Читать книгу Physics of the Terrestrial Environment, Subtle Matter and Height of the Atmosphere - Eric Chassefiere - Страница 13

The words “vapors“ and “exhalations“ were widely used in the scientific literature of the 17th and 18th centuries, and their meaning needs to be clarified. The DUF-1690 provides the following definition of vapors:

Subtle parts of a moist body, which form a kind of smoke that poor heat raises, and cannot dissipate. Clouds and fogs form vapors that rise from the Earth. Meteors are only flaming vapors.

This definition clearly implies that vapors are an emanation of moderately heated wet bodies. The entry FUMÉE (SMOKE) in DUF-1690 provides a definition of smoke:

Moisture that is exhaled in vapor by the action of heat, either external or internal […] Greasy and unctuous woods make a thick, black and stinking smoke […] The fogs are fumes that the Sun raises from the wet soil, and which it cannot resolve.

Smoke is therefore a non-transparent substance that results more or less directly from the evaporation of moisture impregnating the body. In addition, “nitrous and sulphurous exhalations are the main matter of thunder, lightning, and various other meteors that are generated in the air” (Encyclopédie, EXHALAISONS (EXHALATIONS)). The moderate heat that raises this smoke cannot dissipate it, but it is added in DUF-1727 that “the Sun, through its heat, attracts the vapors; resolves, dissipates the vapors”, suggesting that these vapors can change from their smoke form to the form of a transparent substance by the action of the Sun. The Encyclopédie, in its entry VAPEURS (VAPORS), specifies this notion of elevation of a liquid body in the form of vapor:

It is the assembly of an infinite number of small bubbles of water or other liquid matter, filled with air rarefied by heat and elevated by their lightness to a certain height in the atmosphere, after which they fall back, either as rain, dew, snow, etc. The masses formed from this assembly, which float in the air, are what are called clouds.

“Bubbles of water”, as defined in the Encyclopédie in the entry BULLES (BUBBLES), are “small balls of water with air inside”, also known as “water bottles”. An explanation of the mechanism of the water bubble formation is given in the entry BOUTEILLE (BOTTLE):

Bottles or bubbles of water are expandable or compressible; in other words, they take up more or less space, depending on whether the air they contain is more or less heated, or more or less compressed: they are round, because the enclosed air also acts inside them in all directions. The coat that covers them is formed by the smallest particles of the fluid; and since these particles are very thin, and make very little resistance, the bottle will soon burst if the air expands. The mechanism of these small bottles is the same as the mechanism of those that children form with soap by blowing at the end of a wand.

Thus, these water bubbles are the air bubbles covered with a thin film of water that form on the surface of the water when an air bubble that is formed below the surface comes into contact with it before bursting, and subsequently releasing the air they contain into the atmosphere. The author of the entry makes no mention of the fact that these bubbles may come from the water particles themselves transformed into vapor, and not from the air contained in the water, as stated in the entry ÉBULLITION (BOILING) of the Encyclopédie already cited. He continues:

When a liquor is put under the container of the pneumatic machine, and the air starts to be pumped out, bottles or bubbles similar to those produced by rain rise to the surface of the liquor. These bottles are formed by the air in the liquor, which being less compressed when the air in the container begins to be pumped out, is released from between the particles of the fluid, and rises to the surface.

The same happens to a fluid that boils violently, because the air contained in it, being rarefied by the heat, seeks to expand and get out into the open, and promptly escapes to the surface of the fluid, where it forms bottles.

The author of the Encyclopédie’s entry VAPEURS (VAPORS) therefore assumes a boiling-type mechanism for their formation, during which the air bubbles, surrounded by their membrane of water, emerging on the surface of the water, rise (without bursting), because of their lightness, “to a certain height in the atmosphere”. The entry VAPORS in the Lexicon gives an almost identical definition: “Watry Exhalations raised up either by the Heat of the Sun, the Subterraneal, or any other accidental Heat, Fire.” It dwells at length on the process of raising these vapors by referring to the work of Edmond Halley in England and Guillaume Homberg in France. For Halley, “if an Atom of Water be expanded into a Shell or Bubble, whose Diameter will be ten times as great as before, such an Atom will be Specifically lighter [less dense] than Air, and will rise so long as that Flatus [the effervescent principle] or warm Spirit which first separated it from the Mass of Water, will continue to distend it to the same degree. But then that Warmth declining, and the Air growing cooler, and withal Specifically lighter; these Vapors will top at a certain Region of the Air, or else descend.” According to Halley, aqueous vapors dissolved in the air, in the same way that salt is in water, are more abundant during the day, because the Sun heats the air and increases its dissolving power, and are discharged at night in dew, because of the cooling of the atmosphere which is no longer illuminated by the Sun.

