Feb. 17, 1 881] 



NA TURE 



375 



meter, it occurred to me several weeks ago that the results thus 

 obtained might be checked by a more direct and simple form of 

 experiment. Placiug the gases and vapours in diathermanous 

 bulbs, and exposing the bulbs to the action of radiant heat, the 

 heat absorbed by different gases and vapours ought, I con- 

 sidered, to be rendered evident by ordinary expansion. I 

 devised an apparatus with a view of testing this idea. But at 

 this point, and before my proposed gas-thermometer was con- 

 structed, I became acquainted with the ingenious and original 

 experiments of Mr. Graham Bell, wherein mu4cal sounds are 

 obtained thrjugh the action of an intermittent beam of light 

 upon solid bodies. 



From the first I entertained the opinion that these singular 

 sounds were caused by rapid changes of temperature, producing 

 corresponding changes of shape and volume in the bodies im- 

 pinged upon by the beam. But if this be the case, and if gases 

 and vapoui's really absorb radiant heat, they ought to produce 

 sounds more intense than those obtainable from solids. I 

 pictured every stroke of the beam responded to by a sudden 

 expansion of the absorbent gas, and concluded that when the 

 pulses thus e.xcited followed each other with sufficient rapidity, 

 a musical note must be the result. It seemed plain, moreover, 

 that by this new method many of my previous results might be 

 brought to an independent test. Higlily diathermanous bodies, 

 I reasoned, would produce faint sounds, while highly ather- 

 manous bodies would produce loud sounds ; the strength of the 

 sound being, in a sense, a measure of the abs irption. The first 

 experiment made with a view of testing this idea, was executed 

 in the presence of Mr. Graham Bell ' ; and the result was in 

 exact accordance with what I had foreseen. 



The inquiry has been recently extended so as to embrace most 

 of the gases and vapours emplo)ed in my forme/ researches. 

 My first source of rays was a Siemens' lamp connected with a 

 dynamo-machine, worked by a gas-engine. A glass lens was 

 used to concentrate the rays, and afterwards two lenses. By the 

 first the rays were rendered parallel, while the second caused 

 them to converge to a point about seven inches distant from the 

 lens. A circle of sheet zinc provided first with radial slits and 

 afterwards with teeth and interspaces cut through it, was 

 mounted vertically on a whirling table, and caused to rotate 

 rapidly across the beam near the focus. The passage of the 

 slits produced the desired intermitteace," wh'Ie a flask contain- 

 ing the gas or valour to be examined received the shocks of the 

 beam immediately behind the rotating disk. From the flask a 

 tube of india-rubber, ending in a tapering one of ivory or box- 

 wood, led to the ear, which was thus rendered keenly sensitive 

 to any sound generated within the flask. Compared with the 

 beautiful apparatus of Mr. Graham Bell, the arrangement here 

 described is rude ; it is, however, very effective. 



With this arrangement the nmaber of sounding gases and 

 vapours was rapidly increased. But I was soon made aware 

 that the glass lenses withdrew from the beam its most effectual 

 rays. The silvered mirrors employed in my previous researches 

 were therefore invoked ; and with them, acting sometimes singly 

 and sometimes as conjugate mirrors, the curious and striking 

 results which I have now the honour to submit to the Society 

 were obtained. 



Sulphuric ether, formic ether, and acetic ether being pi iced 

 in bulbous flasks, their vapours were soon diffused in the air 

 above the liquid. On placing these flasl;s, whose bottoms only 

 were covered by the liquid, behind the rotating disk, so that the 

 intermittent beam passed through the vapour, loud musical tones 

 were in each case obtained. These are kntown to be the most 

 highly absorbent vapours which my experiments revealed. 

 Chloroform and bisulphide of carbon, on the other hand, are 

 known to be the least absorbent, the latter standing near the 

 head of diathermanous vapours. The sounds extracted from 

 these two substances were usually weak and s :)metimes barely 

 audible, being more feeble with the bisulphide than with the 

 chloroform. With regard to the vapours of amylene, iodide of 



' On November 29 : see yottmal of the Society of Telegraph Engineers, 

 December 8, 1880. 



2 When the disk rotates the individual slits disappear, forming a hazy zone 

 through which objects are visible. 1 hrowing by the clean hand, or better 

 still by white paper, the beam back upon the disk, it appears to stand still, 

 the slits forming so many dark rectangles. The reason is obvious, but the 

 experiment is a very beautiful one. 



