Physics of the Terrestrial Environment, Subtle Matter and Height of the Atmosphere E-Book
Eric Chassefiere
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
The discovery, in the middle of the 17th century, of both the weight of air and the law governing its elasticity transformed the status of the atmosphere from that of a purely mathematical object to that of a complex and highly variable physical system. In the context of rapidly intensifying experimentation and observation, the nature of the atmosphere was therefore the subject of a host of hypotheses, which 18th century scholars tried to reconcile with a coherent physical approach. In particular, this was achieved by the conceptualization of invisible or "subtle" materials, thought to be closely linked to atmospheric stratification. Subtle matter was introduced, largely to reconcile contradictory results concerning the estimation of the height of the atmosphere. These estimations were based on different methods, mainly using the observation of meteors and the refracted and reflected light of stars. Taking as its common thread the question of the height of the atmosphere, which was omnipresent in the texts at the time, this book traces the history of the discovery of the atmosphere and the many questions it generated.
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Table of Contents
Cover
Title Page
Copyright
Introduction
1 Words Used to Describe the Atmosphere and Subtle Matter
1.1. Introduction
1.2. Air and the atmosphere
1.3. Vapors and exhalations
1.4. Coarse and subtle matters
1.5. The triptych of heat, fire and light
1.6. Ether
1.7. Fundamental properties of air
2 Refractive Matter
2.1. Introduction
2.2. State of knowledge in the 17th century
2.3. Arguments for the introduction of a refractive matter other than air
2.4. Discussion
2.5. Conclusion
3 Solar Matter
3.1. Introduction
3.2. State of knowledge of the Sun in the 17th century
3.3. Solar matter and height of the atmosphere
3.4. Conclusion
4 Magnetic Matter
4.1. Introduction
4.2. Main concepts of magnetism in the 17th century
4.3. The explanation of the aurora borealis by magnetic matter
4.4. Magnetism in the second half of the 18th century
4.5. Conclusion
5 Electrical Matter
5.1. Introduction
5.2. Highlighting the link between electricity and thunderstorm activity
5.3. Knowledge of the nature of electricity in the mid-18th century
5.4. Precursory work on fiery meteors
5.5. Explanation using electricity
5.6. Elucidation of the origin of fiery meteors and falling stars
5.7. Conclusion
6 Subtle Air
6.1. Introduction
6.2. Difference in mercury heights between different barometers
6.3. Suspension of water and mercury from the tops of inverted tubes
6.4. Gravity theories and the impulse system
6.5. Light barometers
6.6. Conclusion
7 Results and Theories on the Height of the Atmosphere in the 18th Century
7.1. Introduction
7.2. Representation of the atmosphere inherited from previous centuries
7.3. Two major paradigms for the composition and vertical extension of the atmosphere in the 18th century
7.4. The three main inconsistencies between estimates of atmospheric height made by different methods
7.5. Two other methods for estimating the height of the atmosphere
7.6. Conclusion
8 Atmospheres of Earthly Bodies
8.1. Introduction
8.2. Porosity of bodies
8.3. Atmospheres of bodies
8.4. Conclusion
Conclusion
References
Index
End User License Agreement
List of Illustrations
Chapter 2
Figure 2.1.
Riccioli’s twilight table (Riccioli 1651, p. 39)
Figure 2.2. Experimental device used by Lowthorp for his refraction measurements...
Figure 2.3. Representation of marine instruments (“English quarter”, Figures 9 a...
Chapter 3
Figure 3.1. Cometary tail patterns (Oliver 1777, p. 148). For a color version of...
Figure 3.2. Drawing of an aurora observed in Breuillepont, France (Mairan 1733, ...
Figure 3.3. Table of auroras borealis observed between 500 and 1731. Totals per ...
Chapter 4
Figure 4.1.
Explanatory diagram of vortex formation (Descartes 1681, p. 212)
Figure 4.2. Explanatory diagram of the orientation of a magnet in the vicinity o...
