XLIII
§ 97. 2) Dangerous gases. Carbonic acid produced by
fermentation, 239; carbonic acid, sulphuretted hydrogen, car
buretted hydrogen gas produced in mines, wells and sewers;
how do we prove the existence of such gases? Davy's Safety
Lamp, 240; people killed by choke-damp. 241.
CHAPTER VIII.
Sound Acoustics. 242-257.
§ 98. Sound produced by a vibratory motion; how this
may be verified see: 242.
§ 99. 1.) The Propagation of sound, 243; sound-waves
or condensed and expanded layers of air. The origin and pro
pagation of sound-waves are similar to the manner in which
waves of water come into existence and extend. Just as that part
of the water in a lake, which has been agitated by throwing in a
stone, does not proceed to the shore, but after it has put the
neighbouring parts in motion it recedes, so is it with the propa
gation of sound, 244; thus sound is not propagated in a vacu
uım. 245.
§ 100. Causes which influence the intensity of sound.
a) The intensity of sound is inversely as the square of the
distance of the sounding body from the ear. 246, Sound-waves
are hollow balls and the surfaces of balls are in proportion to the
squares of their radii.
b). It is influenced by the mass, elasticity and density
of the body, which vibrates or yields a sound. 247.
c) The intensity of sound depends on the density of the
air, in which it is produced and propagated. 249.
d) It is modified by the direction of the wind. 249.
