58 



NATURE 



\_May 1 6, 1878 



up while under the influence of strain, so as to maintain 

 the pressure constant, all gave similar and invariable 

 results, but the results varied %yith the materials used. 

 All metals, however, could be made to produce identical 

 results provided the division of the metal was small 

 enough, and that the material used does not oxidise by 

 contact with the air filtering through the mass. Thus 

 platinum and mercury are very excellent and unvarying 

 in their results, whilst lead soon becomes of such high 

 resistance through oxidisation upon the surface, as to be 

 of little or no use. A mass of bright round shot is pecu- 

 liarly sensitive to sound whilst clean, but as the shot 

 soon become coated with oxide this sensitiveness ceases. 

 Carbon, again, from its surface being entirely free from 

 oxidation, is excellent, but the best results I have been 

 able to obtain at present have been from mercury in a 

 finely divided state. I took a comparatively porous 

 non-conductor, such as the willow charcoal used by 

 artists for sketching, heating it gradually to a white 

 heat and then suddenly plunging it in mercury. The 

 vacua in the pores, caused by the sudden cooling, 

 become filled with innumerable minute globules of 

 mercury, thus, as it were, holding the mercury in a fine 

 state of division. I have also tried carbon treated in a 

 similar manner with and without platinum deposited upon 

 it from the chloride of platinum. I have also found 

 similar effects from the willow charcoal heated in an iron 

 vessel to a white heat, and containing a free portion of 

 tin, zinc, or other easily vaporised metal. Under such 

 conditions the willow carbon will be found to be metal- 

 lised, having the metal distributed throughout its pores in 

 a fine state of division. Iron also seems to enter the 

 pores if heated to a white heat without being chemically 

 combined with the carbon as in graphite, and, indeed, 

 some of the best results have been obtained from willow- 

 charcoal containing iron in a fine state of division. 



" Pine charcoal treated in this manner (although a non- 

 conductor as a simple charcoal) has high conductive 

 powers, due to the iron ; and from the minute division of 

 the iron in the pores, is a most excellent material for the 

 purpose." 



The substances above referred to are in practice con- 

 fined in a glass tube or box, and provided with wires to 

 enable them to be easily inserted into a circuit. This is 

 called a transmitter. 



The resistance of the conductors employed is affected 

 by sounds absolutely inaudible, and it is this quality 

 which Prof. Hughes utilises in what he calls ^^r excellence 

 his microphone. This marvellous instrument, of which 

 we shall hear so much in the future, consists of a lozenge- 

 shaped piece of gas-carbon one inch long, \ inch wide at 

 its centre, and \ of an inch thick ; the lower pointed end 

 pivots upon a similar block, the upper rounded end plays 

 free in another carbon block ; all these pieces of carbon are 

 tempered in mercury, and carbon is used in preference to 

 any other material, as its surface does not oxidise. Prof. 

 Hughes, in his paper, states : — 



"The best form and materials for this instrument, 

 however, have not yet been fully experimented on. Still, 

 in its present shape, it is capable of detecting very faint 

 sounds made in its presence. If a pin, for instance, be 

 laid upon or taken off a table, a distinct sound is emitted, 

 or, if a fly be confined under a table-glass, we can hear 

 the fly walking, with a peculiar tramp of its own. The 

 beating of a pulse, the tick of a watch, the tramp of a 

 fly, can thus be heard at least a hundred miles distant 

 from the source of sound. In fact, when further deve- 

 loped by study, we may fairly look for it to do for us, 

 with regard to faint sounds, what the microscope does 

 with matter too small for human vision." 



The construction of the tube-transmitter exhibited to 

 the Royal Society will be seen from the annexed wood- 

 cut. It consists of an e.\terior glass tube, G, two inches 

 long and \ inch in diameter ; in it are four separate 

 pieces of willow charcoal, a is made to press on B, c 

 D, E, and F, until the resistance offered to the current is 

 about one-third that of the line on 'which it is to be 

 employed. 



Fig. 2. 



As Prof. Hughes properly remarks, it is as yet mipos- 

 sible to say what effect will flow from this wonderful 

 discovery, a discovery which shows that it is possible to 

 transmit clear and intelligent articulate speech, and to 

 render the inaudible audible by the mere impact of sound 

 waves upon matter along which an electric current is 

 flowing. 



It is not too early, however, to see that we have in the 

 microphone a new method of attaching and quantifying 

 molecular motions. 



PHYSICAL SCIENCE FOR ARTISTS^ 

 II. 



THE examples I gave in my last paper were tested by 

 a reference to the probable action of the aqueous 

 vapour of our atmosphere in absorbing the various con- 

 stituents of sunlight — the sun being the great source of 

 light with which artists are specially concerned. 



The reason that such a test was not applied long ago 

 was because we are only just now beginning to under- 

 stand why it is that the sun shines ; why its light is white, 

 and again why it is that this white light in passing through 

 a great thickness of our atmosphere as it must do at sun- 

 rise and sunset — when the beams graze the surface of the 

 earth instead of impinging upon it at a high angle — is in 

 great measure absorbed or used up before it gets to the 

 eye. The result of the condition to which I have just 

 referred is familiar to all ; at sunrise and sunset the sun 

 is red and not white. 



The light of the sun we know now is due to the quiver- 

 ing or vibration of the molecules of the matter of which 

 the sun is composed. No molecular vibration no light ; 

 given molecular vibration, the more intense it is the more 

 intense is the light produced. The absorption of the sun- 

 light by our air in the manner I have stated is due to the 

 molecules of our air already in vibration being set in still 

 stronger vibration by the sunlight passing through them. 

 Here again then we have molecules and vibrations. In 

 short the vibration of molecules, so far as light is given 

 out or reflected or quenched by them, sharply defines the 

 physical region in which artists are chiefly interested. 



In a work which recently appeared,^ I have tried 

 to show how the actions involved in sending a telegraphic 

 message may help us to form a mental image of what 

 goes on before the sensation of light is produced; we 

 have a sending instrument, a medium, and a receiving 

 instrument. 



« Continued fr m p. 31. 



2 " Stud'es in Spectrum Analysis." 



