XLIX
is produced by successive explosions produced by a periodical
combination of Oxygen with Hydrogen. After each explosion
the flame is almost extinguished, as the looking-glass shows.
§ 118. If sound-waves meet each other in such a manner,
that the condensation of one wave fills up the expansion of an
other they neutralize each other and no sound is heard. (Inter
ference). Most of the investigations on sound we owe to Helmholtz.
CHAPTER IX.
HEAT
A. Sources of Heat and its propagation.
258-290
§ 119. Definition. The duo hypotheses as to the nature of
heat: 1) Theory of emission, according to which heat is a subtle,
imponderable substance, surrounding the molecules of bodies
and passing from one body to another. 2). Theory of undu
lation, according to which heat is caused by a vibratory motion
in the bodies themselves. This vibration causes waves of
warmth in a kind of ether pervading the whole universe. Hence
the phenomenon of warmth is similar to that of sound in its
organ as well as in its propagation (through ether instead of the
air). The vibrations of this ether finally cause the sensation of
heat in our body. The waves of heat and those of light are
not different from each other, if still not all the waves of heat
produce the sensation of light, we must consider that our organ
of sight is only affected by such vibrations of ether, which show
a certain length of their waves. The atoms of all bodies are
more or less in a state of such vibration. One and the same
D