I may add that when I stand with open eyes in the flashing beam, at a 

 definite velocity of recurrence, subjective colours of e.vtraordinary gorgeous- 

 ness are produced. With slower or quicker rates of rotation the colours 

 disappear. The flashes also produce a giddiness sometimes intense enough 

 to cause me to grasp the table to keep myself erect. 



ethyl, iodide of methyl and benzol, other things being equal, 

 their power to ]iroduce musical tones appeared to be accurately 

 expressed by their ability to absorb radiant heat. 



It is the vapour, and not the liquid, that is effective in 

 producing the sounds. Taking, for example, the bottles in 

 which my volatile substances are habitually kept, I permitted 

 the intermittent beam to impinge upon the liquid in each of 

 them. No sound was in any case produced, while the moment 

 the vapour-laden space above an active liquid was traversed by 

 the beam, musical tones made themselves audible. 



A rock-salt cell filled entirely with a volatile liquid and sub- 

 jectei to the intermittent beam produced no sound. Th's cell was 

 circular and closed at the top. Once, while operating with a 

 higlily athermanous sub^tance, a distinct mu-ical note was heard. 

 On examining the cell however a small bubble was found at its 

 top. The bubble was less than a quarter of an inch in diameter, 

 but still sufficient to produce audilile sounds. When the cell 

 was completely filled the sounds disappeared. 



It is hardly necessary to state that the pitch of the note 

 obtained in each case is determined by the velocity of rotation. 

 It is the same as that produced by blowing against the rotating 

 disk and allowing its slits to act like the perforations of a syren. 



Thus, as regarls vapours, prevision has been justified by 

 experiment. I now turn to gases. A small flask, after having 

 been heated in the spirit-lamp so as to detach all moisture from 

 its sides, was carefully filled with dried air. Placed in the inter- 

 mittent beam it yielded a musical note, but so feeble as to be 

 heard only with attention. Dry oxygen and hydrogen behaved 

 like dry air. This agrees with my former experiments, which 

 assigned a hardly sensible absorption to these gases. When the 

 dry air was displaced by carbmic acid, the sound was far louder 

 than that obtained from any of the elementary gases. When 

 the carbonic acid was displaced by nitrous oxide the sound was 

 much more forcible still, and when the nitrous oxide was dis- 

 placed by olefiant gas it gave birth to a musical n ite which, 

 when the beam was in good condition nnd the bulb well chosen, 

 seemed as loud as that of an ordinary organ-pipe. We have 

 here the exact order in which my former experiments proved 

 these gases to stand as absorbers of radiant heat. The amount 

 of the absorption and the intensity of the sound go hand in hand. 



In 1859 I proved gaseous ammonia to be extremely impervious 

 to radiant heat. My interest in its deportment when subjected 

 to this novel test was therefore great. Placing a small quantity 

 of liquid ammonia in one of the flasks, and warming the liquid 

 slightly, the intermittent beam was sent through the space above 

 the liquid. A loud musical note was immediately produced. 

 By the proper application of heat to a liquid the sounds may be 

 always intensified. The ordinary temperature however suffices 

 in all the cases thus far referred to. 



In this relation the vapour of water was that which interested 

 me most, and as I could not hope that at ordinary temperatures 

 it existed in sufiicient amount to produce audible tones, I heated 

 a small quantity of water in a flask almost up to its boiling- 

 point. Placed in the intermittent beam, I heard — I avow with 

 delight — a powerful musical sound produced by the aqueous' 

 vapour. 



Small wreaths of haze, produced by the partial condensation 

 of the vapour in the upper and cooler air of the flask, were how- 

 ever visible in this experiment ; and it was necessary to prove 

 that this haze was not the cause of the sound. The flask was 

 therefore heated by a spirit-flame beyond the temperature of 

 boiling water. The closest scrutiny by a condensed beam of 

 lii^ht then revealed no trace of cloudiness above the liquid. 

 From the perfectly invisible vapour however the musical sound 

 issued, if anything, more forcible than before. I placed the 

 flask in c ild water until its temperature was reduced from about 

 90° to 10° C, fully expecting that the sound would vanish at 

 this temperature ; but notwithstanding the tenuity of the vapour, 

 the sound extracted from it was not only distinct but loud. 



Three empty flasks filled with ordinary air were placed in a 

 freezing mixture for a quarter of an hour. On being rapidly 

 transferred to the intermittent beam, sounds much louder than 

 those obtainable from dry air were produced. 



Warming these flasks in the flame of a spirit-lamp [until all 

 visible humidity had been removed, and afterwards urging dried 

 air through them, on being placed in the intermittent beam the 

 sound in each case was found to have fallen almost to silence. 



Sending, by means of a glass tube, a puff of breath from the 

 lungs into a dried flask, the power of emitting sound was 

 immediately restored. 



