by experiments on electricity. In so doing, he compares the matter of fire to electrical matter, a position that is held by Jean-Antoine Nollet, among others. After having reviewed various physical phenomena of fire, the importance of air for fire to be preserved is highlighted: “the experiment proves that fire is extinguished very quickly in the vacuum machine; and all the more quickly as the air is pumped out faster, and the container is smaller and better closed.” It is also necessary that the coarse parts of the food of the fire [“the bodies which serve to increase or maintain the fire”], such as smoke, be diverted from the fire, “a body remaining all the longer lit the less smoke it gives off, as can be seen in the wick and the peat coals.”
But it is in phosphorus that fire matter is present in the greatest quantities. The entry PHOSPHORE (PHOSPHORUS) in DUF-1690 describes phosphorus as “a matter that burns, or becomes luminous, without the need to approach a sensitive fire. It is natural, or artificial. [...] Natural phosphorus are materials that, without the help of art, at certain times become luminous, without ever having any detectable heat”, such as glowworms, or sugar when it is stored in a dark place. Natural phosphorus do not always glow, and never emit any heat. It is therefore an example of light that occurs without heat. “Artificial phosphorus are materials that become luminous by artifice, without needing to be lit by a sensitive fire.” Some artificial phosphorus burn and consume everything they come into contact with, such as animal oils produced by distillation (e.g. from urine); others have no heat, such as the Bologna stone, which, when heated beforehand, emits a faint glow in the dark. As the Encyclopédie says:
The general cause of the light of phosphorus is that the matter of fire or light is generally more abundant in this body than in others, so that the simple friction can put it into action, or that the simple action of the particles of fire or light spread in the air can awaken it. Phosphorus phenomena have much to do with electrical phenomena.
Five orders of phosphorus are described. The first order is that of bodies made into phosphorus “by the electric fluid that penetrates them”, such as glowworms, certain flies, the sting of the irritated viper, the eyes of certain live animals, certain live fish and shellfish, or even hair, or hair that has been rubbed vigorously. The second order is that of bodies that have become phosphorus “by shocks or rough rubbing that bring into play the fire contained in their interiors”, for example, stones that are rubbed vigorously against each other, or certain metal alloys that are roughly filed, or hardwoods and softwoods that are rubbed with great force. The third order is that of “bodies that have been exposed to the heat of the Sun or a violent fire, have absorbed light when they expanded, and then hold it back and let it out only little by little, or only when a soft warmth brings them closer to the state they were in when they admitted it”, bodies that today would be described as fluorescent, for example, the stone of Bologna, certain topaz, alabaster, gypsum, which the author considers that they “return the light as they received it; I mean colored, according to the color that was given to the fire that burned them.” The fourth order is that of the bodies made into phosphorus “by fermentation, dissolution, and everything that is produced by these actions, such as exhalations, effervescence, etc.” The fourth order is that of the bodies that have become phosphorus “by fermentation, dissolution, and all that is produced by them, such as exhalations, effervescence, etc.”, among which we can cite wet hay, flour, mine exhalations, meteors such as will-o’-the-wisp, falling stars, lightning or the aurora borealis. The fifth order includes “phosphorus produced by the union of a particular acid with phlogistics [i.e. elemental fire]”, such as nitrous acid, which, when ignited, forms a phosphorus, but which cannot be preserved by any known means.
These bodies, which can shine and even ignite without any external contribution in the form of heat, were considered an important class of luminous bodies at the beginning of the 18th century, as we will see with the mercurial phosphorus, that is, the glow produced by the mercury rubbing against the glass of the barometer, a phenomenon in this case of an electrical nature. The contribution of heat by friction, which characterizes certain phosphorus of the second order, and which falls into the category of mechanical heat-generating actions described by Boyle, is not recognized as such by the author of the article, for whom the origin of the light emission is internal to the phosphorus, due to a pre-existing content of igneous matter.
1.5.3. Light
On the subject of light, the entry LUMIÈRE (LIGHT) in the DUF-1690 says:
It is a very subtle, prompt, and uncluttered body that causes clarity, that illuminates, that gives color to all things, that shakes the eyes, and makes objects visible. Philosophers distinguish between primitive, or radical light, and second or derivative light. Primitive or radical light is that which is in the luminous body, and which consists of a certain movement of its parts, which makes them capable of pushing the subtle matter around, which fills the pores of transparent bodies. And the second or derived light is nothing else than the inclination to move, or the tendency of this subtle matter to move away in a straight line from the center of the luminous body.
Against the Cartesian conception expressed above, borrowed from Jacques Rohault, the author opposes the atomist conception of Pierre Gassendi, which is also that of Newton, for whom light consists of the flow of an infinite number of light corpuscles “spreading with incredible speed on all sides”. René Descartes and Christiaan Huygens rejected the corpuscular approach to light, because how can such a considerable quantity of corpuscles be emitted by luminous bodies without being exhausted, and, on the other hand, how is it that inflamed corpuscles do not heat up the optic nerve at the same time as they illuminate it?
Mr. Huygens designed a long series of globules that form like little sticks, with one end touching the sun and the other touching the back of the eye. After this, it must follow, that at the same instant that the Sun presses the end that is contiguous to it, the one that presses on the eye is also pressed. Thus the light reaches us from the luminous body by some movement imprinted on the subtle matter in between.
DUF-1727 presents some additional elements, mentioning the equivocal character of light, which can be taken as “the particular feeling that the soul receives by the impression that luminous bodies make on the eyes”, or to designate “what is in these bodies by which they cause this particular feeling in the soul.” The same questioning is expressed for heat, as we have seen. The works of Nicolas Malebranche treating light by analogy with sound are mentioned. The time it takes light to pass from the Sun to the Earth, estimated at 11 minutes by Ole Christensen Roemer, thanks to the measured time lag of the moments when Jupiter eclipses some of its satellites, is indicated, and some considerations on the propagation of light and its colors are stated. The Encyclopédie takes up exactly the definition of the DUF, with the ambiguity between purely sensory effect and physical phenomenon. The Cartesian doctrine of the matter of the first element being agitated, which presses in all directions the small globules of the second element, compared to hard spheres that touch each other and instantaneously transmit the action of light to our eyes, is briefly described for criticism. The globules must be elastic, and not hard, for at least two reasons: first of all, the light is not transmitted instantaneously, as thought Descartes, then the light is reflected, which is not compatible with perfectly hard globules. Malebranche, for whom the parts of the luminous body in fast movement excite vibrations of pressure in the subtle matter which is between him and our eye, the amplitude and the frequency of these vibrations conditioning, respectively, the intensity and the color of the transmitted light, replaces the hard globules by swirls, while preserving the general design of Descartes and Huygens of a propagation of the light in the form of wave on a substrate of subtle matter. Huygens’ theory makes it possible to explain the refraction and the reflection, but does not account simply for the propagation of the light in a straight line, which, like sound, should propagate in all directions according to the undulatory assumption. Newton explained the straight-line propagation by the corpuscular hypothesis, “the action by which the body produces in us the sensation of clarity, consisting not in an effort to move, but in the real movement of these particles which move away from all sides of the luminous body
