-NRLF Gd pn r r QC ^ of Oi IN MEMORIAM FLOR1AN CAJORI With the Authors Compliments. UPON THE Production of Sound by Radiant Energy, BY ALEXANDER GRAHAM BELL. PAPER READ BEFORE THE NATIONAL ACADEMY OF SCIENCES, APRIL 21, 1881. WASHINGTON, D. C. GIBSON BROTHERS, PRINTERS. 1881. CAJORI 34-3 UPON THE PRODUCTION OF SOUND BY RADIANT ENERGY. BY ALEXANDER GRAHAM BELL. [A Paper read before the National Academy of Sciences, April, 21, 1881.] IN a paper read before the American Association for the Advancement of Science, last August, I described certain ex- periments made by Mr. Sumner Tainter and myself which had resulted in the construction of a " Phoiophone" or apparatus for the production of sound by light;* and it will be my ob- ject to-day to describe the progress we have made in the inves- tigation of photophonic phenomena since the date of this com- munication. In my Boston paper the discovery was announced that thin disks of very many different substances emitted sounds when exposed to the action of a rapidly-interrupted beam of sunlight. The great variety of material used in these experiments led me to believe that sonorousness under such circumstances would be found to be a general property of all matter. At that time we had failed to obtain audible effects from masses of the various substances which became sonorous in the condition of thin diaphragms, but this failure was explained upon the supposition that the molecular disturbance produced * Proceedings of American Association for the Advancement of Science, Aug. 27th, 1880; see, also, American Journal of Science, vol. xx, p. 305; Journal of the American Electrical Society, vol. iii, p. 3 ; Journal of the Society of Telegraph Engineers and Electricians, vol. ix, p. 404 ; Annales de Chimie et de Physique, vol. xxi. by the light was chiefly a surface action, and that under the circumstances of the experiments the vibration had to be trans- mitted through the mass of the substance in order to affect the ear. It was therefore supposed that, if we could lead to the ear air that was directly in contact with the illuminated surface, louder sounds might be obtained, and solid masses be found to be as sonorous as thin diaphragms. The first experiments made to verify this hypothesis pointed towards success. A beam of sunlight was focussed into one end of an open tube, the ear being placed at the other end. Upon interrupting the beam, a clear, musical tone was heard, the pitch of which depended upon the frequency of the interruption of the light and the loudness upon the material composing the tube. At this stage our experiments were interrupted, as circum- stances called me to Europe. While in Paris a new form of the experiment occurred to my mind, which would not only enable us to investigate the sounds produced by masses, but would also permit us to test the more general proposition that sonorousness, under the in- fliience of intermittent HqJit, is a property common to all matter. The substance to be tested was to be placed in the interior of a transparent vessel, made of some material which (like glass) is transparent to light, but practically opaque to sound. Under such circumstances the light could get in, but the sound produced by the vibration of the substance could not get out. The audible effects could be studied by placing the ear in communication with the interior of the vessel by means of a hearing tube. Some preliminary experiments were made in Paris to test this idea, and the results were so promising that they were com- municated to the French Academy on the llth of October, 1880, in a note read for me by M. Antoine Breguet.* Shortly after- wards I wrote to Mr. Tainter, suggesting that he should carry on the investigation in America, as circumstances prevented me from doing so myself in Europe. As these experiments seem to have formed the common starting point for a series of independent researches of the most important character ? car- * Comptes Rendux, vol. xcl, p. 595. 3 ried on simultaneously, in America by Mr. Tainter, and in Europe by M. Mercadier,* Prof. Tyndall,t W. E. RontgenJ and W. II. Preece, I may be permitted to quote from my letter to Mr. Tainter the passage describing the experiments referred to : " METROPOLITAN HOTEL, HUE C AMBON, PARIS, " Nov. 2, 1880. " DEAR MR. TAINTER : * * * "I have devised a method of producing sounds by " the action of an intermittent beam of light from substances " that cannot be obtained in the shape of thin diaphragms or " in the tubular form ; indeed, the method is specially adapted " to testing the generality of the phenomenon we have discov- " ered, as it can be adapted to solids, liquids, and gases. " Place the substance to be experimented with in a glass test- " tube, connect a rubber tube with the mouth of the test-tube, " placing the other end of the pipe to the ear. Then focus " the intermittent beam upon the substance in the tube. I have " tried a large number of substances in this way with great " success, although it is extremely difficult to get a glimpse of " the sun here, and when it does shine the intensity of the light " is not to be compared with that to be obtained in Washing- " ton. I got splendid effects from crystals of bichromate of " potash, crystals of sulphate of copper, and from tobacco " smoke. A whole cigar placed in the test-tube produced a " very loud sound. I could not hear anything from plain water, " but when the water was discolored with ink a feeble sound " was heard. I would suggest that you might repeat these ex- u periments and extend the results," &c., &c. Experiments with Solids. Upon my return to Washington in the early part of January ,|| Mr. Tainter communicated to me the results of the experiments he had made in my laboratory during my absence in Europe. * " Notes on Radiophony," Comptes Rendus, Dec. G and 13, 1880; Feb. 21 and 28, 1881. See, also, Journal de Physique, vol. x, p. 53. " Action of an Intermittent Beam of Radiant Heat upon Gaseous Matter." roc. Royal Society, Jan. 13, 1881, vol. xxxi, p. 307. I "On the tones which arise from the intermittent illumination of a gas." See Annakn der Phyx. und Chemie, Jan., 1881, No. 1, p. 155. " On the Conversion of Radiant Energy into Sonorous Vibrations." Proc. Royal Society, March 10. 