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.
1.7. Fundamental properties of air
We will end this chapter by returning to the main constituent of the atmosphere, air, and by analyzing what the entry AIR from the Encyclopédie tells us about its three fundamental properties, namely its fluidity, its gravity and its elasticity. The idea that heat promotes the fluidity of the body by the agitation of the parts, a very high degree of agitation, beyond the level necessary to produce fluidity, producing heat, and at the extreme limit the burning, which we find, for example, in the entry HEAT of the Lexicon, finds its exact translation in the following passage of the Encyclopédie:
Some modern philosophers attribute the cause of the fluidity of the air to the fire which is intermingled with it, without which the whole atmosphere, according to them, would harden into a solid and impenetrable mass; and indeed, the greater the degree of fire, the more fluid, mobile and permeable it is; and according to whether the different positions of the Sun increase or decrease this degree of fire, the air always receives a proportionate temperature from it.
Concerning the gravity of air, the author of the entry cites experiments conducted in pneumatic machines, which prove the weight of air. But where does its weight come from? “Some people may doubt that air is heavy by itself, and believe that its gravity may come from the vapors and exhalations it is filled with. There is no reason to doubt that the gravity of air does indeed depend in part on the vapor.” In support of this view, the author describes an experiment in which a glass ball full of air, closed at the top by a partition with small holes, is pumped out completely. Afterward, the partition is covered with salt of tartar, and the air is allowed to enter slowly through the salts into the ball. And this is what the author says:
If the air in the atmosphere is dry, it will be found that the air which had previously filled the ball was of the same gravity as the air which entered through the salts; and if it is humid, it will be found that the air which has passed through the salts is lighter than the air which had previously filled the ball. But although this experiment proves that the gravity of the air depends in part on the vapors that swim through it, one cannot help but recognize that the air is heavy by itself; for otherwise it would not be possible to conceive how the clouds that weigh a lot could remain suspended in it, more often than not only floating in the air with which they are in equilibrium.
Then, different values of the weight of air relative to that of water, estimated by Riccioli, Marin Mersenne, Galileo or Boyle, are provided, all on the order of 1:1000. Measurements made in the presence of the Royal Society of London gave a proportion varying between 1:840 and 1:885. But Musschenbroek gave a much wider range of variation:
Musschenbroek says he sometimes found the gravity of air to be as heavy as water, like 1 to 606, when the air was very heavy. He adds that by doing this experiment during different years and in different seasons, he observed a continuous difference in this proportion of gravity; so that according to the experiments made in various places of Europe he believes that the ratio of the gravity of air to that of water must be reduced to certain limits, which are about 1 to 606, and from there to 1000.
Such a variation, much greater than the natural variation of the barometric pressure, suggested a considerable contribution of vapors and exhalations to the weight of the atmosphere, on the order of at least 30%, this conception of an atmosphere heavily charged with impurities having, as we will see, a strong impact on the understanding by French scientists of the functioning of the atmosphere at the beginning of the 18th century.
Concerning the elasticity of air, which was considered at the time to be unique, contrary to gravity and fluidity, which it shares with other fluids, it was noted that the law of expansion, known today as Boyle–Mariotte’s law, was not entirely accurate when the air was reduced to a volume four times smaller, and that if it were compressed even more, there must be a limit beyond which the parts touched and formed a solid mass, preventing any compression. Similarly, the rule could not be perfectly accurate at large expansions because when the air is as thin as possible, it is not loaded with any weight, yet it occupies a certain amount of space. The spring of the air does not weaken with time: “Mr. de Roberval, having left a wind gun loaded for 16 years with condensed air, and finally releasing this air, pushed a bullet with as much force as very recently condensed air could have done.” How much air can be compressed?
Boyle has found a way to make the air 13 times denser by compressing it: others claim to have seen it reduced to a volume 60 times smaller. Mr. Hales made it 38 times denser by using a press: but by freezing water in a grenade or iron ball, he reduced the air to a volume to 1838 times smaller, so that it must have been more than twice as heavy as water; so, because water cannot be compressed, it follows that the air parts must be of a very different nature than water: for otherwise air could only have been reduced to a volume 800 times smaller; it would then have been precisely as dense as water, and it would have resisted all kinds of pressure with a force equal to that found in water.
This result from Stephen Hales was contested by Halley, who thought that air could not be compressed more than 800 times, because it then reached the density of water, and could not be compressed any more, as water was considered incompressible. Concerning the dilatability of air, “one can […] conclude, according to Musschenbroek, from some rather crude experiments, that air which is close to our globe, can expand to occupy a space 4,000 times larger than the one it occupied.” By making successive expansions of a volume of air, Boyle concluded that “the air we breathe near the surface of the Earth is condensed by the compression of the upper column into a space at least 13,679 times smaller than the space it would occupy in a vacuum.” He seemed to suppose that we could reduce by compression a volume of gas to 1/40th of its value, when he posited that “if this same air is condensed by art, the space it will occupy when it is as much as it can be, will be at the space it occupied in this first state of condensation, as 550,000 is at 1.” Finally, the degree of expansion of the air influences its capacity to penetrate bodies, such as wood: according to Musschenbroek, “when the