July 23, 1874] 



NATURE 



233 



VIBRATIONS OF AIR PRODUCED BY HEAT 

 TOURING the past session an interesting experiment was made 

 by some students uf tlie College of Pliysical Science, Nciv- 

 castle-on-Tyne, engaged in their practical course of chemistry in 

 the laboratory, sufficiently striking and remarkable to secure it, 

 I have little doubt, a short notice among the records of a scien- 

 tific journal. While testing the inllammable properties of the 

 explosive mixture of air and coal-gas proceeding from the mouth 

 of an unlight^d Bunsen-burner, and observing its flame kindle 

 and flashing back along a glass tube, it occurred to one of the 

 students and to the chemical demonstrator, Mr. Haigh, to check 

 the flame in its descent by inserting a piece of wire-gauze in the 

 tube. On reaching the wire-gauze the flame rested there, as 

 they expec'ed ; not silently, however, but bursting to their sur- 

 piise with remarkable clearness and loudness into the peculiar 

 singing strain of the chemical harmonicon. Mr. Ilaigh made 

 several experiments on the flame with tubes of different sizes, 

 which, if more immediate engagements had not prevented me 

 from pursuing them, it had been my intention to have varied, 

 and to have examined them more completely. In the form in 

 which it first presented itself, a convenient and easily intelligible 

 arrangement of which is hei"e sketched, it appears, however, to 

 offer all the attractions and the remarkable strength and variety 

 of singing properties with which it seems to be abundantly 

 endowed. A cylindrical lamp-glass mounted with a cork and 

 wirc-lriangle on a Bunsen-burner serves to shield the mouth of 

 the tube from draughts of air, and to preserve a steady flow of 

 the entering gas. The tube is first lowered over this and lighted 

 at the top ; by raising it gradually sufficient air soon enters with 

 the gas below to make the flame waver on the top of the tube, 

 and finally descend to the wire-gauze, where it then burns most 

 vociferously, especially if the wire-gauze is placed at the best 

 position in the tube to produce some of its harmonic notes. The 

 highest notes are sounded when it is above the middle, or even 

 near the top of the tube, and the lowest when it is not far from 

 the bottom of the tulie ; the stronger draught arising from the 

 long column of heated air, which soon greatly assists the sound, 

 appearing in the latter case to fivour the production of notes of 

 the ileeper pitch. A glass tube about 2 ft. long and nearly I in. 

 in diameter inside furnished a very powerful note, the ^^•ire-gauze 

 being placed a short distance below the middle of th^ tube. By 

 bending down the edges of a square or circular piece of wire- 

 gauze over the flat end of a round ruler so as to fit the tube cor- 

 rectly, all passage of the flame between it and the tube is 

 prevented, but when, as quickly happens with the increasing 

 hect and updraught of the tube, the agitation of the flame grow s 

 more and more intense, it at length red-heats the wire-gauze, 

 and passing through it lights the Bunsen-lamp below. A very 

 instinctive illustration is thus afforded of certain conditions in 

 which the security of Davy-lamps in a fiery atmosphere can no 

 longer be assured, where a sufficiently quick draught, or in this 

 case the pressure of continued vibrations, carries the flame 

 against the meshes of the wire-gauze until they are ignited. In 

 one case danger arises of the wind carrying the flame of one side 

 of the interior of the lamp over to the other side, which it red- 

 heats ; in the present case the vibrations carry the flame back upon 

 itself. If in the former case a red-heated Davy-lamp is not 

 turned round quickly to face the draughf, explosion does not 

 always follow ; but in this case the current of explosive gas is 

 immediately presented to the heated gauze, and not having 

 undergone any previous combustion it is of course quickly 

 kindled. On the other hand, another source of insecurity of 

 safety lamps when exposed to sudden vibrations, or to the shock 

 produced by a fall, is well shown, when it sometimes appears to 

 happen, if the flame flutters very strongly, th.it it strikes through 

 the wire-gauze without red-heating it, and lights the lamp below. 