1881, vol. xxxi, p. 506. )] On the 7th of January. He had commenced by examining the sonorous properties of a vast number of substances enclosed in test-tubes in a simple empirical search for loud effects. He was thus led gradually to the discovery that cotton-wool, worsted, silk, and fibrous materials generally, produced much louder sounds than hard rigid bodies like crystals, or diaphragms such as we had hitherto used. In order to study the effects under better circumstances he enclosed his materials in a conical cavity in a piece of brass closed by a flat plate of glass. A brass tube leading into the cavity served for connection with the hearing-tube. When this conical cavity was stuffed with worsted or other fibrous materials the sounds produced were much louder than when a test- tube was employed. This form of receiver is shown in Figure I. Mr. Tainter next collected silks and worsteds of different colors, and speedily found that the darkest shades produced the best effects. Black worsted especially gave an extremely loud sound. As white cotton- wool had proved itself equal, if not superior, to any other white fibrous material before tried, he was anxious to obtain colored specimens for comparison. Not having any at hand, however, he tried the effect of darkening some cotton- wool with lamp-black. Such a marked reinforcement of the sound resulted that he was induced to try lamp-black alone. About a teaspoonf ul of lamp-black was placed in a test-tube and exposed to an intermittent beam of sunlight. The sound produced was much louder than any heard before. Upon smoking a piece of plate-glass, and holding it in the intermittent beam with the lamp-black surface towards the sun, the sound produced was loud enough to be heard, with atten- tion, in any part of the room. With the lamp-black surface turned from the sun the sound was much feebler. Mr. Tainter repeated these experiments for me immediately upon my return to Washington, so that I might verify his results. Upon smoking the interior of the conical cavity shown in Figure I, and then exposing it to the intermittent beam, with the glass lid in position as shown, the effect was perfectly startling. The sound was so loud as to be actually painful to an ear placed closely against the end of the hearing-tube. The sounds, however, were sensibly louder when we placed some smoked wire gauze in the receiver, as illustrated in the drawing, Figure 1. When the beam was thrown into a resonator, the interior of which had been smoked over a lamp, most curious alternations of sound and silence were observed. The interrupting disk was set rotating at a high rate of speed, and was then allowed to come gradually to rest. An extremely feeble musical tone was at first heard, which fell in pitch as the rate of interrup- tion grew less. The loudness of the sound produced varied in the most interesting manner. Minor reinforcements were con- stantly occurring, which became more and more marked as the true pitch of the resonator was neared. When at last the fre- quency of interruption corresponded to the frequency of the fundamental of the resonator, the sound was so loud that it might have been heard by an audience of hundreds of people. The effects produced by lamp-black seemed to me to be very extraordinary, especially as I had a distinct recollection of ex- periments made in the summer of 1880 with smoked diaphragms, in which no such reinforcement was noticed. Upon examining the records of our past photophonic experi- ments we found in vol. vii, p. 57, the following note: " Experiment Y. Mica diaphragm covered with lamp-black on side exposed to light. "Result : distinct sound about same as without lampblack. A. G. B., July 18^, 1880. "Verified the above, but think it somewhat louder than when used without lamp-black." #. T., July ISth, 1880. Upon repeating this old experiment we arrived at the same result as that noted. Little if any augmentation of sound re- sulted from smoking the mica. In this experiment the effect was observed by placing the mica diaphragm against the ear arid also by listening through a hearing-tube, one end of which was closed by the diaphragm. The sound was found to be more 6 audible through the free air when the ear was placed as near to the lamp-black surface as it could be brought without shading it. Thus the vibrations produced in lamp-black under the above circumstances do not appear to be communicated to any very appreciable extent to the diaphragm on which the lamp-black is deposited. At the time of my communication to the American Associa- tion I had been unable to satisfy myself that the substances which had become sonorous under the direct influence of inter- mittent sunlight were capable of reproducing the sounds of articulate speech under the action of an undulatory beam from our photophonic transmitter. The difficulty in ascertaining this will be understood by considering that the sounds emitted by thin diaphragms and tubes were so feeble that it was im- practicable to produce audible effects from substances in these conditions at any considerable distance from the transmitter; but it was equally impossible to judge of the effects produced by our articulate transmitter at a short distance away because the speaker's voice was directly audible through the air. The extremely loud sounds produced from lamp-black have enabled us to demonstrate the