192 THE POPULAR SCIENCE MONTHLY. 



a hearing-tube. The beam of light is interrupted by its passage through 

 the two slotted disks shown at B, and in operating the instrument mu- 

 sical signals like the dots and dashes of the Morse alphabet are pro- 

 duced from the sensitive receiver (A) by slight motions of the mirror 

 (C) about its axis (D). 



In place of the parabolic reflector shown in the figure a conical 

 reflector like that recommended by Professor Sylvanus Thompson * 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 are two conditions that markedly influence the intensity 

 of the sonorous effects. The loudest sou7ids are produced from sub- 

 stances in a loose, i^orous, spongy condition, and from those that have 

 the darkest or most ahsorhent colors. 



The materials from which the best effects have been produced are 

 cotton-wool, worsted, fibrous materials generally, cork, sponge, plati- 

 num, and other metals in a spongy condition, and lampblack. 



The loud sounds produced from such substances may perhaps be 

 explained in the following manner : Let us consider, for example, the 

 case of lampblack 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 with air instead of water. When- 

 a beam of sunlight falls upon this mass, the particles of lampblack 

 are heated, and consequently expand, causing a contraction of the air- 

 sj^aces 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 increased 

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

 ticles of lampblack. When the light is cut off, the converse process 

 takes place. The lampblack particles cool and contract, thus enlarg- 

 ing 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 condensation 

 is started in the atmosphere each time a beam of sunlight falls upon 

 lampblack, and a wave of rarefaction is originated when the light is 

 cut off. We can thus understand how it is that a substance like lamp- 

 black p>roduces 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 ivhich it rests. 



This curious fact was independently observed in England by Mr. 

 Preece, and it led him to question whether, in our experiments with 



* "Philosophical Magazine," April, 1881, vol. xi, p. 286. 



