Dr. Young’s Lecture on 
32 
PROPOSITION VII. 
If equidistant Undulations be supposed to pass through a Medium , 
of which the Parts are susceptible of permanent Vibrations some - 
what slower than the Undulations , their Velocity will be some- 
what lessened by this vibratory Tendency ; and , in the same 
Medium , the more , ay the Undulations are more frequent. 
For, as often as the state of the undulation requires a change 
in the actual motion of the particle which transmits it, that 
change wilt be retarded by the propensity of the particle to 
continue its motion somewhat longer : and this retardation will 
be more frequent, and more considerable, as the difference be- 
tween the periods of the undulation and of the natural vibration 
is greater. 
Corollary. It was long an established opinion, that heat con- 
sists in* vibrations of the particles of bodies, and is capable of 
being transmitted by undulations through an apparent va- 
cuum. (Newt. Opt. Qu. 18.) This opinion has been of late 
very much abandoned. Count Rumford, Professor Pi ciet, and 
Mr. Davy, are almost the only authors who have appeared to 
favour it ; but it seems to have been rejected without any good 
grounds, and will probably very soon recover its popularity. 
Let us suppose that these vibrations are less frequent than 
those of light; all bodies therefore are liable to permanent 
vibrations slower than those of light; and indeed almost all are 
liable to luminous vibrations, either when in a state of ignition, 
or in the circumstances of solar phosphori ; but much less easily, 
and in a much less degree, than to the vibrations of heat. It will 
follow from these suppositions, that the more frequent luminous 
undulations will be more retarded than the less frequent ; and 
