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

time to refer to a component of small particles emanating from the Earth or water, the union of which can eventually lead to the formation of clouds or mists. The term air takes on different meanings depending on whether it is, for example, coarse air or elementary air. There are similarly varying definitions of ether, which surrounds and, according to some, penetrates the atmosphere, in the latter case providing it with such characteristics as, for example, its elasticity. The proliferation of scientific ideas in the 18th century is accompanied by a multiplicity of meanings given to different words, which is necessary to keep in mind to fully understand the nature of the mechanisms that are analyzed. Chapter 1, which is not intended to be an exhaustive study, provides illuminating observations on the basis of the analysis of a number of articles on different words concerning the atmosphere in several dictionaries: the Dictionnaire Universel de Furetière, whose first edition dates back to 1690; the Encyclopédie by Diderot and d’Alembert, published from 1751; and, as a comparison between the terms used by the French and English scientific communities, the Lexicon Technicum, whose first edition dates back to 1704. There are many parallels between the words used in France and across the channel in England, with dictionary articles from either country frequently quoting authors from the other country, but there are also differences, which are due to the scientific conceptions underlying the use of the words, which in the case of the French articles are infused with Cartesian doctrine. The use of the three dictionaries also makes it possible to note certain progressions that took place in the definition of scientific words between the dawn and the middle of the Enlightenment, in a period of rapid development of scientific thought.

Chapters 2–6 focus on five subtle matters of great importance to our subject, namely refractive matter (Chapter 2), proposed in particular by Jacques Cassini at the turn of the 18th century to remove the inconsistencies of the theory attributing to vapors and exhalations, a major role in refraction; solar matter (Chapter 3), which Jean-Jacques Dortous de Mairan used to explain the aurora borealis, based on the observations of zodiacal light made by Jean-Dominique Cassini a few decades earlier; magnetic matter (Chapter 4), initially proposed by Descartes on the basis of his theory of magnetism, and which Edmond Halley invoked to give his own explanation of the aurora borealis; electrical matter (Chapter 5), which was suggested to account for meteoroids, which ignite as they enter the atmosphere (“fiery meteors”, as we will call them in this book), and shooting stars (“falling stars”, as we will call them), objects that remained mysterious during most of the 18th century, and to which an electrical origin was attributed, as well as the aurora borealis after the theories of Mairan and Halley; and finally, subtle air (Chapter 6), invoked by Mairan in support of his theory of the aurora borealis to explain the suspension of solar matter at great heights.

      Chapter 2 is devoted to refractive matter. After setting the context in terms of representations of the atmosphere at the end of the 17th century, following the discovery of its heavy nature and the elasticity of air, which made the atmosphere a physical object directly observable and measurable in a laboratory and in nature, we analyze the arguments used at the beginning of the 18th century in favor of the existence of a specific refractive matter that escapes the measurements of the barometer, in order to explain the observations of the refraction of starlight by the atmosphere. We show that this idea of a refractive matter fits well with the Cartesian thought dominant in the French Academy of Sciences at the time, and its contradictions, arising in particular from the inconsistency between the supposed major role, at the theoretical level, of condensed vapors in the process of atmospheric refraction, and the observation which, on the contrary, does not show a link between refraction and the presence of particles in suspension. This idea did not take hold or see any development across the Channel, where, at the end of the 17th century, Isaac Newton understood the essential role played by air temperature, and Halley the role of winds in the modulation of atmospheric pressure, without having to resort to the effect of vapors and exhalations. A beneficial side effect of the introduction of refractive matter has been the development of parametric models, using the differential approach, of refraction, such as that of Pierre Bouguer. These models, initially developed by French and English scientists, allowed for the creation of detailed models, including, at the middle of the 18th century, the precise consideration of temperature, and leading at the end of the century to the totally coherent model by Pierre Simon de Laplace, which signaled the definitive abandonment of refractive matter.

Chapter 3 deals with the solar atmosphere. We first examine the rich and abundant landscape at the end of the 17th century for the conceptions of sunspots, zodiacal light and comets, seen as phenomena in close relation to each other through an active solar atmosphere in many compartments of interplanetary space and planets. Then we consider the theory of the aurora borealis formulated by Jean-Jacques Dortous de Mairan at the beginning of the 18th century, namely the episodic precipitation in the Earth’s atmosphere of a subtle solar matter purported to mix with atmospheric matter and become luminous as a result of this mixture. We show how this theory fits into the framework of thought resulting from the previous two centuries, give the estimates of the height of the auroral structures made by Mairan and other scientists, put the theory in the context of the major currents of thought of the time, in particular the tendency of Cartesians to considerably increase the height of the atmosphere at the beginning of the 18th century, and detail the competing theories developed by Edmond Halley and Leonhard Euler. We also examine the impact of the existence of a solar atmosphere on the height of the atmosphere deduced from the duration of twilight periods, a question addressed at the beginning of the previous century by Johannes Kepler, who did not give the atmosphere a height greater than a few kilometers, and to which Philippe de La Hire, a century later, provides elements for an answer.

      In Chapter 4, we examine the question of magnetic matter. At the beginning of the 18th century, Halley, witness to an aurora borealis, had the intuition that the luminous figures of the aurora are the visual manifestation of magnetic matter that circulates from one pole to the other in the upper atmosphere of the Earth, or the ether, following the Cartesian representation of the vortex of the magnet. This intuition is dictated to him by the disposition of the iron filings spread in the vicinity of a magnet, reminiscent of the auroral beams. This idea was taken up again by Charles François de Cisternay du Fay, studying a few years later the properties of magnets, as a proof of the circulation of the magnetic matter in only one direction, and not in both directions, as Descartes supposed in his system of the world. This question of the circulation of magnetic matter, and in particular that of its direction of flow in the magnet, which is at the heart of Halley’s system, preoccupied many scientists during this period, who carried out experiments to try to make magnetic matter apparent and to characterize it. Thus, the aurora borealis, as a life-size experiment revealing the Earth’s magnetic matter, took a central place in this period of progressive evolution in the understanding of the nature of the magnet, which led in the second half of the 18th century to the abandonment of the notion of the circulation of magnetic matter. In this chapter, we present the 17th-century context of Halley’s thought, the details of his explanation and the consequences of his work in the field of magnetism, as well as the more general evolution of the understanding of the magnet in his time and up to the end of the 18th century.

Chapter 5 concentrates on electrical matter. It clarifies the nature of “fiery meteors” (the bodies entering the atmosphere, or meteoroids in the current scientific terminology), whose first documented observations date back to the 17th century, and which provide information on the height of the atmosphere, extended over the entire 18th