Eric Chassefiere - Physics of the Terrestrial Environment, Subtle Matter and Height of the Atmosphere

The same physicist assures us that he has drawn a truly elastic substance from several other bodies; such as bread, grapes, beer, apples, peas, beef, etc. and from a few bodies, by burning them in a vacuum, and singularly from paper, from deer horn: but nevertheless this substance, on close examination, was so far from the nature of pure air, that the animals enclosed in it, not only could breathe only with difficulty, but even died there faster than in a vacuum, where there would have been no air at all.

Thus, the nature of the air, its primary character, or on the contrary, secondary to other bodies from which it would be derived, its resemblance to the elastic substances that we draw from humid bodies, by evaporation, or by burning dry bodies, constituted in the middle of the 18th century still unresolved questions, which made the definition of air, and of the atmosphere which was its mass, fluctuating and multiple. Air, depending on whether it is described as elementary, heterogeneous, permanent or transient, designates different substances, subtle or, on the contrary, coarse, resulting or not from the transformation of other matters, the mixture of which constitutes the atmosphere. This idea is particularly well expressed in the entry ATMOSPHERE in the Encyclopédie :

A modern author sees the atmosphere as a great chemical vessel, in which the matter of all species of sublunar bodies floats in large quantities. This vessel is, he says, like a great furnace, continuously exposed to the action of the Sun; from which it results an innumerable amount of operations, sublimations, separations, compositions, digestions, fermentations, putrefactions, etc., on the nature, constitution, properties, uses, different states of the atmosphere.

The entry ATMOSPHERE in the Lexicon depicts the atmosphere as “the lower part of the Region of the Air or Ether, with which our Earth is encompassed all round; and up into which the Vapours are carried, either by Reflection from the Sun’s Heat, or by being forced up by the Subterranean Fire”. The allusion to ether must be compared to Robert Hooke’s definition of air, as we can read in the entry AIR in the Lexicon , where it is said that Hooke “seems to think the Air to be nothing else but a kind of Tincture or Solution of Terrestrial and Aqueous Particles dissolved in, and agitated by the Ether; and these Particles he supposes to be of a Saline nature.” Thus, according to Hooke, air is a mixture of ether and vapors, and therefore it does not exist as such, other than by these vapors dissolved in ether. The definition of the atmosphere is therefore consistent, since indeed we can consider that the vapors rise in the ether, as much as in the atmosphere which is its mixture with the ether. The terms “Reflection from the Sun’s Heat” are not perfectly clear, but we can verify in the entry VAPORS that it is indeed the heat of the Sun that makes water and other bodies evaporate. After this definition, the author turns to the question of the effect of atmospheric pressure, as demonstrated by Boyle through various experiments. He cited the experiment of two polished marble slabs three inches in diameter, placed in contact with each other, and that in air, required a weight of 80 pounds to separate, while in a vacuum they separated effortlessly.

The entry devotes a paragraph to the question of the height of the atmosphere, which Johannes Kepler estimated, according to the author, to be of the order of eight miles, or 13 km (from the refraction of starlight, Kepler actually proposed a much smaller height of 3.7 km; see Lehn and van der Werf 2005), whereas Giovanni Battista Riccioli estimated at least 50 miles, or 80 km (from the duration of twilight). A value of seven miles, or 11 km, was assigned by Boyle to this height, assuming a homogeneous atmosphere of uniform density from the Earth’s surface to the top of the atmosphere. This is not the case, since air is expandable, and this height must be much greater. The author indicates that, in his reply to Linus, Boyle said that the atmosphere could reach hundreds or even thousands of miles, if it is not a limited portion of the air, but extends as high as it does (thus not being “the lower part”, but the whole). And, if we refer to Hooke’s idea of an atmosphere formed by the dissolution of vapors in the ether, to extend the atmosphere to the totality of the air is to confuse its limits with those of the ether itself, whose vertical extension is potentially infinite, and it is in this sense that we must take Boyle’s hypothesis of such considerable heights. The author stated the law of air expansion and determines the height of the atmosphere to be 45 miles, or 72 km, above which the air must lose all elasticity, and where it is therefore in its natural, uncompressed state. This height corresponds to that deduced from the duration of twilight and, moreover, at such a height, the air occupies 3000 times more volume than near the ground. Knowing that its volume could be reduced 60 times in a pneumatic machine, it is thus likely to be compressed in the 180,000th part of the space it occupies at the top of the atmosphere, in its natural state.

The entry AIR in the Lexicon defines air as a diaphanous (transparent), compressible and expandable (thus elastic) fluid, “covering the Earth and Sea to a great height above the highest Mountains”. As in the Encyclopédie , the following are cited as an essential component of air: (i) vapors, in the broad sense of vapors and exhalations, (ii) subtle matter, which we can assume to come from celestial bodies, as well as magnetic vapors from the Earth, (iii) finally, air in the strict sense, which can be compared to the “elementary” air described in the Encyclopédie , and whose main property is elasticity, a property that air could be the only substance to possess, the elasticity of the other bodies being perhaps due only to the air they contain. This elasticity must be understood as the reaction of air to any compression exerted by the atmosphere which, being located on top of it, weighs on it, or by any other body. Boyle suggested that the same portion of air can take up to 520,000 times more space at some times or places than at others. James Gregory calculated that a globe of air with a diameter of one inch, if it is as rare as it must be, according to the law of expansion, at the distance of half an Earth’s diameter from the Earth, would fill the entire planetary region within the sphere of Saturn (which reconciles the potentially infinite vertical extension of the atmosphere with the absence of friction encountered by the planets in their motion around the Sun).

The author of the entry then emphasized that it is not necessary to use subtle matter to explain the elasticity of air, probably referring to those who, like Newton, hypothesized that ether mixed with air ensures its elasticity. It would be enough, according to him, to imagine that each particle of air is a small spring, as it is also reported in the entry AIR of the Encyclopédie : “Borelli says that air is composed of corpuscles, or small, hard, flexible, springy leaves, capable of springing, and which, making several turns in a spiral line, form the figure of a hollow cylinder”. The author of the Lexicon ’s entry AIR presents a theory according to which these particles would be in rotation around their axis, with the speed of rotation, by the centrifugal force that it causes, being responsible for the greater or lesser extension of the spring of which they are composed. Notably, this could explain the fact that heat expands air, since the rapid movement of caloric particles can act on air particles by making them move away from the axis of their movement, and thus occupy more space. Boyle shows, in his discourse in response to Linus, that “the Strengths required to compress Air, are in Reciprocal Proportion (nearly) to the Spaces comprehending the same Portion of Air”. The Lexicon ’s entry THE WEIGHT OF AIR, which immediately follows, traces the history of Torricelli’s discovery of the weight of air, and describes a number of experiments by Boyle and others.