ON LIGHT. CHAP. I.

B 2

...the famous experiment of TorricelliEvangelista Torricelli (1608–1647), the Italian physicist who invented the barometer.; where the glass tube, from which the quicksilverThe historical name for mercury (original: vif argent). has withdrawn, remaining entirely empty of air, transmits light just as it does when there is air present: for this proves that a matter different from air is found in this tube, and that this matter must have pierced the glass, or the quicksilver, or both, which are both impenetrable to air. And when, in the same experiment, a vacuum is created by putting a little water over the quicksilver, one concludes similarly that the said matter passes through the glass, or the water, or through both.

As for the different ways in which I said that the movements of Sound and of light are successively communicated, one can understand well enough how this occurs regarding Sound when one considers that air is of such a nature that it can be compressed and reduced to a space much smaller than it usually occupies; and that as it is compressed, it strives to expand again: for this, combined with its penetrability, which remains despite its compression, seems to prove that it is made of small bodies that swim and are agitated very quickly in the ethereal matter, composed of much smaller parts. Thus, the cause of the expansion of the waves of Sound is the effort made by these small bodies, which collide with one another, to expand again when they are a bit more crowded within the circuit of these waves than elsewhere.

But the extreme speed of light, and other properties it has, could not admit such a propagation of movement, and I will show here in what manner I conceive it must be. To do this, one must explain the property that hard bodies have in transmitting movement to one another.

When one takes a number of balls of equal size, made of some very hard material, and arranges them in a line