Chapter 5
Figure 5.1. Representation of the explosion of the fiery meteor of July 17, 1771...
Figure 5.2. Diagrams of the fiery meteor of 1758 from the various testimonies co...
Figure 5.3. Trajectory of the 1771 meteor between its appearance in Sussex Count...
Chapter 6
Figure 6.1. Device used by Huygens to show water suspension (Huygens 1672, p. 13...
Figure 6.2. Schematic diagrams of various experiments carried out by Hauksbee to...
Chapter 8
Figure 8.1. Devices used for the experiment with the wooden vase (left) and the ...
Figure 8.2. Charcoal pores (top) and petrified wood pores (bottom) under the mic...
Guide
Cover
Table of Contents
Title Page
Copyright
Introduction
Begin Reading
Conclusion
References
Index
End User License Agreement
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Series Editor
Denis-Dider Rousseau
Physics of the Terrestrial Environment, Subtle Matter and Height of the Atmosphere
Conceptions of the Atmosphere and the Nature of Air in the Age of Enlightenment
Eric Chassefière
First published 2021 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:
ISTE Ltd27-37 St George’s RoadLondon SW19 4EUUK
www.iste.co.uk
John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USA
www.wiley.com
© ISTE Ltd 2021
The rights of Eric Chassefière to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.
Library of Congress Control Number: 2021940067
British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-78630-717-0
Introduction
This book presents the methods developed in the 17th and 18th centuries for estimating the height of the atmosphere, methods essentially based on the observation of the light refracted and reflected by the atmosphere, and of certain meteors. As a result, these methods gave rise to a number of contradictions and advances. This question of determining the height of the atmosphere is inseparable from how the atmosphere is represented, with the atmosphere passing from the status of an idealized and purely mathematical object up to the middle of the 17th century, to that of a complex, eminently variable physical object, whose nature was the subject of a multitude of hypotheses, and which 18th-century scientists tried to reconcile within a coherent overall vision. The quest to determine how high the atmosphere reached, which the different phenomena studied did not all place at the same height, proved a powerful motivator of new research and debate during this time, as well as for the necessary syntheses that resulted. Rather than addressing, one by one, the different methods used to estimate the height of the atmosphere, in this book, we adopt a transverse approach by examining the various matters of a more subtle nature introduced by the scientists of the time to explain the phenomena from which they derived this height – the milestones in the construction of the representation of the atmosphere as a physical object. These subtle matters, most of which are part of the legacy of René Descartes, offer a broad field of investigation, which allows us to set the stage for the evolution of the scientific thinking of the time in relation to the understanding of atmospheric phenomena. There is a strong overall coherence between the hypotheses formulated at that time, far beyond the question of the validity of these hypotheses with respect to today’s knowledge, showing to what extent their abundance and the confrontation of points of view allow knowledge to mature, until the conceptual leap that overturned old ideas and allowed for an objective advance took place. These processes occurred over a considerable period of time, on the scale of over a century. It is this work of progressive maturation – the product of a permanent tension between the legacy of old ideas and the new ideas resulting from ever more numerous, rich and precise observations – which we present in this book.
Chapter 1 provides an analysis of the meaning of words used in the 18th century to characterize air, atmosphere, ether and, more generally, subtle matters. Understanding the literature of the time on the atmosphere and subtle fluids requires a good understanding of the exact meaning of the terms used, which often differs from the meaning given to them today. For example, the term vapor, which we apply today to a gas, was used at that 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 century. The extraterrestrial origin 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.
