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

of these bodies, hypothesized as early as the end of the 17th century, but which violates the Cartesian doctrine of meteors inherited from Aristotle, took a very long time to gain acceptance, between the first scientifically substantiated proposal made by Halley at the beginning of the 18th century and the essentially accurate theory proposed by Ernst Chladni at the end of the same century. Few articles on the subject that mark out the 18th century show that, around the middle of the century, there was a period of rapid development in the experimentation on atmospheric electricity using instruments such as metal spikes and kites, which led to the discovery of the electrical nature of thunderstorms, the flow of electricity as an essential agent of fiery meteors and falling stars (i.e. shooting stars), as well as the aurora borealis, with these different phenomena being considered by many scientists of the time as closely related. In the second half of the 18th century, there was a general tendency to attribute many phenomena involving fire to electricity, and the meteors mentioned above are no exception to this rule. The electrical nature of fiery meteors was hotly debated during this period, the electrical hypothesis having its fierce supporters and detractors, who expressed doubts on the basis of observations, some aspects of which they considered contradictory with the electrical nature of the phenomenon.

      Chapter 6 addresses the question of subtle air, more delicate than the air we breathe, but less subtle than ether, in relation to the aurora borealis theory of Mairan. The extremely tall estimate of the height of the auroral structures, which rotate with the Earth and were assumed by most scientists of the time to be immersed in its upper atmosphere, suggests that the auroral matter, which Mairan supposed to come from the solar atmosphere, mixes with very fine air, extending much higher than the coarse air, whose pressure is given to us by the mercury barometer. In this chapter, we analyze the conceptual framework from the 17th century in which Mairan’s system must be placed, as well as the experimental evidence on which he and his contemporaries relied to postulate the existence of subtle air: unequal mercury levels in different barometers, suspension of mercury at great heights in inverted tubes, “mercurial phosphorus” (the luminous barometers), considerable degree of adhesion between joined polished planes and theories of the coherence of solid bodies. We show how the subtle air hypothesis allowed Mairan to overcome difficulties resulting from divergent estimates of the height of the atmosphere using different methods, and how, thanks to the introduction of subtle air, a coherent picture of the vertical structure of the atmosphere emerged from the work carried out in the second half of the 17th century and throughout the 18th century.

Chapter 7 is devoted to a synthesis of the estimates of the height of the atmosphere made from the various methods used, the interpretation of which involves one or more of the subtle matters analyzed in the previous chapters: atmospheric refraction (refractive matter), the duration of twilight periods (solar matter), the aurora borealis (solar and magnetic matters, subtle air), fiery meteors, and falling stars (electrical matter). We describe the context, in terms of the representations of the atmosphere and the interpretation of the different phenomena used to estimate its height. We analyze, in the light of the then-dominant representations of the atmosphere, in particular the presence of a component considered to be of major importance, in terms of weight and refractive power, of vapors and exhalations, or the existence of a finer, even subtle component, of the atmosphere extending at a very great height above the Earth, and of the conceptions developed at the turn of the century on subtle matters, the contradictions that affected the evolution of how the height of the atmosphere was estimated by the various different methods, and the arguments developed to solve them. More generally, the introduction of these many subtle matters gave rise to very lively debates, which at the dawn of the 19th century led to a relatively unified, scientifically supported vision of the Earth’s atmosphere and its vertical extension. The numerous concepts developed at the time, following their own logic, within a different frame of thought from that of today, were translated into advances and tested by the observation of the atmosphere, constantly tending towards the search for an overall coherence of the representation of the physical object that, for the scientists of the Enlightenment, made up the atmosphere that surrounds us. In particular, this included the idea of a vertical stratification of the atmosphere, no longer based on Aristotelian categorizations, but on criteria of a physical nature relating to different characteristics such as density or the electrical state, for example.

      In Chapter 8, we look at the question of the atmospheres of terrestrial bodies, which are at the juncture point between the atmosphere and subtle matter. We explore the vast field of the various exchange processes envisaged at that time between solid bodies and the Earth’s atmosphere, which are responsible for the particular atmospheres surrounding these bodies. These processes presuppose a circulation of matter through the pores of the bodies; large pores for thick vapors and air, and small pores for subtle matter. The porosity of bodies, in nature and in terms of physical characteristics, is an essential aspect of this question, and we first present the various observations and theories of the time on this subject. Then, we move on to an examination of the atmospheres of the bodies themselves, again detailing different observations and theories, and reviewing the various materials involved in the formation of these atmospheres: air, various vapors, electrical and magnetic fluids, igneous or caloric fluids, or luminous fluids, etc. These theories are in some cases based on particular representations of the atmosphere and the ether, and on principles of physics such as dissolution, applied to the ether or to the igneous fluid as solvents, and to earthly or aqueous bodies as dissolved substances within an air forming the mixture, which we try to put into perspective in the context of the time. Out of the abundance of ideas in this field, we see emerging a closely interconnected world, where solid bodies, their particular atmospheres and the Earth’s global atmosphere interact permanently through the circulation of subtle, or thick, materials within solid bodies and their atmospheres, through which they are in contact with each other, exchanging matter and movement.

      In order to provide clarity on both the vocabulary and the scientific ideas of the time, we felt it necessary to illustrate our subject with numerous excerpts from texts. These excerpts, concerning sources in French, are translated into English. Concerning the sources in English, we have endeavored to provide the original source text.

      1

      Words Used to Describe the Atmosphere and Subtle Matter

      1.1. Introduction

      This chapter presents an examination of the words used in the early and mid-18th century to describe the atmosphere, and everything related to its functioning, as well as ether and subtle matter that are closely involved in its environment and composition. This knowledge of the precise meaning of the words referencing the atmosphere and its various components, which fit into a framework of thought very different from that of today, is necessary to understand the evolution of the thinking of the time. Here, we will voluntarily limit ourselves to the definitions given in specific dictionaries: (i) the Dictionnaire Universel de Furetière, whose first volume of the first edition dates back to 1690 (Furetière 1690–1701), and a fourth edition dates back to 1727 (Furetière 1727), (ii) the Encyclopédie by Diderot and d’Alembert, published beginning in 1751 (Encyclopédie 1751–1772), and (iii) the English Lexicon Technicum, of which the first publication dates back to 1704 (Harris 1704). More in-depth information will be provided in the following chapters.

Furetière’s dictionary has this exact title: Dictionnaire Universel, Contenant généralement tous les mots français, tant vieux que modernes, et les Termes de toutes les Sciences et des Arts. Designed to compensate for the lack of consideration of scientific, technical and artistic words in the Dictionnaire by the Académie (French Academy of Sciences), which did not appear until 1694, this dictionary, initially written by Antoine de Furetière in the 1650s, constitutes a significant sum of scientific and technical knowledge at the turn of the 18th century. Its publication, first as an excerpt in 1684, then as a complete publication in 1690, two years after Furetière’s death, although endorsed by