Читать книгу Physics of the Terrestrial Environment, Subtle Matter and Height of the Atmosphere - Eric Chassefiere - Страница 12

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 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.

Although English scientists recognized the presence of subtle matter in the atmosphere, notably magnetic matter (Edmond Halley used magnetic matter to explain the aurora borealis), they nevertheless gave them less prominence than their French counterparts, with the exception, however, of Hooke, who assumed that the atmosphere resulted from the dissolution of vapors in the ether. This point of view, explicitly judged too Cartesian by the author of the ETHER entry in the Lexicon, seemed rather marginal in the English community, whose general tendency, as we will see in the course of this chapter, was to minimize the possible role of subtle matter in favor of physical explanations that favor, in particular, the role of mechanical interactions between bodies.