1Words 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 Louis XIV, earned its author his exclusion from the Académie Française, a (short) majority of whose members did not look favorably on this initiative, which was in competition with that of the institution. In addition to the content of the 1690 edition, a comparison of this content with the enriched 1727 edition provides interesting outlooks on the evolution of the meaning of words, and of the underlying knowledge, during the first quarter of the 18th century. The Encyclopédie of Diderot and d’Alembert, published in the middle of the 18th century, provides a much larger compendium, which reflects the rapid development of scientific thought in the first half of the Enlightenment. We limit ourselves in this chapter to this particular period, to the 18th century, during which reflections on the height of the atmosphere, and on the subtle matters invoked to explain the rapidly emerging discrepancies between the estimates made by the different methods, are developed. The progression of knowledge between 1727, the date of the publication of the fourth edition of Le Furetière, and 1751, date of the first edition of the Encyclopédie by Diderot and d’Alembert, is very significant, and we will try to outline the primary paths of this evolution. In this chapter, we will note DUF-1690 and DUF-1727, the editions of the Dictionnaire Universel Furetière of 1690–1701 and 1727.
The first English alphabetical encyclopedia is the Lexicon Technicum or, An Universal English Dictionary of Arts and Sciences: Explaining not only the Terms of Art, but the Arts Themselves (Harris 1704), referred to simply as the Lexicon here. Although the emphasis is on mathematical subjects, it does not only contain terms of a scientific or technical nature, but also includes entries relating to law, commerce, music and the arts in general. It is the work of Pastor John Harris, who claimed for his dictionary not only the function of giving the meaning of scientific and artistic words, but also elements of knowledge about the sciences and arts referred to by words, in a universalist approach that would be that of Diderot and d’Alembert’s Encyclopédie half a century later. We have limited ourselves in this chapter to the first edition of the Lexicon dating to 1704, composed of a first volume, a second volume consisting of additions, composed mainly of mathematical and astronomical tables, being published in 1710. Three other editions of the first volume were created in a short period of time, in 1708, 1716 and 1725, with the two volumes being published together only in 1736. In his preface, Harris insists on the fact that the contents of his dictionary do not come from other dictionaries, but are collected from the original works of the best authors, thus making it a source of information in its own right, with contents distinct from the Furetière, and otherwise specific to the English scientific context, differing in many respects from the French context, as we will see.
In section 1.2, we examine the definitions of air and atmosphere, as these two terms are far from being equivalent. Sections 1.3 and 1.4, respectively, deal with vapors and exhalations, as well as with subtle matter, two essential components of the atmosphere, as the treatment of subtle matter also requires attention to the definition of “normal” (“coarse” as opposed to “subtle”) matter. Section 1.5 is devoted to the triptych made up of the three subtle matters heat, fire and light, closely linked by multiple relations, whether demonstrated or conjectural. Section 1.6 deals with the question of the meaning given to the word ether, in its complex relationship with the atmosphere. We end with a conclusion, which is also the introduction to the main body of the work, which deals with subtle matters, particularly in relation to the question of the height of the atmosphere. Naturally, in this analysis, we seek to investigate the scientific concepts underlying the words used, since they partly determine their meaning, the nascent scientific approach contributing to inflecting this meaning according to the characters, proven or supposed, of the designated environments and phenomena. However, this investigation is only minimal, in the very spirit of the dictionaries used, as the scientific contents specific to the primary subject of the work, namely the height of the atmosphere, are to be developed in the following chapters. We have not presented all of the matter considered in the various articles analyzed, focusing on those that are directly related to the theme of this book.
1.2. Air and the atmosphere
The word ATHMOSPHÈRE (ATMOSPHERE) is only briefly defined in the DUF-1690:
It is the part of the air that is charged with vapors, or clouds, and that does not have the purity of the ethereal region: this is what causes the refraction of the light of the stars. The Moon appears larger at its rising, because of the vapors of the Atmosphere.