 This may, however, have occurred from imperfect fitting of the 

 wire-gauze to the sides of the tube, and it would be interesting 

 to repeat it if possible with precautions for making the surrounel- 

 ing junction quite secure. A lighted Davy-lamp suspended by 

 a wire in a tin tube 3 ft. or 4 ft. long and wide enough to admit 

 it easily, through which a stream of coal-gas mixed with air was 

 passing made the tube hum very loudly, but no explosion fol- 

 lowed, perhaps because it was not found possible to produce in 

 the lamp a sufliciendy violent agitation of the flame. A 

 remark.able example of the ease with which the wire-gauze flame 

 excites the notes of even very short, wide-mouthed tubes can 

 easily be shown by inserting a well-fitting piece of wire-gauze 

 I or 2 in. from the lower end of a straight lamp-glass, as shown 

 in the sketch, and supporting this a few inches above an un- 



lighted Bunsen jet. When the gas is lighted on the top of the 

 wire-gauze and the heat of tlie glass chimney becomes sufficient 

 to increase the draught, which may also be adjusted by varying 

 the gas supply to the glass, its shrill treble note is sounded at 

 once with overpowering loudness. The sensitiveness of the 

 wire-gauze flame to acoustical impressions was, I believe, de- 

 monstrated very recently by Prof. Barrett, by many new and 

 striking experiments 01 the depression of its luminous cap or top 

 in obedience to the voice and to other sounds ; and I have been 

 assured both by Prof. Tait and by Prof. Marreco that the use of 

 the smokeless wire-gauze burners, common in laboratories before 

 the introduction of Bunsen's lamps, for exciting the hoarse music 

 of singing flames in lubes of large calibre has long been familiar 

 to them as a thoroughly eftective means of reproducing the che- 

 mical harmonicon with common coal-gas. The easily inllammable 

 nature of well aerated coal-gas combined with the conducting 

 and quenching power of wire-gauze on flames which ic supports, 

 supplies an obvious explanation of the responsive vibrations of 

 the flame to any description of rh)thmical surrounding agitations 

 and impulses. I was not, however, prepared for an equally 

 remarkable and peculiar property of heated wire-gauze to the 

 above, which, lik.- the last experiment, was also shown to me 

 by Mr. Haigh in some of his trials of the sounding tubes. When 

 the flame had been sounding strongly and the gas was turned off 



-^ 



to extinguish it, instead of ceasing immediately the musical note 

 continued for a considerable time, sometimes even gaining a 

 little in strength before it died away, the tube then appear- 

 ing to have the power of intoning spontaneously without the 

 presence of any visible exciting cause. That the source of 

 these prolon;;ed vibrations is the heat communicated to the wire- 

 gauze, which enables it to expand the air by impulses in the 

 tube as the ascending current gradually passes through its 

 meshes was confirmed by a variety of experiments, all pointing 

 to this origin of the sound as its real explanation. It happened 

 on one occasion, when the flame passed through the gauze, 

 lighting the Bunien-lamp below, and leaving the gauze red-hot, 

 that on putting out the lamp the after-note sounded so long and 

 loudly as quite to equal, if it did not even surpass what had just 

 been emitted by the flame. To reproduce the same note it is in 

 fact only necessary to red-heat a wire-gauze diaphragm inserted 

 a few inches above the lower end of a pretty wide glass tube 

 over a Bunsen-flame, and to remove it from the lamp, when the 

 gravest note of the tube will immediately be sounded with all 

 the strength and purity that can be desired. Somewhat coarser 

 wire gauze than that used for the singing-flame succeeds the best, 

 as, besides being more easily red-heated by the Bunsen-flame, 

 it furnishes a larger store of heat to the ascending air-current, 

 which, in parsing through its meshes, produces the singing 

 sound. If the tube is raised quickly, the draught through it 

 being thus checked it stops, and as soon as it is brought to rest 



