1.4. Coarse and subtle matters
Besides vapors and exhalations, we have seen that heterogeneous air contains another type of substance, namely “the matter of light or fire, which emanates perpetually from celestial bodies; to which some physicists add the magnetic emanations of the Earth, true or alleged”. These subtle substances, of which there are many other kinds, play a fundamental role in the representation of the atmosphere in the 18th century, as we will see in the chapters devoted to them. But first, we must define what matter was considered to be at the time in the most general sense. We find a definition of matter in the entry MATIÈRE (MATTER) of the DUF-1690:
First principle: substance whose mixture, or bond, composes all natural bodies […] Celestial matter is very subtle and liquid: it rotates and carries with it the globes of the planets […].
The raw matter is that which one conceives by making abstraction of all its various forms. […]
Extent, divisibility, figure, and impenetrability are essential properties of matter. Extent is conceived before the other three, and therefore extent is what constitutes the essence of matter. It is certain that the idea of extent is inseparable from matter: and as soon as no extent is conceived, no idea of matter remains. It is disputed whether matter is infinitely divisible. The subtlety of philosophers has made the question problematic. Rohault posits the divisibility of matter to infinity.
The celestial matter in question is the ether. Matter, characterized by its four properties, of which “extent” is the predominant one is, according to the Cartesians, is a first principle, a substance that enters the composition of all natural bodies. The question of its divisibility, which placed Cartesians and atomists at odds, was considered problematic. The only addition to this definition in DUF-1727 concerns movement: “Some have argued that movement is essential to matter, and that all its parts have always been in motion, and cannot be otherwise.” The Encyclopédie gives the following definition of matter, which effectively includes the notion of motion under the term mobility:
substance extended, solid, divisible, mobile and passible, the first principle of all natural things, and which by its various arrangements and combinations forms all bodies.
The pre-eminence of extent was questioned by some authors, such as Samuel Clarke, because “if extent were the essence of matter, and therefore matter and space were one and the same thing, it would follow that matter is infinite and eternal, that it is a necessary being, which can neither be created nor annihilated, which is absurd.” Thus, in Clarke’s Newtonian conception, impenetrability is the essence of matter:
It seems, either by the nature of gravity, or by the movements of comets, or by the vibrations of pendulums, etc., that empty and non-resistant space is distinguished from matter, and that consequently matter is not a simple expanse, but a solid, impenetrable expanse, endowed with the power to resist.
Concerning matter as a first principle, some people think that matter, when it was created, was divided into several orders of corpuscles differing from each other in substance, hardness, figure, etc., the various combinations of these corpuscles resulting in the variety of bodies and their characteristics. Others, such as Newton, considered that this variety resulted from “different arrangements of the same matter, which he believes to be homogeneous and uniform in all bodies”. According to Newton, all bodies, even the most volatile, and even light, are composed of hard parts, with hardness being as essential as impenetrability. This can be seen in the hardness of some porous bodies, which suggests that their simple parts, devoid of pores, are even harder. It would take much less force to separate the hard parts of a porous body from each other than it would take to break a solid corpuscle without pores. The force that, according to Newton, holds the hard corpuscles in contact with each other in a porous body, “a force that is greater at the point of contact than anywhere else, and which then decreases so rapidly that it is no longer sensitive at a very small distance”, is gravitational attraction, responsible for the cohesion of natural bodies.
The Cartesians, who did not accept the existence of emptiness, gave this particular matter “which they suppose to pass through and freely penetrate the pores of all bodies, and fill these pores so as not to leave any void or interstices between them” the name subtle matter. But, says the author of the entry about Cartesians:
In vain, they resort to this machine to support their belief in an absolute fullness, and to attune it to the phenomenon of movement, etc., in a word, to make it act and move as they wish. Indeed, if such a matter existed, it would be necessary, in order for it to fill the voids of all other bodies, that it be itself entirely free of emptiness; that is to say, perfectly solid, much more solid, for example, than gold, and consequently, much heavier than gold, and more resistant […]; which cannot be in harmony with phenomena.
By “phenomena”, the author meant the movement of planets and other bodies, which takes place without measurable resistance, within interplanetary space. Although Newton did not share the Cartesians’ point of view on the non-existence of emptiness, he nevertheless agreed that there is a subtle matter, not to fill the totality of empty spaces, but to transmit heat and light through its vibrations, thus both caloric and luminous matter. Newton’s subtle matter is much looser than air, and penetrates the densest bodies:
He [Newton] inferred the existence of this matter from the experiments of two thermometers enclosed in two glass vessels, from one of which air was drawn out, and which we both carry from a cold to a warm place. The thermometer which is in the vacuum becomes hot, and rises almost as soon as the one in the air, and if they are brought back to the cold place, they both cool down, and both fall to about the same point. Does this not show, he says, that the heat of a warm place is transmitted through the vacuum by the vibrations of a medium much more subtle than air, a medium which remains in the vacuum after the air has been drawn from it? And isn’t this medium the same one that breaks and reflects the rays of light? […]
The same philosopher still speaks of this subtle medium or fluid at the end of his principles. This fluid, he says, penetrates the densest bodies; it is hidden in their substance; it is by its force and by its action that the particles of the bodies attract each other at very small distances, and that they attach themselves strongly when they are contiguous; this same fluid is also the cause of the action of electric bodies, either to repel or to attract neighboring corpuscles.
Thus, Newton also attributed to this subtle matter, which transmits heat and light, the attraction of matter, whether gravitational or electrical in nature. The author of the entry notes the contradiction between Newton’s first point of view, which was to consider gravitational attraction as a property of matter, and his attribution of the cohesion of bodies to subtle 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