This definition expresses that the atmosphere is only a part of the air, the one in which vapors and clouds are mixed. Purity is here a criterion used to distinguish the atmosphere from the ether, or ethereal region, and the phenomenon of atmospheric refraction seems to be associated with the presence of vapors and clouds. The second sentence, which refers to what is known today as “the lunar illusion”, that is, the fact that the Moon appears larger when it is close to the horizon, long attributed to refraction, confirms the link that the first sentence seems to establish between the presence of vapors and atmospheric refraction. The definition of the atmosphere in DUF-1727 is a little more detailed. It further notes that the part of the air charged with vapors and clouds is “the coarsest and heaviest”, an idea that we will see plays an essential role in the representation that scientists, especially French scientists, had of our atmosphere and some of its characteristics, such as atmospheric pressure. The following sentence about the atmosphere was also added in the 1727 edition: “It ends at a certain distance, and forms like a globe that surrounds and encloses that of the Earth.” Thus, the idea of a physical top of the atmosphere, located “at a certain distance”, became more clearly apparent at the turn of the 18th century. The attribution to vapors of the perceived enlargement of the Moon near the horizon remained in the 1727 edition. The entry RÉFRACTION (REFRACTION) from DUF-1727 is instructive in this respect. It provides information about atmospheric refraction and about the light during the twilight hours that is reflected from the sky (the coarse air charged with impurities, as was the vision held at that time) once the Sun sets, or before it rises:
and if the coarseness of the air, which seems to cause this great refraction, also gives longer twilight hours, as it would appear: in the longest periods of darkness, the six-month nights experienced at the poles, there will still be a fairly great twilight even without the Moon, and this utility compensates them [the inhabitants of the polar zone] for the inconvenience of the coarse air they breathe.
Thus, with respect to both the refraction of the light of the stars and the reflection of the light of the Sun during twilight, it is necessary to attribute these to the coarseness of the air in its lower part, thus to the impurities it contains (vapors, exhalations, clouds), to which it owes precisely its name, the “atmosphere“ (“sphere of the vapors”). But what exactly is air? The DUF-1690 tells us, with regard to air:
A fluid and light element that surrounds the globe, the sea and the Earth. The air is divided into lower, middle, and upper regions. Water resolves and evaporates into air. One cannot live without breathing air. We cannot live on air. The ancients did not know the gravity of the air. We know the gravity of the air from using a barometer, its heat from a thermometer, its dryness from a hygrometer. We found the invention of pumping air to make vacuum, by Mr. Boyle’s machine. Mr. Mariotte, in his Essays on Physics, says that air can expand more than four thousand times more than it is near the Earth before it is in its natural expansion, as it has it at the top of the atmosphere, where it is not loaded with any weight. Its height, according to his calculation, is only 20 leagues [≈80 km]: and it would not be 30 when it is eight million times more rarefied than the one near the Earth.
In the 1727 edition, it is not the element that is described as fluid and light, but the matter, or substance, and a more precise definition of the vertical stratification of air is provided:
An element; fluid, light, transparent matter that surrounds the Earth; fluid, moist substance used for breathing. The mass of air is divided into lower, middle, and upper regions. The lower or inferior region of the air is that which we inhabit, and which is bounded by the reflection of the Sun’s rays. It is sometimes cold and sometimes warm, depending on the diversity of climates and seasons. The middle region of the air is the air space from the top of the highest mountains to the lower region of the air we breathe. It is cold, and humid, because of the vapors and exhalations that the Sun raises there. The upper region of the air is that which extends from the top of the mountains to the region of the elemental fire; it is purer, rarer, and lighter than the others.
Air is certainly an “element” in the sense of the ancients: “the ancient philosophers recognized four elements, fire, air, water, and earth. These four popular elements are not elements, strictly speaking, because they are composed bodies; and not simple, unmixed bodies” (F1727). But, as is most important to note, it was considered at the beginning of the 18th century to be a physical matter that can be characterized by measurement, and whose distribution according to height can be predicted by applying the law of expansion, still known today as the law of Boyle-Mariotte. And this matter is subject to being mixed, as shown, for example, by the fact that air contains a certain degree of humidity. The stratification of the atmosphere expressed above is inherited from the Aristotelian design, but relatively precise criteria are provided to define the boundaries between layers. The middle region is the region that contains vapors and clouds, and which gives rise to the reflection of the Sun’s rays. The atmosphere thus consists of the union of the lower and middle regions, and extends from the Earth’s surface to the tops of the highest mountains. The supreme region, that of “pure” and “rare” air, above the highest mountains, extends to the elemental fire, therefore to the element fire, which fills, in the Aristotelian conception, the sub-lunar space above the air.
