matter, which certainly reflected his lack of certainty about the existence of subtle matter, a position that would rather be a concession to the Cartesians. Especially since Newton also supported, and not without hesitation, the Cartesian hypothesis of magnetic matter to explain the properties of the magnet. Still, Cartesians and Newtonians agreed on the more or less nuanced assertion, based on different arguments, of the existence of a subtle matter filling the world.
Another subtle matter is igneous matter, or fire matter. The Encyclopédie gives the following definition: “a principle that some chemists use in explaining several effects, especially to account for the increase in weight that some bodies experience in calcination”. The author gives the example of 20 pounds of lead that is melted over a fire until it is reduced to dust. The resulting lead powder increases in volume and weighs 25 pounds. The increase in weight does not come from the residue of the wood or coal used to maintain the fire, as shown in the same experiment carried out at the hearth of a burning glass, a convex lens placed under the sun:
It was therefore concluded that this increase in weight could only be due to the Sun’s rays, which were concentrated in the matter exposed to their action during the entire time of the operation, and that it was to the condensed matter of these rays of light that the excess gravity observed there was to be attributed; and for this purpose it was assumed that the matter which serves to transmit light and heat to us, the action of the Sun or fire, was heavy, that it was capable of great condensation, that it indeed condensed prodigiously in the pores of certain bodies, without being constrained by any weight.
But this explanation, which appealed to the subtle matter transmitting light and heat, does not hold, according to the author, because there was evidence that the matter of light is not heavy. For example, the matter of light in a glass vacuum chamber, whose presence can be known by the fact that we can still see the objects behind the chamber through it, does not support the mercury suspended in the barometer at the slightest height. Or light does not offer any perceptible resistance to a globe passing through it, unlike a fluid with a certain density. It is therefore necessary to look elsewhere for the matter responsible for the increase in the mass of lead. And it is in the vapors and exhalations dissolved in the air (a concept that we have seen is used to explain the evaporation of bodies), and which are therefore contained in its pores, that we must necessarily find it:
According to Mr. Boerhaave’s very detailed remarks, air contains a large number of heavy molecules in its pores: water, oil, volatile salts, etc. With respect to water, we know how any amount of salt of tartar, exposed to air, becomes charged in a very short time with an equal weight of water molecules. This heavy matter is therefore contained in the pores of the air. The presence of sulfur molecules, salts, etc. is no more difficult to observe. Without resorting to any still, one need only be in the open country in stormy weather, raise one’s eyes to the sky to see the large number of lightning bolts shining from all sides: they are fires, they are lit sulphur, they are volatile salts, no one can disagree.
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.
1.5. The triptych of heat, fire and light
1.5.1. Heat
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
