Vibrations of Air Produced by Heat

Abstract

DURING the past session an interesting experiment was made by some students of the College of Physical Science, Newcastle-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 scientific journal. While testing the inflammable properties of the explosive mixture of air and coal-gas proceeding from the mouth of an unlighted 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 expected; not silently, however, but bursting to their surprise with remarkable clearness and loudness into the peculiar singing strain of the chemical harmonicon. Mr. Haigh 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 forum in which it first presented itself, a convenient and easily intelligible arrangement of which is here 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 wire-triangle 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 must vociferously, especially if the wire-gauze is placed at the best position in the tube to produce some of its harmonic notes. The lighest 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 fl-nm the bottom of the tube; the stronger draught arising from the long column of heated air, which soon greatly assists the sound, appearing in the latter case to favour the production of notes of the deeper pitch. A glass tube about 2 ft. long and nearly 1 in. in diameter inside furnished a very powerful note, the wire-gauze being placed a short distance below the middle of the 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 correctly, all passage of the flame between it and the tube is prevented, but when, as quickly happens with the increasing heat and updraught of the tube, the agitation of the flame grows more and more intense, it at length red-heats the wire-gauze, and passing through it lights the Bunsen-lamp below. A very instructive illustration is thus afforded of certain conditions in which she 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 time 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 nut turned round quickly to face the draught, explosion does not always follow; but in this case the current of explosive gas is immediately presented to the heated gauze, and nut having undergone any previous combustion it is of course quickly kindled. On the other band, another source of insecurity of safety lamps when exposed to sudden vibrations, or to the shock produced by a fall, is avail shown, when it sometimes appears to happen, if the flame flutters very strongly, that it strikes through the wire-gauze without red-heating it, and lights the lamp below. This may, however, have occured 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 surrounding junction quite secure. A lighted Davy-lamp suspended by a wire in a tin tube 3 ft. or 4ft. 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 followed, perhaps because it was not found possible to produce in the lamp a sufficiently violent agitation of the flame. A remarkable 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 1 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 unlighted Bunsen jet. When the gas is lighted on the top of the wire-gauze and the heat of the 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, demonstrated very recently by Prof. Barrett, by many new and striking experiments o-s 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. Msrreco 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 tubes of large calibre has long been familisr to them as a thoroughly effective means of reproducing the chemical hsrmonicon with common coal-gas. The easily intlammable nature of well aërated coal-gas combined with the conducting and quenching power of wire-gauze on flames which it supports, supplies an obvious explanation of the responsive vibrations of the flame to any description of rhythmical surrounding egitations and impulses. I was not, however, prepared fur an equally remarkable and peculiar property of heated wire-gauze to the above, which, like 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 appearing to have the power of intoning spontaneously without the presence of any visible exciting cause. That the source of these prolonged vibrations is the heat cummunicated 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 Bunsen-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 passing 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 it begins to sing again; by lowering it quickly the note is much 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 through the air, which increases the draught and the strength of 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 Bunsen-flame, which was then extinguished, and the gas again turned on immediately, causing the platinum wire to continue to glow by catalytic action. As long as its red-heat continued, the musical sound of the tube also continued to be produced. A glass tube 2 ft. long by 11/4 in. 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 way 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 sufficient to maintain the sound) a tube thus fitted with pieces of platinum sponge laid upon wire-gauze would start and continue to sound by itself.