feasibility of using this substance in an articulating photophone in place of the electrical receiver for- merly employed. The drawing (Fig. 2) illustrates the mode in which the experi- ment was conducted. The diaphragm of the transmitter (A) was only 5 centimetres in diameter, the diameter of the re- ceiver (B) was also 5 centimetres, and the distance between the two was 40 metres, or 800 times the diameter of the transmit- ting diaphragm. We were unable to experiment at greater distances without a heliostat on account of the difficulty of keeping the light steadily directed on the receiver. Words and sentences spoken into the transmitter in a low tone of voice were audibly reproduced by the lamp-black receiver. In Fig. 3 is shown a mode of interrupting a beam of sunlight for producing distant effects without the use of lenses. Two similarly-perforated disks are employed, one of which is set in rapid rotation, while the other remains stationary. This form of interrupter is also admirably adapted for work with artificial 11 light. The receiver illustrated in the drawing consists of a parabolic reflector, in the focus of which is placed a glass ves- sel (A) containing lamp-black or other sensitive substance, and connected with a hearing-tube. The beam of light is inter- rupted by its passage through the two slotted disks shown at B, and in operating the instrument musical signals like the dots and dashes of the Morse alphabet are produced from the sensi- tive receiver (A) by slight motions of the mirror (C) about its axis (D.) In place of the parabolic reflector shown in the figure a coni- cal reflector like that recommended by Prof. Sylvanus Thomp- son* can be used, in which case a cylindrical glass vessel would be preferable to the flask (A) shown in the figure. In regard to the sensitive materials that can be employed, our experiments indicate that in the case of solids the physical condition and the color markedly influence the intensity of the sonorous effects. The loudest sounds are produced from substances in a loose, porous, spongy condition, and from those that have the darkest or most absorbent colors. The materials from which the best effects have been obtained are cotton-wool, worsted, fibrous materials generally, cork, sponge, platinum and other metals in a spongy condition, and lamp-black. The loud sounds produced from such substances may per- haps be explained in the following mariner: Let us consider, for example, the case of lamp-black a substance which be- comes heated by exposure to rays of all refrangibility. I look upon a mass of this substance as a sort of sponge, with its pores filled with air instead of water. When a beam of sunlight falls upon this mass, the particles of lamp-black are heated, and consequently expand, causing a contraction of the air-spaces or pores among them. Under these circumstances a pulse of air should be expelled, just as we would squeeze out water from a sponge. The force with which the air is expelled must be greatly in- creased by the expansion of the air itself, due to contact with the heated particles of lamp-black. When the light is cut off *Pbil. Mag., April, 1881, vol. xi, p. 286. 12 the converse process takes place. The lamp-black particles cool and contract, thus enlarging the air spaces among them, and the enclosed air also becomes cool. Under these circum- stances a partial vacuum should be formed among the particles, and the outside air would then be absorbed, as water is by a sponge when the pressure of the hand is removed. I imagine that in some such manner as this a wave of con- densation is started in the atmosphere each time a beam of sun- light falls upon lamp-black, and a wave of rarefaction is origi- nated when the light is cut off. We can thus understand how it is that a substance like lamp-black produces intense sonorous vibrations in the surrounding air, while at the same time it communicates a very feeble vibration to the diaphragm or solid bed 'upon which it rests. This curious fact was independently observed in England by Mr. Preece, and it led him to question whether, in our experi- ments with thin diaphragms, the sound heard was due to the vibration of the disk or (as Prof. Hughes had suggested) to the expansion and contraction of the air in contact with the disk confined in the cavity behind the diaphragm. In his paper read before the Royal Society on the 10th of March, Mr. Preece de- scribes experiments from which he claims to have proved that the effects are wholly due to the vibrations of the confined air, and that the disks do not vibrate at all. I shall briefly state my reasons for disagreeing with him in this conclusion : 1. When an intermittent beam of sunlight is focussed upon a sheet of hard rubber or other material, a musical tone can be heard, not only by placing the ear immediately behind the part receiving the beam, but by placing it against any portion of the sheet, even though this may be a foot or more from the place acted upon by the light. 