Diderot and d’Alembert’s Encyclopédie (which we will refer to simply as the “Encyclopédie”) shows a significant evolution in the definition of the atmosphere. This word is presented as the name “given to the air that surrounds the Earth, that is, to this rare and elastic fluid with which the Earth is covered everywhere at a considerable height, which gravitates towards the center of the Earth and weighs on its surface, which is carried along with the Earth around the Sun, and which shares its annual as well as diurnal movement.” The elasticity of the air is emphasized, and the air no longer rises there only “at a certain distance”, but “at a considerable height”. The atmosphere has become, for most scientists, “the entire mass of air surrounding the Earth”, even if some writers call the atmosphere only “that part of the air close to the Earth which receives vapors and exhalations, and which substantially breaks up the rays of light” (by refraction). The atmosphere is not bounded at its top by elemental fire, but by “a more subtle matter called ether”, and “the space above the coarse air, though perhaps not entirely empty of air, […] is called the ethereal region or ethereal space.” Thus, we cannot exclude, upwards in the atmosphere, henceforth the whole mass of air, the existence of pure air, that of the “supreme region”, as defined in the DUF.
The definition of air in the Encyclopédie also shows a notable evolution. It is defined there as “a light, fluid, transparent body, capable of compression and expansion”. This body cannot be considered as an element, “although it may have parts that deserve this name”. There are two types of air: (i) vulgar or heterogeneous air and (ii) clean or elementary air: “Heterogeneous or vulgar air is an assembly of corpuscles of different kinds, which together constitute a fluid mass, in which we live and move, and which we inhale and exhale alternately.” We find air loaded with vapors and clouds (“coarse air”), composed of heterogeneous substances, which it is said can be reduced to two kinds:
1. The matter of light or fire, which emanates perpetually from celestial bodies. To this, some physicists add the magnetic emanations of the Earth, true or alleged.
2. This infinite number of particles that rise in the form of vapors or dry exhalations from the Earth, water, minerals, plants, animals, etc. either by the heat of the Sun, or by that of underground fires, or by that of fireplaces.
What is called “elemental air” is air itself, “a subtle, homogeneous and elastic matter, which is the basis, so to speak, and the fundamental ingredient of all the air in the atmosphere, and which gives it its name”. It is therefore pure air from above, mixed in the lower part of the atmosphere with the vapors and exhalations emanating from the Earth. We also see appearing among the heterogeneous substances various subtle matters, to which we will return.
The entry ATMOSPHERE found in the Encyclopédie devotes several paragraphs to the question of the height of the atmosphere, which we will address at length in the following chapters. The lower limit to the height of the atmosphere can be calculated by assuming that the air is homogeneous, without elastic force, and therefore of the same density everywhere. The measurement of the height of mercury in the barometer provides the weight of the air column, and knowing the ratio of the density of mercury to that of the “air we breathe here below”, namely 10,800, the height of the atmosphere, assumed homogeneous, is estimated to be 2 leagues ¼ (≈8.5 km). This value is a lower limit, by virtue of the elasticity of the air:
Air, by its elasticity, has the virtue of compressing and expanding: it has been found by various experiments frequently repeated in France, England and Italy, that the different spaces it occupies, when compressed by different weights, are reciprocally proportional to these weights: that is, the air occupies less space at the same time as it is more compressed; hence, it follows that in the upper part of the atmosphere, where the air is much less compressed, it must be much more rarefied than it is close to the surface of the Earth; and that consequently the height of the atmosphere must be much greater than that which we have just found.
