were noticed soon after the discovery of the gas, and have 

 been the subject of several elaborate inquiries. The fact that 

 the notes are substantially the same as those which may be elicited 

 from the tube in other ways, e.g. , by blowing, was announced by 

 Chladni. Faraday proved that other gases \\ere competent to 

 take the place of hydrogen, though not without disadvantage. 

 Bat it is to Sondhauss that we owe the most detailed examination 

 of the circumstances under which the sound is produced. His 

 experiments prove the importance of the part taken by the 

 column of gas in the tube which supplies the jet. For example, 

 sound cannot be obtained with a supply tube which is plugged 

 with cotton in the neighbourhood of the jet, although no differ- 

 ence can be detected by the eye between the flame thus obtained 

 and others which are competent to excite sound. When the 

 supply tube is unobstructed, the sounds obtainable are limited as 

 to pitch, often dividing themselves into detached groups. In 

 the intervals between the groups no coaxing will induce a main- 

 tained sound, and it may be added that, for a part of the interval 

 at any rate, the influence of the flame is inimical, so that a 

 vibration started by a blow is damped more rapidly than if the 

 jet were not ignited. 



Partly in consequence of the peculiar behaviour of flames, and 

 partly for other reasons, the thorough explanation of these phe- 

 nomena is a matter of some difficulty ; but there can be no doubt 

 that they fall under the head of vibrations maintained by heat, 

 the heat being communicated periodically to the mass of air 

 confined in the sounding tube at a place where, in the course of 

 a vibration, the pressure varies. Although some authors have 

 shown an inclination to lay stress upon the effects of the current 

 of air passing through the tube, the sounds can readily be 

 produced, not only when there is no through draught, but 

 even when the flame is so situated that there is no sensible 

 periodic motion of the air in its neighbourhood. In the course 

 of the lecture a globe intended for burning phosphorus in oxygen 

 gas was used as a resonator, and, when excited by a hydrogen 

 flame well removed from the neck, gave a pure tone of alx)ut 

 ninety-five vibrations per second. 



In consequence of the variable pressure within the resonator, 

 the issue of gas, and therefore the equivalent of heat, varies 

 during the vibration. The question is under what circumstances 

 the variation is of the kind necessary for the maintenance of the 

 vibration. If we were to suppose, as we might at first be 

 inclined to do, that the issue of gas is greatest Avhen the pressure 

 in the resonator is least, and that the phase of greatest develop- 

 ment of heat coincides with that of the greatest issue of gas, we 

 should have the condition of things the most unfavourable of 

 all to the persistence of the vibration. It is not difficult, how- 

 ever, to see that both suppositions are incorrect. In the supply 

 tube (supposed to be unplugged, and of not too small bore) 

 stationary, or approximately stationary, vibrations are excited, 

 whose phase is either the same or the opposite of that of the 

 vibration in the resonator. If the length of the supply tube from 

 the burner to the open end in the gas -generating flask be less than a 

 quarter of the wave length in hydrogen of the actual vibration, 

 the greatest issue of gas precedes by a quarter period the phase of 

 greatest condensation; so that if the development of heat is 

 retarded somewhat in comparison with the issue of gas, a state 

 of things exists favoiwable to the maintenance of the sound. 

 Some such retardation is inevitable, because a jet of inflammable 

 gas can burn only at the outside, but in many cases a still more 

 potent cause may be found in the fact that during the retreat of 

 the gas in the supply tube small quantities of air may enter from 

 the interior of the resonator, whose expulsion must be effected 

 before the inflammable gas can again begin to escape. 



If the length of the sujoply tube amounts to exactly one quarter 

 of the wave length, the stationary vibration within it will be of 

 such a character that a node is formed at the burner, the vari- 

 able part of the pressure just inside the burner being the same as 

 in the interior of the resonator. Under these circumstances 

 there is nothing to make the flow of gas, or the development of 

 heat, variable, and therefore the vibration cannot be maintained. 

 This particular case is free from some of the difficulties which 

 attach themselves to the general problem, and the conclusion is 

 in accordance with Sondhauss' observations. 



