Читать книгу Physics of the Terrestrial Environment, Subtle Matter and Height of the Atmosphere - Eric Chassefiere - Страница 15

Heat is defined in the entry CHALEUR (HEAT) of the DUF-1690 as “the sensation that results from the action and movement of small atoms of fire in bodies, when they act on others”, and also as “the fire’s unique substance, as there are several atoms or parts together that spread out into the surroundings to cause the sensation of warmth.” An important addition was made in the 1727 edition, namely that it “is all the more violent the more numerous [the fire atoms] are and the more agitated they are.” For the Encyclopédie, heat is “one of the primary qualities of bodies, and the one opposed to cold” as well as “a physical being whose presence is known and whose degree is measured by the rarefaction of air, or of some spirit contained in a thermometer,” and still a sensation, but “relative” in that it “should only be considered in relation to the organ of touch, since there is no object that seems warm to us, unless its heat exceeds that of our body.” As for the exact nature of the heat that is in the bodies, “some claim that it is a quality; others claim that it is a substance; and some claim that it is a mechanical affection”:

Our latest and best authors in mechanical, experimental, and chemical philosophy think very differently about heat. The main question they propose is whether heat is a particular property of a certain immutable body called fire; or whether it can be produced mechanically in other bodies by altering their parts.

Boerhaave supported the first hypothesis, that fire is an immutable substance. For him, “what we call fire is a body by itself, sui generis, which has been created as such from the beginning, which cannot be altered in its nature or properties, which cannot be produced again by any other body, and which cannot be changed into any other, nor cease to be fire.” According to him, fire was present everywhere, in all parts of space and bodies, but it remained hidden and imperceptible, and could only be discovered by certain effects it produces, such as “heat, light, colors, rarefaction, and burning.” Luminous bodies, such as the Sun, ordinary fire or lamps, “do not hurl fire from their own substance,” but “direct and determine the corpuscles of fire that surround them to form parallel rays.” This action of collecting fire is done by “pushing an infinite series of igneous atoms towards the same place, or the same body, so that each atom strikes its blow, and seconds the effort of those who have gone before it.” A second way of collecting fire is to simply pile it up in a narrower space; this is done by quickly rubbing one body against another. This rubbing must be done at high speed, so that only the particles of fire floating in the surrounding air are mobile enough to slip into the empty spaces left around the rubbed bodies, accumulate in them and form “a kind of fire atmosphere”: “This is how the axles of cartwheels and millstones, the ropes of vessels, etc., receive heat from friction, catch fire, and often throw flame.” Guillaume Homberg, Willem Jacob’s Gravesande and Nicolas Lemery agreed.

Supporters of the second hypothesis, Francis Bacon, Robert Boyle and Isaac Newton “do not […] conceive [heat] as a property originally inherent in any particular species of body, but as a property that can be produced mechanically in a body.” Boyle argued that heat could be produced by mechanical action, for example, in a piece of iron pounded intensely with a hammer, where cause of the heat is found in the force of the hammer’s movement, which imparts a violent and variously determined agitation to small parts of the iron. The piece of iron first becomes hot in relation with other bodies, in comparison with which it was previously cold, and then becomes noticeably hot when the agitation imparted by the hammer is stronger than that of the parts of a person’s fingers that are holding it, since the hammer and the anvil may continue to be cold during the operation. The relative character of the heat actually felt is inseparable from this conception of heat communicated by mechanical action, which exists only in comparison with the heat of bodies not subjected to the same mechanical action. The fact that the hammer remains cold shows that it is not by transmitting its heat that it heats the piece of metal. A body can therefore provide heat without being hot itself:

One proof, says the same author [Boyle], that heat can be produced mechanically is that one only has to reflect on its nature, which seems to consist mainly in this mechanical property of matter, which is called motion: but this requires that the motion be accompanied by several conditions or modifications.

In the first place, the agitation of the body parts must be violent; for this is what distinguishes the bodies that are called warm, from those that are simply fluid […].

Another condition is that the determination of the movement is diversified, and that it is directed in all directions […].

