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

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.

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.