51(3 Trof. A. G. Boll on the Production of 



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 

 increased by the expansion of the air itself, due to contact 

 with the heated particles of lampblack. When the light is 

 cut off, the converse process takes place. The lampblack 

 particles cool and contract, thus enlarging the air-spaces 

 among them, and the enclosed 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 the lampblack, and a wave of rarefaction is ori- 

 ginated when the light is cut off. We can thus understand how 

 it is that a substance like lampblack produces intense sonorous 

 vibrations in the surrounding air, while at the same time it com- 

 municates 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 ex- 

 periments 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 describes 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 focused 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 primary circuit of an induction-coil, the tele- 

 phone (C) being placed in the secondary circuit. 