The results of Homberg’s experiments on water evaporation are then briefly presented. For him:

the Fiery or Æthereal Matter first puts the small Particles of the Water into an Agitation, and then mingles itself with it; which Mixture is what we call Vapors; this being Specifically lighter than Air, will rise in it, till it come to such an height, as that the Air is there of the same Relative Gravity with itself, and there it will swim about, till by the Motion of Winds, or other Causes, its Constitution is broken, and so the Watry Parts uniting together in greater Drops, it descends in Dew, or Rain.

The author notes the similarity between Halley’s and Homberg’s conclusions, the “flatus” of one being the “ethereal matter” of the other. But Homberg also conducted experiments of evaporation under a vacuum from a wet soil, which show that water evaporates even in a vacuum, without the help of air, since water drops form on the inner wall of the container emptied of its air (Homberg 1693). He concluded that air did not participate in the first ascensional phase of the water bubbles, since the water was projected onto the walls by the agitation of the ethereal matter. Thus, it must be assumed that the mechanism for the rise and stabilization of the water bubbles, due to their specific lightness relative to the ambient air, only occurred during the second phase.

This representation of evaporation, that is, the formation of steam, is dictated by the only directly observable form of evaporation, that is, boiling. The resulting emanations of humid bodies are again referred to as smoke, which is consistent with the conception of a cloud of particles (water bubbles) rising in the atmosphere. On the contrary, the Encyclopédie’s entry ÉVAPORATION (EVAPORATION), as we will see, concludes that a transparent vapor is being formed, which may not be air. Some scholars, as the author of the entry VAPEURS (VAPORS) says, also use vapor to refer to the fumes sent out by dry bodies, such as sulfur, but Newton and a few others used exhalations in this case. Here is how exhalations are defined in the DUF-1690:

In dogmatic terms, it is particularly said of the dry bodies and small atoms that the Earth continually pushes into the air, or that the stars attract. Vapors rise from the waters, and exhalations from the Earth: these are fatty, oleaginous and sulfurous parts that serve as matter for lightning and some other meteors.

DUF-1727 adds to “dry bodies or atoms”, “those sulfurous particles that have been separated from earthly bodies by the heat of the Sun or by the agitation of subtle matter.” The Encyclopédie defines exhalations as “smoke or vapor that is exhaled or comes out of a body and spreads in the air”, and clearly distinguishes between vapors and exhalations:

The words “exhalation” and “vapor” are usually taken interchangeably, but the authors themselves distinguish between them. They refer to as vapors the wet fumes that rise from water and other liquid bodies; and exhalations, the dry fumes that come from solid bodies, such as soil, fire, minerals, sulfur, salts, etc., which are not in contact with the air.

The exhalations, taken in the latter direction, are dry corpuscles or flows, which rise from hard, earthly bodies, either by the heat of the Sun, or by the agitation of the air, or by some other cause.

The entry EXHALATION in the Lexicon, very briefly, provides a similar definition: “whatever is raised up from the Surface of the Earth or Water by means of the heat of the Sun, that of the Subterraneal Fire, etc. such as Vapours, Mists, Fogs, etc.” It should be noted that vapors are considered here as a special case of exhalations, and that no distinction of the type proposed by the Encyclopédie is made between vapors and exhalations.