2. When the beam is thrown upon the diaphragm of a " Blake Transmitter," a loud musical tone is produced by a telephone connected in the same galvanic circuit with the carbon button, (A,) Fig. 4. Good effects are also produced when the carbon button (A) forms, with the battery, (B,) a portion of the pri- mary circuit of an induction coil, the telephone (C) being placed in the secondary circuit. Fig. 6. Fig. 5. n In these cases the wooden box and mouth-piece of the trans- mitter should be removed, so that no air-cavities may be left on either side of the diaphram. It is evident, therefore, that in the case of thin disks a real vibration of the diaphragm is caused by the action of the in- termittent beam, independently of any expansion and contrac- tion of the air confined in the cavity behind the dw/phrd^m. Lord Rayleigh has shown mathematically that a to-and-fro vibration, of sufficient amplitude to produce an audible sound, would result from a periodical communication and abstraction of heat, and he says: "We may conclude, I think, that there " is at present no reason for discarding the obvious explanation " that the sounds in question are due to the bending of the " plates under unequal heating." (Nature, xxiii, p. 274.) Mr. Preece, however, seeks to prove that the sonorous effects cannot be explained upon this supposition ; but his experimental data are not sufficient to support his conclusion. Mr. Preece expected that if Lord Rayleigh's explanation was correct, the expansion and contraction of a thin strip under the influence of an intermittent beam could be caused to open and close a galvanic circuit so as to produce a musical tone from a tele- phone in the circuit. But this was an inadequate way to test the point at issue, for Lord Rayleigh has shown (Proc. of Roy. Soc., 1877) that an audible sound can be produced by a vibra- tion whose amplitude is less than a ten-millionth of a centime- tre, and certainly such a vibration as that would not have suf- ficed to operate a " make-and-break contact" like that used by Mr. Preece. The negative results obtained by him cannot, therefore, be considered conclusive. The following experiments (devised by Mr. Tainter) have given results decidedly more favorable to the theory of Lord Rayleigh than to that of Mr. Preece : 1. A strip (A) similar to that used in Mr. Preece's experi- ment was attached firmly to the centre of an iron diaphragm, (B,) as shown in Figure 5, and was then pulled taut at right angles to the plane of the diaphragm. When the intermittent beam was focussed upon the strip (A) a clear musical tone could be heard by applying the ear to the hearing-tube (C.) 18 This seemed to indicate a rapid expansion and contraction of the substance under trial. But a, vibration of the diaphragm (B) would also have re- sulted if the thin strip (A) had acquired a to-and-fro motion, due either to the direct impact of the beam or to the sudden expansion of the air in contact with the strip. 2. To test whether this had been the case an additional strip (D) was attached by its central point only to the strip under trial, and was then submitted to the action of the beam, as shown in Fig. 6. It was presumed that if the vibration of the diaphragm (B) had been due to a pushing force acting on the strip (A,) that the addition of the strip (D) would not interfere with the effect. But if, on the other hand, it had been due to the longitudinal expansion and contraction of the strip (A,) the sound would cease, or at least be reduced. The beam of light falling upon strip (D) was now interrupted as before by the rapid rotation of a perforated disk, which was allowed to come gradually to rest. No sound was heard excepting at a certain speed of rotation, when a feeble musical tone became audible. This result is confirmatory of the first. The audibility of the effect at a particular rate of interruption suggests/ the explanation that the strip D had a normal rate of vibration of its own. When the frequency of the interruption of the light corres- ponded to this, the strip was probably thrown into vibration after the manner of a tuning-fork, in which case a to-and-fro vibration would be propagated down its stem or central support to the strip (A.) This indirectly proves the value of the experiment. The list of solid substances that have been submitted to experiment in my laboratory is too long to be quoted here, and I shall merely say that we have not yet found one solid body that has failed to become sonorous under proper conditions of experiment.* Experiments with Liquids. The sounds produced by liquids are much more difficult to * Carbon and thin microscope glass are mentioned in my Boston paper as non-responsive, and powdered chlorate of potash in the communication to the French Academy, (Comptes Rendus, vol. xcl, p. 595.) All these substances have since yielded sounds under more careful conditions of experiment. 19 observe than those produced by solids. The high absorptive power possessed by most liquids would lead one to expect in- tense vibrations from the action of intermittent light, but the number of sonorous liquids that have so far been found is ex- tremely limited, and the sounds produced are so feeble as to be heard only by the greatest attention and under the best cir- cumstances of experiment. In the experiments made in my laboratory a very long test-tube was filled with the liquid under examination, and a flexible rubber-tube was slipped over the mouth far enough down to prevent the possibility of any light reaching the vapor above the surface. Precautions were also taken to prevent reflection from the bottom of the test-tube. An intermittent beam of sunlight was then focussed upon the liquid in the middle portion of the test-tube by means of a lens of large diameter. Results. Clear water No sound audible- Water discolored by ink Feeble sound. Mercury No sound heard. Sulphuric ether* Feeble, but distinct sound. Ammonia " " " " Ammonio-sulphate of copper Writing ink Indigo in sulphuric acid Chloride of copper * tt u a .. The liquids distinguished by an asterisk gave the best sounds. Acoustic vibrations are always much enfeebled in passing from liquids to gases, and it is probable that a form of experi- ment may be devised which .will yield better results by com- municating the vibrations of the liquid to the ear through the medium of a solid rod. Experiments with Gaseous Matter. On the 29th ' of November, 1880, I had the pleasure of showing to Prof. Tyndall in the laboratory of the Royal Insti- tution the experiments described in the letter to Mr. Tainter from which I have quoted above, and Prof. Tyndall at once 20 