Читать книгу Physics of the Terrestrial Environment, Subtle Matter and Height of the Atmosphere - Eric Chassefiere - Страница 18

The entry ETHER in both the DUF and the Encyclopédie sheds complementary light. Ether is, according to the DUF-1690, “that pure substance which is above the atmosphere, which fills all the sky where the stars take their course.” The 1727 edition offers further information on the rarity of ether in relation to air, and the existence of different ethers of varying degrees of rarity, “making ether a suitable means of transmitting light and the influences of the most distant stars.” The entry in the Encyclopédie uses a definition of ether quite similar to that in the DUF. Ether, whose necessity of existence is invoked to prevent “the majority of the universe from being entirely empty”, either fills only the space between the celestial bodies, above the atmosphere, or is “of such a subtle nature, that it penetrates the air and other bodies, and occupies their pores and intervals.” Some believed that ether did not exist, and that “air itself, by its extreme tenuousness and by this immense expansion of which it is capable, can spread out into the intervals of the stars and be the only matter there.” The very term ether is ambiguous; some people called ether a fluid of the same nature as other bodies, but distinguished by its tenuousness, whereas ancient tradition attributed to it a purer and more subtle nature than that of “substances around the Earth”. There is thus a continuum of representations of ether, which goes from pure air to the most subtle matter. A subtle matter, as defined in the Encyclopédie, is originally “the name that Cartesians give to a matter that they suppose to pass through and freely penetrate the pores of all bodies, and fill these pores so as not to leave any voids or interstices between them.” For this matter to leave no void, it must not itself contain any, which implies that it is “perfectly solid, much more solid, for example, than gold, and therefore much heavier than gold, and more resistant.” This is judged “not to agree with phenomena”, such as the regular movements of the planets, as well as Newton’s argument against the Cartesian system: “if the heavens were filled exactly with fluid matter, however subtle, they would resist the movement of the planets and comets much more than mercury would”. But Newton nevertheless agreed that there was subtle matter, “or a medium much looser than air, which penetrates the densest bodies”. Following the thermometer experiment already described, he concluded that the heat passes through the glass of the pneumatic machine emptied of its air, which implies the presence of an intermediate body passing through the pores of the glass and propagating heat, just like light. This intermediate body, which he called the ethereal medium, must bathe the whole space since, after passing through the glass, it must pass through all the other bodies. Having established the existence of this ethereal medium, Newton moved on to its properties:

and says that it is not only rarer and more fluid than air, but also much more elastic and more active; and that by virtue of these properties, it can produce many of the phenomena of nature. It is, for example, to the pressure of this medium that Newton seems to attribute the gravity of all other bodies; and to its elasticity, the elastic force of air and nerve fibers, emission, refraction, reflection, and other phenomena of light.

Thus, Newton’s ethereal medium explains not only the propagation of light and heat in a vacuum, but also the gravity of bodies and the elasticity of the air, which is conferred on it by the ether with which it is mixed.

The Lexicon entry ÆTHER begins with Hooke‘s definition of it as a “Medium or Fluid Body in which all other Bodies do as it were swim and move”, but he adds that this conception is too close to the “Cartesian Doctrine of an Absolute Plenum, which by many Infallible Reasons and Experiments is proved to be impossible”. And this is how, with many nuances, he defines ether:

As therefore we call the Medium in which we breath and live, the Air, by which we understand an Elastical Fluid Body, either having its very large Interstices devoid of all Matter, or else filled in part with a Fluid which is very easily moved out of them by Compression, and which readily returns into them again when that Compression is taken of: So we agree to call that finer Fluid Body, if it be a Body, which is extended round our Air and Atmosphere, above it and beyond it, up to the Planets, or to an Indefinite Distance; this, I say, we call the Æther, tho’ what we mean by that word, we scarce well understand. For that there can be no Fluid whose parts do resist the Motions of Bodies thro’ them (as our Air does) in the Planetary Regions, we are certain almost to a Demonstration; because the Motion of the Heavenly Bodies is by no means impeded or altered by any such Resistance, but they move as freely as if they were in an absolute Void. But that which is often meant by the word Æther or Æthereal Matter, is a very fine thin Diaphanous Fluid, which some will have to surround the Earth up to as far as the Interstellary World, and which easily penetrates and runs thro’ all things, and lets all things run as easily thro’ it.

Thus, the conception of ether expressed in the Lexicon differs little from that in the DUF and the Encyclopédie, and it differs from the Cartesian system, of which even Hooke’s position is considered too close, in favor of an extremely light ether filling all space, as introduced by Newton, at the risk of the incoherence of his thought. At the beginning of the 18th century, ether was therefore a vague notion, as well as a proliferating one, whose existence was far from being unanimously accepted.