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

movement of a subtle matter, like the first element of Descartes, but about the movement of the ordinary matter likely to give place to a light emission. The author refers to Hooke, who judges that movement, to produce light, must satisfy two conditions: (i) it must be extremely fast, like that of fermentation and putrefaction, which makes brine and rotten wood shine and (ii) it must be vibratory, with vibrations of an extremely short period, as well as those of rubbed diamonds which become shiny. These phenomena, which today would be called chemiluminescence and triboluminescence, are characteristic of phosphorus, which we have previously described. There is no question here of fire and heat as sources of light, the latter being found in movement, from which of course it can result some heat, according to Boyle’s theory that we have exposed, and some fire, for example, by friction. The author then states elements of Newton’s work on light (refraction, colors), and notes that Newton, because of the straight-line propagation of light, considers that this propagation cannot consist only in the Cartesian principle of action.

An essential aspect of the article, largely absent from the DUF and the Encyclopédie, is the long development devoted to the demonstration made by William Molyneux that light is a “body”, and therefore matter, in the sense of physical matter, and not subtle, as considered by most French scientists of the time. Molyneux, in his Dioptrique, identifies three properties of light that show its material nature. First of all, the phenomenon of refraction, which shows that light passing through diaphanous bodies is resistant to it. What, if not a body, can be resisted by a medium?

      This resistance to the passage of light through different diaphanous bodies can be understood as resulting from the fact that the medium prevents light from diffusing and distributing itself in all parts of the medium, and therefore the medium can be said to be less illuminable: because, by its nature, light tends to diffuse. And, conversely, the more the light affects the parts of the medium that it illuminates equally and uniformly, the greater the number of particles of the illuminated medium to which it transmits its energy, the more the medium can be said to be illuminable, less resistant to the progression of light. Hence the fact that, the more solid and small the affected parts of the medium are, the less space they admit between them for any other heterogeneous matter, the more important is the illumination of the medium. And it is certain that resistance must result from the contact of two bodies, and that contact, whether active or passive, is the property of the bodies.

      Thus, the interaction between light and the medium is conceived in terms of mechanical action and reaction between bodies, the resistance of the medium being all the weaker as it is cohesive and admits fewer impurities in its pores, potentially slowing down the light by preventing it from diffusing. The second property confirming that light is a body, and a “body moved and projected forward”, is that its passage from one place to another is not instantaneous, but takes a certain amount of time, its movement being the fastest of all. The author cites Roemer’s observation of the eclipses of the satellites of Jupiter, which allowed him to estimate the speed of light. A third piece of evidence put forward by Molyneux is that “light cannot, by any technical or artificial process, be increased or decreased”. One cannot, for example, enlarge the light of the Sun or of a candle, any more than 1 cubic inch of gold can be enlarged:

For every time we see the light increase, it is at the expense of some other part of the environment that has lost light, or that light, which naturally should have spread to other parts, has been brought to the brighter place. Thus, for example, in a magnifying glass that concentrates the Sun’s light at its focal point, or point of combustion; we must first consider that the image of the Sun is projected at the focal point on a separate base from that of the glass. And secondly, we can observe all around the bright spot of the image of the Sun the marked shadow that the full width of the magnifying glass casts on it: for all the rays of the Sun, which would have fallen on this wide space of shadow, are now gathered and concentrated in the bright spot, producing vigorous light and violent heat.

      An objection can be made that the light is increased by reflection, without depriving any other place of the light it would otherwise have received. It is easy to see that this objection does not hold. If we imagine a candle placed in a room facing a small opening to the outside, half of the light from the flame illuminates the room, while the other half illuminates the outside space. If we now place in front of the opening a mirror with its reflective side facing inwards, the light that previously illuminated the outside of the room illuminates the inside of the room, to the detriment of the outside space which is no longer illuminated. We thus see that the mirror subtracts the light in the middle behind it, and the contradiction raised falls.

      The content of the Lexicon entry thus significantly differs from the contents of the articles of the DUF and the Encyclopédie, in that it focuses, with the exception of the passages devoted to the laws of propagation of light, on the question of the character of light as a body, and therefore as matter. The Lexicon entry makes almost no reference to heat, and to the possible relations between heat and light, a subject of concern to French scientists, and cites as examples, in support of the idea that light is born of movement, only those of phosphorus whose light is not accompanied by fire or heat. Light is presented as a matter in its own right, in mechanical interaction with its environment.

1.6. Ether

The entry ETHER in both the DUF and the Encyclopédie sheds complementary light. Ether is, according to the DUF-1690, “that pure substance which is above the atmosphere, which fills all the sky where the stars take their course.” The 1727 edition offers further information on the rarity of ether in relation to air, and the existence of different ethers of varying degrees of rarity, “making ether a suitable means of transmitting light and the influences of the most distant stars.” The entry in the Encyclopédie uses a definition of ether quite similar to that in the DUF. Ether, whose necessity of existence is invoked to prevent “the majority of the universe from being entirely empty”, either fills only the space between the celestial bodies, above the atmosphere, or is “of such a subtle nature, that it penetrates the air and other bodies, and occupies their pores and intervals.” Some believed that ether did not exist, and that “air itself, by its extreme tenuousness and by this immense expansion of which it is capable, can spread out into the intervals of the stars and be the only matter there.” The very term ether is ambiguous; some people called ether a fluid of the same nature as other bodies, but distinguished by its tenuousness, whereas ancient tradition attributed to it a purer and more subtle nature than that of “substances around the Earth”. There is thus a continuum of representations of ether, which goes from pure air to the most subtle matter. A subtle matter, as defined in the Encyclopédie, is originally “the name that Cartesians give to a matter that they suppose to pass through and freely penetrate the pores of all bodies, and fill these pores so as not to leave any voids or interstices between them.” For this matter to leave no void, it must not itself contain any, which implies that it is “perfectly solid, much more solid, for example, than gold, and therefore much heavier than gold, and more resistant.” This is judged “not to agree with phenomena”, such as the regular movements of the planets, as well as Newton’s argument against the Cartesian system: “if the heavens were filled exactly with fluid matter, however subtle, they would resist the movement of the planets and comets much more than mercury would”. But Newton nevertheless agreed that there was subtle matter, “or a medium much looser than air, which penetrates the densest bodies”. Following the thermometer experiment already described, he concluded that the heat passes through the glass of the pneumatic machine emptied of its air, which implies the presence of an intermediate body passing through the pores of the glass and propagating heat, just like light. This intermediate body, which he called the ethereal medium, must bathe the whole space since, after passing through the glass, it must pass through all the other bodies. Having established the existence of this ethereal medium, Newton moved on to its properties:

      and says that it is not only rarer and more fluid than air, but also much more elastic and more active; and that by virtue of these properties, it can produce many of the phenomena of nature. It is, for example, to the pressure of this medium that Newton seems to attribute the gravity of all other bodies; and to its elasticity, the elastic force of air and nerve fibers, emission, refraction,