470 A. G. Bell— Production of Sound by Radiant Energy. 
and in operating the instrument musical signals like the dots 
and dashes of the Morse alphabet are produced from the sensi- 
pahte receiver (A) by slight motions of the mirror (C) about its 
xis (D). 
ie place of the parabolic reflector shown in the figure a coni- 
cal reflector like that recommended by Professor Sylvanus 
Thompson * can be used, in which case a cylindrical glass ves- 
sel 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 mania and from those that have the 
darkest or most absorbent colo 
The materials from isoh ‘the best effects have been obtained 
are cotton-wool, worsted, fibrous materials generally, cork, 
ie platinum and other metals in a spongy condition, and 
amp- 
The loud sounds produced from such substances may per- 
haps be explained in the following manner: 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 
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 remove 
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 com- 
municates a very feeble vibration to the diaphragm or solid bed 
upon whieh it rests. 
_*Phil. Mag., April, 1881, vol. xi, p. 286. 