expressed the opinion that the sounds were due to rapid changes of temperature in the body submitted to the action of the beam. Finding that no experiments had been made at that time to test the sonorous properties of different gases, he sug- gested filling one tes4ube with the vapor of sulphuric ether, (aTgood absorbent of heat,) and another with the vapor of bi- sulphide of carbon, (a poor absorbent,) and he predicted that if any sound was heard it would be louder in the former case than in the latter. The experiment was immediately made, and the result veri- fied the prediction. Since the publication of the memoirs of Rontgen * and Tyn- dall f we have repeated these experiments, and have extended the inquiry to a number of other gaseous bodies, obtaining in every case similar results to those noted in the memoirs re- ferred to. The vapors of the following substances w^ere found to be highly sonorous in the intermittent beam: Water vapor, coal gas, sulphuric ether, alcohol, ammonia, amylene, ethyl bromide, diethylamene, mercury, iodine, and peroxide of nitrogen. The loudest sounds were obtained from iodine and peroxide of nitrogen. I have now shown that sounds are produced by the direct action of intermittent sunlight from substances in every physi- cal condition, (solid, liquid, and gaseous,) and the probability is therefore very greatly increased that sonorousness under such circumstances will be found to be a universal property of matter. Upon Substitutes for Selenium in Electrical Receivers. At the time of my communication to the American Associa- tion the loudest effects obtained were produced by the use of selenium, arranged in a cell of suitable construction, and placed in a galvanic circuit with a telephone. Upon allowing an in- termittent beam of sunlight to fall upon the selenium a musical * Ann. der Phys. und Chem., 1881, No. 1, p. 155. tProc. Roy. Soc., vol. xxxi, p. 307. Pigr- 7. tone of great intensity was produced from the telephone con- nected with it. But the selenium was very inconstant in its action. Two pieces of selenium (even of the same stick) seldom yielded the same results under identical circumstances of annealing, &c. While in Europe last autumn, Dr. Chichester Bell, of Univer- sity College, London, suggested to me that this inconstancy of result might be due to chemical impurities in the selenium used. Dr. Bell has since visited my laboratory in Washington, and has made a chemical examination of the various samples of se- lenium I had collected from different parts of the world. As I understand it to be his intention to publish the results of this analysis very soon, I shall make no further mention of his in- vestigation than to state that lie has found sulphur, iron, lead, and arsenic in the so-called " selenium," with traces of organic matter ; that a quantitative examination has revealed the fact that sulphur constitutes nearly one per cent, of the whole mass ; and that when these impurities are eliminated the selenium ap- pears to be more constant in its action and more sensitive to light. Prof. W. G. Adams* has shown that tellurium, like selenium, has its electrical resistance affected by light, and we have at- tempted to utilize this substance in place of selenium. The arrangement of cell (shown in Fig. 7) was constructed for this purpose in the early part of 1880 ; but we failed at that time to obtain any indications of sensitiveness with a reflecting gal- vanometer. We have since found, however, that when this tellurium spiral is connected in circuit with a galvanic battery and telephone, and exposed to the action of an intermittent beam of sunlight, a distinct musical tone is produced by the tele- phone. The audible effect is much increased by placing the tellurium cell with the battery in the primary circuit of an in- duction coil, and placing the telephone in the secondary circuit. The enormously high resistance of selenium and the ex- tremely low resistance of tellurium suggested the thought that an alloy of these two substances might possess intermediate electrical properties. We have accordingly mixed together *Proc. Roy. Soc., vol. xxiv, p. 163. selenium and tellurium in different proportions, and while we do not feel warranted at the present time in making definite statements concerning the results, I may say that such alloys have proved to be sensitive to the action of light. It occurred to Mr. Tainter before my return to Washington last January that the very great molecular disturbance pro- duced in lamp-black by the action of intermittent sunlight should produce a corresponding disturbance in an electrical cur- rent passed through it, in which case lamp-black could be em- ployed in place of selenium in an electrical receiver. This has turned out to be the case, and the importance of the discovery is very great, especially when we consider the expense of such rare substances as selenium and tellurium. The form of lamp-black cell we have found most effective is shown in Fig. 8. Silver is deposited upon a plate of glass, and a zigzag line is then scratched through the lilm, as shown, dividing the silver surface into two portions insulated from one another, having the form of two combs with interlocking teeth. Each comb is attached to a screw-cup, so that the cell can be placed in an electrical circuit when required. The surface is then smoked until a good film of lamp-black is obtained, filling the interstices between the teeth of the silver combs. When the lamp-black cell is connected with a telephone and galvanic battery, and exposed to the influence of an intermittent beam of sunlight, a loud musical tone is produced by the telephone. This result seems to be due rather to the physical condition than to the nature of the conducting material employed, as metals in a spongy condition produce similar