The action by which we cause the humidity of the bodies to exhale is evaporation, says the DUF-1690 in the entry EVAPORATION. For example, “Salt is formed by the evaporation of moisture, either by the heat of the sun, as in salt marshes; or by means of fire, as in places where there are salt wells.” DUF-1727 adds that in chemistry, evaporation, (i.e. the dissipation of superfluous moisture) differs from “exhalation”, which “is practiced only on dry matter”, following the same distinction that has been made between vapors and exhalations. In the Encyclopédie’s entry EVAPORATION, the following definition is given:

Almost all liquid bodies and most solids exposed to the air, by the action of this fluid alone, or with the help of moderate heat, gradually rise in the atmosphere. Some do so fully, others only partially: this passage, or this total or partial rise of bodies in the atmosphere, physicists refer to as evaporation.

Thus, the presence of air, which allows a lighter matter to rise, possibly aided by the action of heat, is the condition for evaporation. Following this one:

Bodies raised in the air by evaporation are supported in such a state that they are absolutely invisible, until by some change in the atmosphere, their particles gather together in small masses that noticeably disturb the transparency of the air: for example, air is […] at all times full of water that has risen by evaporation, and remains there, invisible, until new circumstances reunite its dispersed molecules in small masses that substantially disturb its transparency. This is what distinguishes evaporation from the rise in the atmosphere of certain small and light bodies, such as dust, which only rise and sustain themselves there by the mechanical impulse of the agitated air, which retain their same volume, their opacity, and fall back as soon as the air ceases to be agitated.

The matter being evaporated is thus recognized as being invisible, no longer being compared, as half a century earlier in DUF-1690, to smoke. It is the subsequent reunion of molecules dispersed in the air into small clusters of matter that is recognized as being responsible for the appearance of opaque matter, such as fogs or clouds. Evaporation is clearly distinguished from the lifting of pre-existing particles, in that it proceeds from an operation of decomposition of a liquid or solid body. The notion of evaporation is extended to “the rising of certain bodies in the atmosphere, produced by a degree of heat sufficient to decompose them, or by the calcination itself”. The particles raised by these means in the air are of the same nature as those that rise by evaporation; they also support themselves in such a state of division that they are perfectly invisible. The author gives the example of sulfur, which decomposes as it burns, releasing “vitriolic acid [sulfuric acid] and the flammable principle [the matter of fire, or igneous matter]”, which “rise in the atmosphere and become invisible there”, as well as that of the decomposition of animal and vegetable matter by which “volatile principles are released, capable of rising and sustaining themselves in the atmosphere”. The term “evaporation” is thus generalized, which is equivalent to putting vapors (literally, products of evaporation) on the same level as the exhalations released by dry matter:

By these examples it is clear that evaporation does not differ essentially from the rise of volatile particles released by the application of sufficient heat to decompose bodies, or by calcination; that these operations only dispose the bodies to the rise of some of their parts; that, in addition, the particles which rise in the air in this manner are of the same nature, and support themselves there as well as those which rise by evaporation. However, it has been customary not to call evaporation the rise of the particles detached through these operations which decompose bodies; it has restricted the meaning of this word to the elevation of the free volatile parts, free of principles which can fix them, and which, in order to rise in the atmosphere, either require no artificial heat, or require only moderate heat, which hardly exceeds that of boiling water.

We must understand here how the action of heat, and the presence of air, allows the evaporation of volatile parts of the body, whether water, air, the inflammable principle or molecules of an earthy nature, the latter only acquiring the property of rising in the air “as long as they contract an intimate union with water molecules”. The flammable principle itself, that is, the matter of fire (or igneous), although its molecules are in a very loose free state, is fixed so strongly in bodies, where it is not combined with water, that it is not able to evaporate by itself. On the other hand, when combined with water molecules, igneous molecules make them evaporate much faster. And here is what the author of the entry tells us about the mechanism of evaporation, as it is commonly accepted at the time:

Bodies susceptible to evaporation evaporate all the more quickly the more they are heated. It is probably this very simple observation that gave rise to the most generally adopted hypothesis on the mechanism of evaporation. It has been assumed that as water molecules are rarefied by heat, or, what amounts to heat, by the adhesion of igneous particles, their specific gravity decreased to such an extent that the molecules, having become lighter than air, could rise in this fluid, until they reached a layer of the atmosphere whose specific gravity was equal to their own.