When the supply tube is somewhat longer than a quarter of 



the wave, the motion of the gas is materially different from that 

 first described. Instead of preceding, the greatest outward flow 

 of ga.s follows at a quarter period interval the phase of greatest 

 condensation, and therefore if the development of heat be some- 

 what retarded, the whole effect is unfavourable. This state of 

 things continues to prevail, as the supply tube is lengthened, 

 until the length of half a wave is reached, after which the motion 

 again changes sign, so as to restore the possibility of mainten- 

 ance. Although the size of the flame and its position in the tube 

 (or neck of resonator) are not without influence, this sketch of 

 the theory is sufficient to explain the fact, formulated by Dr. 

 Sondhauss, that the principal element in the question is the length 

 of the supply tube. 



The next example of the production of sound by heat, shown 

 in the lecture, was a very interesting phenomenon discovered by 

 Rijke. When a piece of fine metallic gauze, stretching across 

 the lower part of a tube, open at both ends and heki vertically, 

 is heated by a gas flame placed under it, a sound of considerable 

 power, and lasting for several seconds, is observed almost imme- 

 diately after the removal of the flame. Differing in this respect 

 from the case of sonorous flames, the generation of sound was 

 found by Rijke to be closely connected with the formation of a 

 through draught, which impinges upon the heated gauze. In 

 this form of the experiment the heat is soon abstracted, and then 

 the sound ceases ; but by keeping the gauze hot by the current 

 from a powerful galvanic battery, Rijke was able to obtain the 

 prolongation of the sound for an indefinite period. In any case 

 from the point of view of the lecture the sound is to be regarded 

 as a maintained sound. 



In accordance with the general views already explained, we 

 have to examine the character of the variable communication of 

 heat from the gauze to the air. So far as the communication is 

 affected directly by variations of pressure or density the influence 

 is unfavourable, inasmuch as the air v/ill receive less heat from 

 the gauze when its own temperature is raised by condensation. 

 The maintenance depends upon the variable transfer of heat due 

 to the varying vwtions of the air through the gauze, this motion 

 being compounded of a uniform motion upwards with a motion, 

 alternately upwards and downwards, due to the vibration. In 

 the lower half of the tube these motions conspire a quarter period 

 before the phase of greatest condensation, and oppose one another 

 a quarter period after that phase. The rate of transfer of heat 

 will depend mainly upon the temperature of the air in contact 

 with the gauze being greatest when that temperature is lov/est. 

 Perhaps the easiest way to trace the mode of action is to begin 

 with the case of a simple vibration without a steady current. 

 Under these circumstances the whole of the air which comes in 

 contact with the metal, in the course of a complete period, 

 becomes heated ; and after this state of things is established 

 there is comparatively little further transfer of heat. The effect 

 of superposing a small steady upwards current is now easily 

 recognised. At the limit of the inwards motion, i.e. at the phase 

 of greatest condensation, a small quantity of air comes into con- 

 tact with the metal, which has not done so before, and is accord- 

 ingly cool ; and the heat communicated to this quantity of air 

 acts in the most favourable manner for the maintenance of the 

 vibration. 



A quite different result ensues if the gauze be placed in the 

 upper half of the tube. In this case the fi'esh air will come 

 into the field at the moment of greatest rarefaction, when the 

 communication of heat has an unfavourable instead of a favour- 

 able effect. The principal note of the tube therefore cannot be 

 sounded. 



A complementary phenomenon discovered by Bosscha and 

 Riess may be explained upon the same principles. If a current 

 of hot air impinge upon cold gauze, sound is produced ; but in 

 order to obtain the principal note of the tube the gauze must be 

 in the upper, and not as before in the lower, half of the tube. 

 An experiment due to Riess was shown in which the sound is 

 maintained indefinitely. The upper part of a brass tube is kept 

 cool by water contained in a tin vessel, through the bottom of 

 which the tube passes. In this way the gauze remains com 

 paratively cool, although exposed to the heat of a gas flame 

 situated an inch or two below it. The experiment sometimes 

 succeeds better when the draught is checked by a plate of wood 

 placed somewhat closely over the top of the tube. 



Both in Rijke's and Riess' experiments the variable transfer 

 of heat depends upon the motion of vibration, while the effect 

 of the transfer depends upon the variation of pressure. The 

 gauze must therefore be placed where both effects are sensible. 



