i88 3 J 
The Velocity of Light. 
513 
and consequent counteraction it increases with regard to 
the centre of its own mass ; it propagates light wherever it 
meets with resistance, with matter which has to accommo- 
date itself to the polar and multipolar motions of molecules, 
to their inter- and en-molecular axes, faces, and stratifica- 
tions, and finally turns to heat entirely absorbed. 
Be S the sun ; a, an atom of his involved in a light gene- 
rating combination, repelling it from S ; c, a molecule of a 
satellite, L, of Jupiter turned towards a, and not separated 
from it by opaque masses; and b, an atom of the earth 
turned towards c : a , in itself a microcosmos, is repelled 
from S and brought nearer c ; it is therefore more attracted 
by L than the mean S, and therefore next exercises an in- 
creased repelling and frictional influence against c, which 
thus repels and presses interior molecules, /, towards the 
centre of L ; c describes an orbit towards the centre of L. 
This motion puts to greater rotation the more resisting, 
denser /, which, thus induced to greater rotary motion, 
renders only part of the acquired orbital motion as reflected 
motion, as light towards b. But all this transmission of 
orbital motion of waves, this propagation of light and radiant 
heat, requires time, — a completed circuit of coming and 
going motion between whole strata of generator and re- 
ceiver. 
Sound does not reach, like light and heat, through space; 
it is not heard through vacuum, but it can be more or less 
communicated by any matter contiguous or made conti- 
guous. The spaces dividing heavenly bodies are therefore 
empty spaces ; even light and radiant heat carry no message 
from them to our ear, but they carry it through our atmo- 
sphere. 
Sound is the result of a propulsion and repulsion origin- 
ated by that attraction which puts the striking body in 
motion. The amount and quality of shock, and the qualities 
of generator and receiver, determine the qualities of sound ; 
thus the distance, power, and qualities of generator and 
receiver of light and heat determine their quantities and 
qualities. 
The velocity of sound does not depend on the shock which 
produces it, but on the medium in which it is propagated ; 
any over motion, for instance by wind, being a transport of 
the medium itself with its vibrations, or a mutation from 
sound to silent different motion, again mutated to sound. 
The velocities of light and radiant heat also depend on 
qualities of the medium in which they travel, and most 
media even exclude the propagation unless reduced to less. 
