May 12, 1881] 



NA TURE 



example, the case of lamp-black — a substance which becomes 

 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 

 wiLh 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 circumstancei 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 increased 

 by the expansion of the air itself, due to contact with the heated 

 particles of lamp-black. When the light is cut off the converse 

 process takes place. The lamp-black particles co )1 and contract, 

 thus enlarging the air-spaces among them, and the inclosed air 

 also becomes cool. Under these circumstances 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 

 originated when the light is cut off. We can thus understand 

 hoiv it is that a substance like lomp-bUck 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 upju which it rests. 



In his paper read before the Royal Society on March 10 Mr. 

 Preece describes experiments from which he claims to have 

 proved that the effects are wholly due to the vibrations of the 

 confined air, and tint the disks do not vibrate at all. 



But for reasons stated Mr. Bell concludes that in the case of 

 thin disks a real vibration of the diaphragm is caused by the 

 action of the intermittent beam, independently of any expansion 

 and con'.ractio.i of the air confined in the cavity behind the 

 diaphragm. Lord Rayleigh has shown mathematically that a 

 to-and-fro vibration, of sufficient am,Jitude to produce an audible 

 sound, would result fro.m a periodical comjiunication and 

 abstraction of heat, and he says : " We may conclude, I think, 

 that there is at present no reaion for discarding the obvious 

 explan.ition that the sounds in question are due to the bending of 

 th* plates under unequal heating" (Nature, vol. xxiii. p. 274). 



[Mr. Bell then describes experiments (devised by Mr. Tainter) 

 which have given re-ults decidedly more favourable, in his 

 opinion, to the theory of Lord Rayleigh than to that of Mr. 

 Preece.] 



The list of solid sub.tances that have been submitted to 

 experiment in my laboratory is too long to be quoted here, and 

 I shall merely say th it we have not yet found one solid body 

 that has failed to become sonorous under proper conditions of 

 experiment.^ 



The sounds produced by liquids are much more difficult to 

 observe than those produced by solids. The high absorptive 

 power possessed by most liquids would lead one to expect 

 i.itense 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 circum- 

 stances of experiment. In the experiments made in my laboratory 

 a very lon^ test-tube was filled with the liquid under examination, 

 and a flexible rubber-tube was slipped over the mouth far enough 

 d nwn to prevent the possibility of any light reaching the vapour 

 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 



Clearwater 



Water discoloured by ink 



Mercury 



No sound audible. 



Feeble sound. 



No .sound heard. 



Feeble, b.it distinct sound. 



Sulphuric ether '■ 



Ammonia 



Ammonio-sulphate of copper ... ,, ,, 



Writing ink ^^ 



Indigo in sulphuric acid ,, ,, 



Chloride of copper * ,, ^^ 



The liquids distinguished by an asterisk gave the best sounds. 



^ Carbon and thin microscope glass are mentioned in my Bjsto.i paper as 

 non-responsive, and powdered chlorate of potash in the communication to 

 the French Academy iCoinptes reiidus, vol. .\cl.. p. 5951 All these sub- 

 stances have since yielded sounds imder more careful conditions of 

 experiment. 



Acou-tic vibrations are always much enfeebled in passing 

 from liquidi 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 hquid to the air through the 

 medium of a solid rod. 



The vapours of the following substances were found to be 

 highly sonorou-i in the intermittent beam : —Water vapour, coal 

 gas, sulphuric ether, alcohol, ammonia, amylene, ethyl bromide, 

 diethylamene, mercury, iodine, and peroxi e of nitrogen. The 

 loudest sounds were obtained from iodine and peroxide of nitro- 

 gen. I have uoiv shown that sounds are produced by the direct 

 action of intermittent sunlight from subitances in every physical 

 condition (solid, liquid, and gaseju-<), and the probability is 

 therefore very greatly increased that sonorousness under such 

 circumstances will be found to be a universal property of matter. 



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. But the sele lium was 

 very inconstant in its action, and from experiments by Dr. 

 Cliichester Bell of University College of London, it was found 

 that all the seleniuoi used was tainted with impurities. 



Prof. W. G. Adams {Proceedings Royal Society, vol. xxiv. p. 

 163) has shown ;that tellurium, like selenium, has its electrical 

 resistance affected by light, and we have found that when this 

 tellurium spiral is connected in circuit with a galvanic batteiy 

 and telephone and exposed to the action of an intermittent beam 

 of sunlight, a distinct musical tone is produced by the telephone. 



It occurred to Mr. Tainter before my return to Washington 

 last January that the very great molecular disturbance produced 

 in Icimp-black by the action of intermittent sunlight should pro- 

 duce a corresponding disturbance in an electric current passed 

 through it, in which case lamp-black could be empljyed 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. 



We have ob erved that different substances produce sounds of 

 very different intensities under similar circumstances of experi- 

 ment, and it has appeared to us that very valuable information 

 might be obtained if we could measure the audible effects pro- 

 duced. For this purpose we have con^tructed several different 

 forms of apparatus for studying ihe effect.*, but our researches 

 are nit yet complete. 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. Pre- 

 liminary experiments were made by Mr. Tainter during my 

 absence in Europe to ascertain the distance from the focal point 

 of a lens at whi-h 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. 



Distance from Focal Point of Lens at which Sounds become 

 Inaudible with Diffei-ent Substances 



Zinc diaphragm (polished) I'SI 



Hard rubber diaphragm I'90 



Tinfoil ,, 2-00 



Telephone ,, (japanned iron) 2'15 



Zinc ,, (unpolished) 2'IS 



White silk (in receiver) 3'I0 



White worsted ,, 4'oi 



Yellow worsted ,, 4"°6 



Yellow silk ,, 4''3 



White cotton-wool ,, 4'38 



Green silk ,, 4'52 



Blue worsted „ 4'69 



Purple silk „ 4'82 



Brown silk „ S'°2 



Black silk „ 5'2I 



Red silk „ 5'24 



Black worsted „ O'SO 



Lamp-black. In receiver the limit of audibility could not 

 be determined on account of want of space. Sound 



perfectly audible at a dist.mce of lo'oo 



Mr. Tainter was convinced from these experiments that this 

 field of research promised valuable results, and he at once de- 



