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

To the objection that the oil does not evaporate, and mixes only with the air with great difficulty, Herman Boerhaave replied that it is precisely because the air contains a lot of oil in its pores that it cannot accept any more. He imagined that the action of fire on the surrounding air expels water molecules from the pores of the air, and that the space left vacant by these molecules releases the denser matters that were confined there, which, due to their gravity, become joined with the lead molecules. He provided a numerical estimate of the increase in weight resulting from the combustion of lead by analogy with the process of tartar formation:

What impossibility is there, then, after it has been seen that air can easily supply twenty pounds of water to twenty pounds of salt of tartar, and that it does indeed supply them in a short time, that the same air can supply to twenty pounds of lead for the whole time of the calcination, I do not say twenty pounds of water molecules, that the action of the fire removes and drives out of the pores of the air, which surrounds the vase in which the lead is calcined, but only five pounds of denser, heavier, and at the same time more subtle molecules of matter, which were contained in the pores of the air among these same molecules of water, which being no longer supported in these pores by the molecules of this water, which the fire has removed from them, will be released from the pores of the air by their own gravity, will come to join the molecules of lead, whose weight and volume they will increase.

The volume of air that provided the matter that comes to be incorporated into the lead made lighter as a result, rises and gives way to new air, “so that in a very short time all the parts of the air contained in a large space, will be able, by this simple and intelligible mechanism, to approach successively one after the other the lead that is calcined, and deposit the heavy molecules that this air contains in its pores”. Thus, the igneous matter is not, in itself, responsible for the increase in weight of lead. It is its movement that causes the disunion of the parts of lead, drives water out of the air pores and allows dense matter to escape from the air pores and come into contact with the lead molecules:

The fire, or the rays of light, gathered in the focus of a magnifying glass, provide here only a great movement that disunites the parts of the metal, by calcining the sulfur, which binds them together, and leaves the heavy particles, which come from the pores of the air, and which do not have the same viscosity, the freedom to surround the molecules of lead, and to reduce this metal to powder.

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?