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

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.