The consideration of the dilatation in the calculation of the vertical structure of the density of the air was carried out by Edme Mariotte, and others, in the second half of the 17th century, leading to a density which forms, with the height, a “continuous geometrical proportion”, that is, which decreases exponentially with the altitude. It follows that “the rule of compression according to the weights cannot give the height of the atmosphere; for this height would have to be infinite, and the density of the air would have to be zero at its upper surface”. But another obstacle prevented the height of the atmosphere from being estimated by this method. Jacques Cassini, during his campaign of measurements intended to extend the meridian of Observatoire de Paris, precisely measured the heights of several mountains, as well as the pressures prevailing at the top of these mountains. He found laws of variation with height that do not correspond to Boyle-Mariotte’s law of expansion. Expansion increases faster than the inverse of the compressive weight at altitude (as the inverse of the square of this weight, according to him). The Academy conducted numerous laboratory experiments at reduced pressure, experimenting with air dilatation much greater than that at work on mountain tops, and found no deviation from Boyle-Mariotte’s law. Hence:
Some physicists have concluded that the air on the mountain tops is of a different nature from the air we breathe down here, and apparently follows other laws in its expansion and compression.
The reason for this difference must be attributed to the amount of coarse vapors and exhalations with which the air is laden, and which is much greater in the lower part of the atmosphere than above. Since these vapors are less elastic and therefore less capable of rarefaction than pure air, the rarefactions of pure air must necessarily increase in greater proportion than the weight decreases.
Thus, the elemental air at the top would by nature be different from the heterogeneous air at the bottom; less elastic, because of the vapors and exhalations it contains, which does not conclude on the height of the atmosphere from pressure measurements made at different heights, since it is not possible to extrapolate at great heights the measurements made near the surface of the Earth from a single law of air expansion:
In any case, it is constant that the rarefactions of the air at different heights do not follow the proportion of the weights with which the air is loaded; therefore the barometer experiments, made at the foot and on the top of the mountains, cannot give us the height of the atmosphere, since these experiments are done only within the lowest part of the air. The atmosphere extends far beyond this; and its rarefactions are all the further away from the previous law, the farther from the Earth it is. This is what prompted de La Hire, after Kepler, to use an older, simpler and safer method to find the height of the atmosphere: this method is based on the observation of the twilight hours.
We will return to the twilight method. The essential fact here is the inference, from the confrontation between pressure measurements made at different altitudes on the mountains and measurements of the relationship between air dilatation and pressure made in the laboratory, of the existence at great heights of air following a law of expansion different from that of the air that directly surrounds us. Thus, the heterogeneous lower air and the elementary upper air do not only differ in their densities, the former being much heavier than the latter because of its load of impurities emanating from the Earth, but also in their nature, the latter extending much higher than it would if it followed the law of expansion of the former. It should be noted that the entry REFRACTION from the Encyclopédie no longer makes any reference to the role of vapors and exhalations, which is emphasized in the Dictionnaire Universel. As we will see, the role of atmospheric impurities in refraction was disproved at the beginning of the 18th century on the basis of observations showing its absence under certain conditions, such as the case of a star seen through a cloud, which led some scientists to postulate the existence of a subtle, lightweight refractive matter. An allusion to refractive matter can be found in the entry REFRACTION of DUF-1727:
The cause of refraction is not yet known; perhaps it will never be known, like many other points in physics. Is it air, is it refractive matter that is in the air, according to Mr. Cassini’s conjecture? This is where we are still on this matter. There are lot of apparent annoyances in one or the other system, and consequently a lot of uncertainties.
The existence of refractive matter was far from being unanimously accepted. It is not mentioned in the entry REFRACTION of the Encyclopédie.