This system is pushed further by Newton. He does not regard fire as a particular kind of body originally endowed with such and such a property; but according to him, fire is only a strongly igneous body, that is to say, hot and heated to the point of casting abundant light. Is a red-hot iron something else, he says, than fire? Is a burning coal anything other than red, burning wood?

According to Newton, fixed bodies heated to a considerable degree emit light, and this emission is done by the vibration of their parts. Bodies rich in “earthly and sulfurous parts” become luminous when these parts are agitated, either by the application of an external fire, or by friction, percussion, or putrefaction:

Thus, sea water in a storm, quicksilver [mercury] stirred in a vacuum, the back of a cat, or the collar of a horse rubbed against the fur in a dark place, wood, flesh, and fish as they putrefy, fumes rising from corrupted waters, commonly called will-o’-the-wisps, wet hay and wheat piles, glowworms, amber and diamond when rubbed, steel beaten with a stone, etc., etc. give off light.

The entry HEAT in the Lexicon provides additional information on the approach of English scholars. In it, heat is defined as “one of the four Primary Qualities, and seems to consist only, or at least chiefly, in the local Motion of the small Parts of a Body Mechanically modified by certain Conditions, of which the Principal is the vehement and various Agitations of those small Insensible Parts.” The question of the existence, or lack thereof, of an immutable body known as fire, which would produce heat, is not raised. The author of the entry sets out three conditions for the production of heat. The first is that the small parts should be agitated with a degree of violence and speed greater than that necessary to produce fluidity. The second is that this violent agitation should also be varied, with the particles having to move in all directions. The third condition is that the particles agitated violently and in all directions must be small enough to be individually insensitive. Because, unless they are extremely fine and subtle, they cannot easily penetrate the pores of adjacent bodies and thus heat or burn them.

An interesting part of the entry HEAT in the Lexicon has to do with the relationship between heat and fire. It is said that Newton, at the end of his Optics, conjectured “That Flame is a Fume, or Exhalation heated red hot; that is, so hot as to shine: Because Bodies don’t flame without emitting a copious Fume, and this Fume burns in the Flame.” The author speaks of the will-o’-the-wisp, a vapor that glows without heat, “and there seems to be the same difference between this Vapour and Flame, as between rotten Wood shining without Heat, and burning Coals of Fire.” This example seems to be intended to show that the glow of will-o’-the-wisp is not a flame, the absence of both smoke and flame must be contrasted with the presence of both smoke and flame above the coals of fire. Another example is that of the distillation of essences: “if the Head of the Still be taken off, the ascending Vapour will take Fire at the Flame of a Candle, and be turned into Flame; and the Flame will run along the Vapour from the Candle to the Still.” In this example, the steam that rises from the still, which is smoked, turns into flame, and then returns as flame to the still, smoke and flame moving into each other. He further mentions the smoke that rises above bodies heated by fermentation which, if the heat becomes sufficient, shine and become flames, and the bodies which, burning, vanish into a fiery smoke, “which Smoke, if the Flame be put out, is very thick and visible, and sometimes smells strongly, but in the Flame loses its Smell by burning.” It is the nature of the smoke, in the latter case, that conditions the color of the flame (blue for sulfur, green for copper, yellow for tallow). And the author notes that the “Smoke passing thro’ Flame cannot but grow red hot, and red hot Smoke can have no other appearance but that of Flame.”

Thus, for English scientists, it was the heat, which is communicated to the bodies by mechanically agitating their parts, or by exposing them to fire, that was responsible for the emission of light. Newton also conjectured that “the Sun and stars are merely excessively heated earthly bodies.” According to others, such as Boerhaave or Gravesande, it was fire, “a particular body as old as the others” that previously existed in all bodies, which is at the origin of the light emitted. Moreover, Newton considered the flame, and thus fire, a matter (in this case not subtle), by comparing it to smoke, whose sufficiently high level of heat causes ignition. Heat and fire are thus closely linked, everyone recognizing the property of fire to give heat, but only some people consider that heat produces fire, while others make the matter of fire pre-existing in bodies. And, obviously, the English academic community, very much focused on the idea of the mechanical generation of heat, dispensed with subtle matter to explain caloric and igneous phenomena.