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