The entry AIR in the Encyclopédie devotes long sections to the different “characteristics” of air. Unlike the vapors contained in a bottle, which, when it is cold, lose their elasticity and attach themselves around the inner walls of the glass, air does not condense. It is the air that provides the means for earthly bodies to burn, while on the contrary, the vapors and exhalations extinguish fire, coals and burning iron. While in stormy weather, the exhalations ignite, producing lightning, air remains intact after a rainstorm. We do not know the nature of the air, because we cannot examine the air alone and purified of the materials mixed in it. For some, air was “a substance sui generis, which does not derive from any other, which cannot be generated, which is incorruptible, immutable, present in all places, in all bodies, etc.”. That is, by definition, elementary air. For others, the elasticity of air, its essential and distinctive character, was conferred on it by the matter of the bodies from which it was derived, “which has become, through the changes made in it, susceptible to permanent elasticity”. This conception was notably that of Robert Boyle, who carried out numerous experiments in the production of air from bodies that did not seem to contain air, the best methods for this purpose being “fermentation, corrosion, dissolution, decomposition, boiling of water and other fluids, and the reciprocal action of bodies, especially saline bodies, on each other”. According to Newton, “particles of a dense, compact, fixed substance, adhering to each other by a powerful attractive force, can only be separated by violent heat, and perhaps never without fermentation; and these bodies, which are eventually rarefied by heat or fermentation, are transformed into truly elastic air”. While the entry AIR in DUF-1690 states that “water resolves itself, evaporates into air”, the Encyclopédie states that not everything that appears to be air is air:
The example of the aeolipile, where water is sufficiently rarefied by fire, comes out with a sharp whistle, in the form of a matter perfectly similar to air; but soon afterwards loses this resemblance, especially in the cold, and becomes water again through condensation, as it was originally. The same behavior can be observed in the spirit of wine, and other subtle and fleeting spirits obtained by distillation; instead of the real air being reduced neither by compression, nor by condensation or any other means, to any substance other than air.
So you can make water take on the appearance of air for a while: but it soon regains its own.
We find the same questioning expressed in the entry for ÉBULLITION (BOILING) in the Encyclopédie:
With regard to the cause of boiling, we have historically related to the word “boiling” that which physicists usually give as the cause of boiling, and which they attribute to the air which is released from the particles of water; but other physicists reject this cause, and believe that boiling comes from the particles of water itself, which are changed by the action of fire into very expanded vapor, and which rise from the bottom of the vessel to the surface. Here are the reasons for their opinion: (1) The boiling is done in the vacuum machine, when water previously purged of air is heated in it. It is therefore not the air that produces it; it is in this case the heat that makes the water scarce: these are the words of Mr. Musschenbroek […] (2) Water does not stop boiling until it is evaporated; but how can one conceive that the air enclosed in water, and which makes up at most one thirtieth the part, can suffice for all this boiling? (3) Although not all liquors contain the same amount of air, all seem to boil equally. (4) The more water is free to evaporate, that is, the more the vase in which it is put is open, the less heat it supports without boiling. (5) The more subtle a liquor is, and therefore easy to reduce to steam, the less heat is needed to boil it. Thus the spirit of wine boils at a lower heat than water, and water at a lower heat than mercury.
Air is divided into “real or permanent” and “apparent or transient”. The vapors, produced by evaporation of water, are apparent air, while dry exhalations are permanent air. Air is to be understood here in the sense of coarse or heterogeneous air, a mixture of elemental air and impurities emanating from the Earth and water. The production of air from solid bodies that appear to be devoid of air is questioned by the author of the entry AIR in the Encyclopédie:
But, after all, there is still reason to doubt whether the matter thus extracted from solid bodies has all the properties of air; whether this air is not transient, or whether the permanent air that is drawn from bodies did not already exist there. Mr. Boyle proves through an experiment conducted in the pneumatic machine with a lit wick, that this subtle smoke, which the fire raises even from dry bodies, does not have as much spring as air, since it cannot prevent the expansion of a little air enclosed in a bladder which it surrounds […] Nevertheless in some later experiments, by dissolving iron in vitriol oil and water, or in etching, he formed a large air bubble which had a real spring and which, as a result of its spring, prevented the neighboring liquor from taking its place; when a warm hand was applied to it, it expanded easily like any other air, and separated in the liquor itself into several bubbles, some of which rose out of the liquor in the open air.
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