effects. For in- stance, when an electrical current is passed through spongy platinum while it is exposed to intermittent sunlight, a dis- tinct musical tone is produced by a telephone in the same cir- cuit. In all such cases the effect is increased by the use of an induction coil; and the sensitive cells can be employed for the reproduction of articulate speech as well as for the production of musical sounds. We have also found that loud sounds are produced from lamp-black by passing through it an intermittent electrical 27 current ; and that it can be used as a telephonic receiver for the reproduction of articulate speech by electrical means. A convenient mode of arranging a lamp-black cell for ex- perimental purposes is shown in Fig. 9. When an intermittent current is passed through the lamp-black, (A,) or when an in- termittent beam of sunlight falls upon it through the glass plate B, a loud musical tone can be heard by applying the ear to the hearing-tube C. When the light and the electrical cur- rent act simultaneously, two musical tones are perceived, which produce beats when nearly of the same pitch. By proper ar- rangements a complete interference of sound can undoubtedly be produced. Upon the Measurement of the Sonorous Effects Produced by Different Substances. We have observed that different substances produce sounds of very different intensities under similar circumstances of ex- periment, and it has appeared to us that very valuable informa- tion might be obtained if we could measure the audible effects produced. For this purpose w r e have constructed several differ- ent forms of apparatus for studying the effects, but as our re- searches are not yet complete, I shall confine myself to a sim- ple description of some of the forms of apparatus we have de- vised. When a beam of light is brought to a focus by means of a lens, the beam diverging from the focal point becomes weaker as the distance increases in a calculable degree. Hence, if we can determine the distances from the focal point at which two different substances emit sounds of equal intensity, we can calculate their relative sonorous powers. Preliminary experiments were made by Mr. Tainter during my absence in Europe to ascertain the distance from the focal point of a lens at which the sound produced by a substance became inaudible. A few of the results obtained will show the enormous differences existing between different substances in this respect. 28 Distance from Focal Point of Lens at which Sounds became Inaudible with Different Substances. Zinc diaphragm, (polished) 1.51 m Hard rubber diaphragm 1.90 " Tin-foil " 2.00 " Telephone " (Japanned iron) 2.15 " Zinc " (unpolished) 2.15 " White silk, (In receiver shown in Fig. 1.). ... 3.10 " White worsted, " " " " . . - - 4.01 " Yellow worsted, " " " .... 4.06 Yellow silk, " " " u .... 4.13 " White cotton-wool, " " " " 4.38 " Green silk, " " " " .... 4.52 " Blue worsted, " " " " . . . . 4.69 " Purple silk, " " " .... 4.82 " Brown silk, " " " .... 5.02 " Black silk, " " " " .... 5.21 " Red silk, " " " ..'.. 5.24 Black worsted, " " .... 6.50 " Lamp-black. In this case the limit of audibility could not be determined on account of want of space. Sound perfectly audible at a distance of 10.00 " Mr. Tainter was convinced from these experiments that this field of reseach promised valuable results, and he at once de- vised an apparatus for studying the effects, which he described to me upon my return from Europe. The apparatus has since been constructed and I take great pleasure in showing it to you to-day. (1.) A beam of light is received by two similar lenses, (A B, Fig. 10,) which bring the light to a focus on either side of the interrupting disk (C.) The two substances, whose sonorous powers are to be compared, are placed in the receiving vessels (D E) (so arranged as to expose equal surfaces to the action of the beam) which communicate by flexible tubes (F G) of equal length, with the common hearing-tube (H.) The re- ceivers (D E) are placed upon slides, which can be moved along the graduated supports (I K.) The beams of light pass- ing through the interrupting disk (C) are alternately cut off by the swinging of a pendulum, (L.) Thus a musical tone is produced alternately from the substance in D and from that 31 in E. One of the receivers is kept at a constant point upon its scale, and the other receiver is moved towards or from the focus of its beam until the ear decides that the sounds pro- duced from 1) and E are of equal intensity. The relative po- sitions of the receivers are then noted. (2.) Another method of investigation is based upon the pro- duction of an interference of sound, and the apparatus employed is shown in Fig. 11. The interrupter consists of a tuning- fork, (A,) which is kept in continuous vibration by means of an electro-magnet, (B.) A powerful beam of light is brought to a focus between the prongs of the tuning-fork, (A,) and the passage of the beam is more or less obstructed by the vibration of the opaque screens (C D) carried by the prongs of the fork. As the tuning-fork (A) produces a sound by its own vibra- tion, it is placed at a sufficient distance away to be inaudible through the air, and a system of lenses is employed for the pur- pose of bringing the undulating beam of light to the receiving lens (E) with as little loss as possible. The two receivers (F G) are attached to slides which move upon the graduated sup- ports (H I) on opposite sides of the axis of the beam, and the receivers are connected by flexible tubes of unequal length (K L) communicating with the common hearing-tube (M.) The length of the tube (K) is such that the sonorous vibra- tions from the receivers (F G) reach the common hearing-tube (M) in opposite phases. Under these circumstances silence is produced when the vibrations in the