Читать книгу Physics of the Terrestrial Environment, Subtle Matter and Height of the Atmosphere - Eric Chassefiere - Страница 19

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 air is expanded to a certain point, it no longer passes through the pores of all kinds of wood.”

Before closing this chapter, let us return to the other essential constituent of the atmosphere, namely ethereal matter, whose existence is postulated, as we have seen, particularly on the basis of the fact that light and heat propagate in the vacuum created in a pneumatic machine. This ethereal matter splits into a large number of subtle matters, which are invoked to explain various phenomena. For example, there is magnetic matter, responsible for the orientation of the magnetic needle and for the attraction between the magnet and iron, which operates in a vacuum, as Boyle’s experiments have shown; or electrical matter, responsible for the glow and the effects of attraction and repulsion between electrified bodies, which is transmitted through the vacuum, as attested by the sparks obtained by Francis Hauksbee in his static electricity generator. Some of these matters are supposed to be identical, for example, those that transmit light and heat, according to Newton. Nollet equated electrical matter with fire matter. Laurent Béraud tried to unify magnetic matter and electrical matter. Many theories were developed to achieve a more integrated vision of ether and subtle matter. Most of the above-mentioned matter, as well as some others, such as Cassini’s refractive matter, Descartes’ solar matter or the subtle air invoked by Jean Bernoulli to explain the mercurial phosphorus (luminous barometer), take part in the representations of the atmosphere which were developed at the end of the 17th century and in the 18th century to explain the natural phenomena being used to estimate its height: the refraction of starlight (refractive matter), the reflection of sunlight (the effect of the solar atmosphere, and therefore of solar matter, on the height of the atmosphere estimated by the twilight method), fiery meteors passing through the atmosphere and falling stars (electrical matter), the aurora borealis (solar matter, subtle air, magnetic matter), etc. The purpose of the following chapters is specifically to examine these subtle matters, and the way they are supposed to interact with the atmosphere to explain the different phenomena that define the quest to determine its height.