As we have seen, this is the explanation given by Halley and Homberg for the formation of the vapors. Thus, it is the addition of igneous particles to water molecules, resulting in the heating of these molecules and their rarefaction, which results in their evaporation, in air heavier than they are. The author of the entry, on the observation that “ice evaporates even in the most severe cold”, and also that condensed particles (clouds) are not lower in winter than in summer, rejects this explanation of evaporation due to the effect of igneous particles making water molecules lighter than those in the air. His explanation is that water dissolves in the air, just as salt dissolves in water. Dissolving in air makes water invisible, just as salt dissolving in water becomes invisible. He therefore considers air as a solvent, all the more effective in dissolving water when it is heated, especially by the vapors themselves in contact with the evaporable body. To the objection made to his theory that evaporation also occurs in a vacuum, he replies that the space above water cannot be completely empty of air:

According to the experiments of some physicists, water evaporates in a vacuum; it can therefore rise without the help of air, without being supported by it, as I said in the state of dissolution. But if the physicist had paid attention to the fact that water contains an immense quantity of air from which it cannot be entirely purged, and that it cannot evaporate without the air it contains developing, he would easily have noticed that this objection contains a paradox, and that it is impossible for a space containing water that evaporates to remain perfectly empty of air.

Thus, in the middle of the 18th century, there was still uncertainty as to the exact nature of the vapors and exhalations, whose mechanisms used for lifting, dissolution in the air or lightening by the igneous particles joined together, were the subject of controversy. It follows from the preceding analysis that the word “vapor”, as used in the first part of the 18th century, should not be taken in the sense of water vapor (or vapor formed from liquid matter in general) in its gaseous form, but rather small particles of water (or liquid) rising in the atmosphere from the Earth’s water, or any other liquid, which evaporates. These lighter than air particles rise in the atmosphere to the level where they are in equilibrium with it, and can be transported there by the wind. They can also lead to the formation of fogs or clouds. The entry NUÉE (FOG) in DUF-1727 says, on this subject: “As the subtle and rarefied vapors are the matter of the winds, also the vapors tightened and condensed form fogs and rain; hence the rain usually brings down and soothes the wind”. Fog (which can be identified with clouds floating in the lowest region of the air) rises to different heights, as the entry NUÉE (FOG) in the Encyclopédie states:

Fogs rise in our atmosphere at different heights. We sometimes see them suspended, one above the other, and they seem very distinct, which depends mainly on the difference in their specific gravity [their density], which keeps them in balance with the air, which is more or less dense. We know that they are suspended one above the other by the different routes they take, with one being carried higher and the other lower, without mixing together. It is said that the highest fogs rarely rise above the height of the tops of the highest mountains; for one can usually see from afar that these peaks rise above the clouds. (2) We learn from various observers who have been on the highest mountains that they have always seen the fogs floating below them, without ever noticing that they are above their heads. Riccioli has calculated that the highest fogs never rise to the height of 5000 steps [≈3000 m]. However, there could perhaps be some subtle exhalations that rise much higher.

As for the dissipation of fog, this takes place through rain, but not exclusively in this way. It can also be dislocated by the wind, or dissolve in the air in proportion to the purity of the air when it rises in the atmosphere due to a local increase in pressure.

Vapors rising from humid bodies, and the exhalations that are the equivalent of them applied to dry bodies, produced, respectively, by evaporation and exhalation, are therefore rather ill-defined substances, of a rather more liquid nature (in the sense of cloud particles) than of a gaseous nature (in the sense of air molecules) in the minds of the scientists of the time, although the gaseous nature is correctly perceived by some of them in the process of evaporation. The supposed liquid nature of vapors is a legacy of the Aristotelian vision, which attributes to vapors, and not to air, the ability to refract light and enlarge the Moon close to the horizon, just as water enlarges by refraction the image of the stick immersed in it. We will generally speak of vapors and exhalations to designate all the marine and terrestrial emanations present in the atmosphere, which are not necessarily transparent to light, as we have detailed. In some cases, the word vapor will be used to describe all emanations, including exhalations.