receivers (F G) are of equal intensity. When the intensities are unequal, a residual effect is perceived. In operating the instrument the position of the receiver (G) remains constant, and the receiver (F) is moved to or from the focus of the beam until complete silence is produced. The relative positions of the two receivers are then noted. (3.) Another mode is as follows: The loudness of a musical tone produced by the action of light is compared with the loudness of a tone of similar pitch produced by electrical means. A rheostat introduced into the circuit enables us to 32 measure the amount of resistance required to render the elec- trical sound equal in intensity to the other. (4.) If the tuning-fork (A) in Fig. 11 is thrown into vibra- tion by an midulatory instead of an intermittent current passed through the electro-magnet, (B,) it is probable that a musical tone, electrically produced in the receiver (F) by the action of the same current, would be found capable of extinguishing the effect produced in the receiver (G) by the action of the undu- latory beam of light, in which case it should be possible to establish an acoustic balance between the effects produced by light and electricity by introducing sufficient resistance into the electric circuit.. Upon the Nature of the Rays that Produce Sonorous Effects in Different Substances. In my paper read before the American Association last August and in the present paper I have used the word "light" in its usual rather than its scientific sense, and I have not hith- erto attempted to discriminate the effects produced by the dif- ferent constituents of ordinary light, the thermal, luminous, and actinic rays. I find, however, that the adoption of the word "photophone" by Mr. Tainter and myself has led to the assumption that we believed the audible effects discovered by us to be due entirely to the action of luminous rays. The meaning we have uniformly attached to the words u photo- phone" and "light" will be obvious from the following pas- sage, quoted from my Boston paper: " Although effects are produced as above shown by forms of " radiant energy, which are invisible, we have named the appa- " ratus for the production and reproduction of sound in this way " the ' photophone ' because an ordinary beam of light contains " the rays which are operative" To avoid in future any misunderstandings upon this point we have decided to adopt the term " radiophone" proposed by M. Mercadier, as a general term signifying an apparatus for the production of sound by any form of radiant energy, limiting the words thermophone, photophone^ and actinophone to appa- X \ N \ v 1 1 1 1 1 1 1 I 1 I JL i~n_i i i i/ ;> .^f^r, i 37 ratus for the production of sound by thermal, luminous, or actinic rays respectively. M. Mercadier, in the course of his researches in radiophony, passed an intermittent beam from an electric lamp through a prism, and then examined the audible effects produced in dif- ferent parts of the spectrum. (Co'mptes Rendus, Dec. 6th, 1 880.) We have repeated this experiment, using the sun as our source of radiation, and have obtained results somewhat different from those noted by M. Mercadier. (1.) A beam of sunlight was reflected from a heliostat (A, Fig. 12) through an achromatic lens, (B,) so as to form an image of the sun upon the slit (C.) The beam then passed through another achromatic lens (D) and through a bisulphide of carbon prism, (E,) forming a spec- trum of great intensity, which, when focused upon a screen, was found to be sufficiently pure to show the principal absorp- tion lines of the solar spectrum. The disk-interrupter (F) was then turned with sufficient ra- pidity to produce from five to six hundred interruptions of the light per second, and the spectrum was explored with the re- ceiver, (G,) which was so arranged that the lamp-black surface exposed was limited by a slit, as shown. Under these circumstances sounds were obtained in every part of the visible spectrum, (excepting the extreme half of the violet,) as well as in the ultra-red. A continuous increase in the loudness of the sound was observed upon moving the re- ceiver (G) gradually from the violet into the ultra-red. The point of maximum sound lay very far out in the ultra-red. Be- yond this point the sound began to decrease, and then stopped so suddenly that a very slight motion of the receiver (G) made all the difference between almost maximum sound and complete silence.* (2.) The lamp-blacked wire gauze was then removed and the interior of the receiver (G) was filled with red worsted. Upon exploring the spectrum as before, entirely different results were obtained. The maximum effect was produced in the green at * The results obtained in this and subsequent experiments are shown in a tabulated form in Fig. 14. 38 that part where the red worsted appeared to he black. On either side of this point the sound gradually died away, becom- ing inaudible on the one side in the middle of the indigo, and on the other at a short distance outside the edge of the red. (3.) Upon substituting green silk for red worsted the limits of audition appeared to be the middle of the blue and a point a short distance out in the ultra-red. Maximum in the red. (4.) Some hard-rubber shavings were now placed in the re- ceiver (G.) The limits of audibility appeared to be on the one hand the junction of the green and blue, and on the other the outside edge of the red. Maximum in the yellow. Mr. Tainter thought he could hear a little way into the ultra-red, and to his ear the maximum was about the junction of the red and orange.