But if the one resembles the other in this respect, they differ in
many other things; to wit, in the first production of the movement
which causes them; in the matter in which the movement spreads; and in
the manner in which it is propagated. As to that which occurs in the
production of Sound, one knows that it is occasioned by the agitation
undergone by an entire body, or by a considerable part of one, which
shakes all the contiguous air. But the movement of the Light must
originate as from each point of the luminous object, else we should
not be able to perceive all the different parts of that object, as
will be more evident in that which follows. And I do not believe that
this movement can be better explained than by supposing that all those
of the luminous bodies which are liquid, such as flames, and
apparently the sun and the stars, are composed of particles which
float in a much more subtle medium which agitates them with great
rapidity, and makes them strike against the particles of the ether
which surrounds them, and which are much smaller than they. But I hold
also that in luminous solids such as charcoal or metal made red hot in
the fire, this same movement is caused by the violent agitation of
the particles of the metal or of the wood; those of them which are on
the surface striking similarly against the ethereal matter. The
agitation, moreover, of the particles which engender the light ought
to be much more prompt and more rapid than is that of the bodies which
cause sound, since we do not see that the tremors of a body which is
giving out a sound are capable of giving rise to Light, even as the
movement of the hand in the air is not capable of producing Sound.
Now if one examines what this matter may be in which the movement
coming from the luminous body is propagated, which I call Ethereal
matter, one will see that it is not the same that serves for the
propagation of Sound. For one finds that the latter is really that
which we feel and which we breathe, and which being removed from any
place still leaves there the other kind of matter that serves to
convey Light. This may be proved by shutting up a sounding body in a
glass vessel from which the air is withdrawn by the machine which Mr.
Boyle has given us, and with which he has performed so many beautiful
experiments. But in doing this of which I speak, care must be taken to
place the sounding body on cotton or on feathers, in such a way that
it cannot communicate its tremors either to the glass vessel which
encloses it, or to the machine; a precaution which has hitherto been
16