The Luminiferous Aether

But sound only travels 180 toises per second.

Hence, the speed of Light is more than 600,000 times greater than that of sound.

• However, it is not instantaneous, since there is a difference between a finite thing and an infinite.

It follows that light spreads by spherical waves, like the movement of Sound.

But it also differs from sound in terms of:

• how its first movement is caused
• how the movement spreads
• how the movement is propagated.

Sound is produced by the agitation of a body which shakes all the contiguous air.

But the movement of the Light originates from each point of the luminous object.

The Luminiferous Aether

This movement is best explained by supposing that all liquid luminous bodies, such as flames, and the sun and the stars, are composed of particles which float in a much more subtle medium.

This medium:

• agitates them with great rapidity
• makes them strike against the ether particles which surrounds them which are much smaller than the light particles.

This same movement occurs in luminous solids such as charcoal or metal made red hot in the fire when the metal or wood particles are violently agitated.

• Those of them on the surface similarly strike against the ethereal matter.

The agitation of the particles which engender the light should be much more prompt and rapid than those of the bodies which cause sound.

This is because we the tremors of a body giving out a sound are not capable of emitting Light just as the movement of the hand in the air is not capable of producing sound.

The movement of light coming from the luminous body is propagated through what I call “Ethereal matter”.*

• It is not the same that serves for the propagation of sound.

The medium for the propagation of sound is the air.

• We feel it when we breathe.

It is removed [in a vacuum] yet it still leaves there the ethereal matter that conveys Light.

This may be proven by shutting up a sounding body in a glass vessel from which the air is withdrawn by the machine from Mr. Boyle.

Care must be taken to place the sounding body on cotton or on feathers so that it cannot communicate its tremors:

• to the glass vessel which encloses it, or
• to the machine

After removing all the air, one hears no Sound from the metal, though it is struck. Yet the light still traverses it as before.

It proves that the air is the matter by which Sound spreads.

Torricelli’s celebrated experiment has a tube of glass from which the quicksilver has withdrawn itself, remaining void of air. It transmits Light just the same as when air is in it.

This proves that a matter different from air exists in this tube. It must have penetrated the glass or the quicksilver even if they are both impenetrable to the air.

When, in the same experiment, one makes the vacuum after putting a little water above the quicksilver, one concludes equally that the said matter passes through glass or water, or through both.

Air can be compressed to a much smaller space. This explains the differnt movements of sound.

The more the air is compressed, the more it exerts an effort to regain its volume.

This property, along with its penetrability, remains despite its compression.

It proves that it is made up of small bodies which:

• float around
• are agitated very rapidly in the ethereal matter composed of much smaller parts.

Sound spreads through the effort which these little bodies make in collisions with one another, to regain freedom when they are a little more squeezed together in the circuit of these waves than elsewhere.

But the extreme speed of Light and its other properties cannot admit of such a propagation of motion.