* (5.) A test-tube containing the vapor of sulphuric ether was then substituted for the receiver (G.) Commencing at the violet end, the test tube was gradually moved down the spec- trum and out into the ultra-red without audible effect, but when a certain point far out in the ultra-red was reached a dis- tinct musical tone suddenly made its appearance, which disap- peared as suddenly on moving the test-tube a very little further on. (6.) Upon exploring the spectrum with a test-tube contain- ing the vapor of iodine the limits of audibility appeared to be the middle of the red and the junction of the blue and indigo. Maximum in the green. (7.) A test-tube containing peroxide of nitrogen was substi- tuted for that containing iodine. Distinct sounds were ob- tained in all parts of the visible spectrum, but no sounds were observed in the ultra-red. The sounds were well marked in all parts of the violet, and I even fancied that the audible effect extended a little way into the ultra-violet, but of this I cannot be certain. Upon exam- ining the absorption spectrum of peroxide of nitrogen it was at once observed that the maximum sound was produced in that part of the spectrum where the greatest number of absorption lines made their appearance. * Iu the diagram Fig. 14 the mean of these readings is shown. 41 (8.) The spectrum was now explored by a selenium cell, and the audible effects were observed by means of a telephone in the same galvanic circuit with the cell. The maximum effect was produced in the red about its junction with the orange. The audible effect extended a little way into the' ultra-red on the one hand and up as high as the middle of the violet on the other. Although the experiments so far made can only be considered as preliminary to others of a more refined nature, I think we are warranted in concluding that the nature of the rays that produce sonorous effects in different substances depends upon the nature of the substances that are exposed to the beam, and that the sounds are in every case due to those rays of the spec- tram that are absorbed by the body. The Spectrophone. Our experiments upon the range of audibility of different substances in the spectrum have led us to the construction of a new instrument for use in spectrum analysis, which was de- scribed and exhibited to the Philosophical Society of Washing- ton last Saturday.* The eye-piece of a spectroscope is re- moved, and sensitive substances are placed in the focal point of the instrument behind an opaque diaphragm containing a slit. These substances are put in communication with the ear by means of a hearing-tube, and thus the instrument is converted into a veritable " spectrophone," like that shown in Fig. 13. Suppose we smoke the interior of our spectrophonic receiver, and fill the cavity with peroxide of nitrogen gas. We have then a combination that gives us good sounds in all parts of the spectrum, (visible and invisible,) except the ultra violet. Now, pass a rapidly-interrupted beam of light through some substance whose absorption spectrum is to be investigated, and bands of sound and silence are observed upon exploring the spectrum, the silent positions corresponding to the absorption bands. Of course, the ear cannot for one moment compete with the eye in the examination of the visible part of the spec- *Proc. of Phil. Soc. of Washington, April 16, 1881. 42 trum ; but in the invisible part beyond tbe red, where the eye is useless, the ear is invaluable. In working in this region of the spectrum, lamp-black alone may be used in the spectro- phonic receiver. Indeed, the sounds produced by this sub- stance in the ultra-red are so well marked as to constitute our instrument a most reliable and convenient substitute for the therrno-pile. A few experiments that have been made may be interesting. (1.) The interrupted beam was filtered through a saturated solution of alum. Result : The range of audibility in the ultra-red was slightly reduced by the absorption of a narrow band of the rays of lowest refrangibility. The sounds in the visible part of the spectrum seemed to be unaffected. (2.) A thin sheet of hard rubber was interposed in the path of the beam. Result : Well-marked sounds in every part of the ultra-red. No sounds in the visible part of the spectrum, excepting the extreme half of the red. These experiments reveal the cause of the curious fact al- luded to in my paper read before the American Association last August that sounds were heard from selenium when the beam was filtered through both hard rubber and alum at the same time. (See table of results in Fig. 14.) (3.) A solution of ammonia-sulphate of copper was tried. Result : When placed in the path of the beam the spectrum disappeared, with the exception of the blue and violet end. To the eye the spectrum was thus reduced to a single broad band of blue-violet light. To the ear, however, the spectrum re- vealed itself as two bands of sound w T ith a broad space of silence between. The invisible rays transmitted constituted a narrow band just outside the red. I think I have said enough to convince you of the value of this new method of examination, but 1 do not wish you to understand that we look upon our results as by any means complete. It is often more interesting to observe the first tot- terings of a child than to watch the firm tread of a full- grown man, and I feel that our first footsteps in this new field 45 of science may have more of interest to yon than the fuller results of mature research. This must be my excuse for having dwelt so long upon the details of incomplete experiments. I recognize the fact that the spectrophone must ever remain a mere adjunct to the spectroscope, but I anticipate that it has a wide and independent iield of usefulness in the investigation of absorption spectra in the ultra-red. 14 DAY USE RETURN TO DESK FROM WHICH BORROWED LOAN OEPT. This book is due on the last date stamped below, or on the date to which renewed. Renewed books are subject to immediate recall. Mis 10 APR 1 2 20QQ LD 21-100m.6,'56