Georges Buffon - Buffon's Natural History. Volume X (of 10)

This, therefore, is sufficient to demonstrate that light is neither particular nor different from common matter; that its essence, and its essential properties are the same; and that it differs only from having undergone, in the point of contact, the repulsion whence its volatility proceeds; and in the same manner as the effect of the force of attraction extends, always decreasing as the space augments, the effects of repulsion extend and decrease the more, but in an inverted order, insomuch that we can apply to the expansive force all that is known of the attractive. These are two instruments of the same nature, or rather the same instrument, only managed in two opposite directions.

All matter will become light, for if all coherence were destroyed it would be divided into molecules sufficiently minute, and these molecules, being at liberty, will be determined by their mutual attraction to rush one against the other. In the moment of the shock the repulsive force will be exercised, the molecules will fly in all directions with an almost infinite volatility, which, nevertheless, is not equal to their velocity acquired in the moment of contact, for the law of attraction being augmented as the space diminishes, it is evident, that at the contact the space is always proportionable till the square of the distance becomes nil, and, consequently, the velocity acquired by virtue of the attraction must at this point become almost infinite: and it would be perfectly so if the contact were immediate, and, consequently, the distance between the two bodies void; but there is nothing in nature entirely nil, and nothing truly infinite; and all that I have observed of the infinite minuteness of the atoms which constitute light, of their perfect spring, and of the nil distance in the moment of contact, must be understood only relatively. If this metaphysical truth were doubted, a physical demonstration may be given. It is pretty generally known that light employs seven minutes and a half to come from the sun to the earth; supposing, therefore, the sun at thirty-six millions of miles, light darts through this enormous distance in that short space, that is (supposing its motion uniform), 80,000 miles in one second. But this velocity, although prodigious, is yet far from being infinite, since it is determinable by numbers. It will even cease to appear so prodigious, when we reflect on the celerity of the motion of the comets to their perihelia, or even that of the planets, and by computing that, we shall find that the velocity of those immense masses may pretty nearly be compared to that of the atoms of light.

So, likewise, as all matter can be converted into light by the division and expulsion of its parts, when they feel a shock one against another, we shall find that all the elements are convertible; and if it have been doubted whether light, which appears to be the most simple element, may be converted into a solid substance, it is because we have not paid sufficient attention to every phenomena, and were infected with the prejudice, that being essentially volatile it can never become fixed. But it is plain that the fixity and volatility depend on the same attractive force in the first case, and become repulsive in the second; and from thence are we led to think that this change of matter into light, and from light into matter, is one of the most frequent operations of Nature.

Having shewn that impulsion depends on attraction; that the expansive force, like the attractive, becomes negative; that light, heat, and fire, are only modes of the common existing matter; in one word, that there exists but one sole force, and one sole matter, ever ready to attract or repel, according to circumstances; let us see how, with this single spring, and this single subject, Nature can vary her works, ad infinitum . In a general point of view, light, heat, and fire, only make one object, but in a particular point of view they are three distinct objects, which, although resembling in a great number of properties, differ nevertheless in a few others, sufficiently essential for us to consider them as three distinct things.

Light, and elementary fire, compose, it is said, only one and the same thing. This may be, but as we have not yet a clear idea of elementary fire we shall desist from pronouncing on this first point. Light and fire, such as we are acquainted with, are two distinct substances, differently composed. Fire is, in fact, very often luminous, but it sometimes also exists without any appearance of light. Fire, whether luminous or obscure, never exists without a great heat, whereas light often burns with a noise without the least sensible heat. Light appears to be the work of nature while fire is only the produce of the industry of man. Light subsists of itself, and is found diffused in the immense space of the whole universe. Fire cannot subsist without food, and is only found in some parts of this space where man preserves it, and in some parts of the profundity of the earth, where it is also supported by suitable food. Light when condensed and united by the art of man, may produce fire, but it is only as much as it lets fall on combustible matters. Light is therefore no more, and in this single instance, only the principle of fire and not the fire itself: even this principle is not immediate, for it supposes the intermediate one of heat, and which appears to appertain more than light to the essence of fire. Now heat exists as often without light as light exists without heat: these two principles might, therefore, appear not to bind them necessarily together; their effects are not contemporary, since in certain circumstances we feel heat long before light appears, and in others we see light long before we feel any heat. Hence is not heat a mode of being, a modification of matter, which, in fact, differs less than all the rest from that of light, but which can be considered apart, and still more easily conceived? It is, nevertheless, certain, that much fewer discoveries have been made on the nature of heat than on that of light; whether man better catches what he sees than what he feels; whether light, presenting itself generally as a distinct and different substance from all the rest, has appeared worthy of a particular consideration; whereas heat, the effect of which is the most obscure, and presents itself as a less detached and less simple object, has not been regarded as a distinct substance but as an attribute of light and fire.

The first thing worthy of remark, is, that the seat of heat is quite different from that of light: the latter occupies and runs through the void space of the universe; heat, on the contrary, is diffused through all solid matter. The globe of the earth, and the whole matter of which it is composed, have a considerable degree of heat. Water has its degree of heat which it does not lose but by losing its fluidity. The air has also heat, which we call its temperature, and which varies much, but is never entirely lost, since its springs subsist even in the greatest cold. Fire has also its different degrees of heat, which appear to depend less on its own nature, than on that of the aliments which feed it. Thus all known matter possesses warmth; and, hence, heat is a much more general affection than that of light.

Heat penetrates every body without exception which is exposed to it, while light passes through transparent bodies only, and is stopped and in part repelled, by every opaque one. Heat, therefore acts in a much more general and palpable manner than light, and although the molecules of heat are excessively minute, since they penetrate the most compact bodies, it seems, however, demonstrable, that they are much more gross than those of light; for we make heat with light, by collecting it in a great quantity. Besides, heat acting on the sense of feeling, it is nececssary that its action be proportionate to the grossness of this sense, the same as the delicacy of the organs of sight appears to be to the extreme fineness of the parts of light; these parts move with the greatest velocity, and act in the instant at immense distances, whereas those of heat have but a slow progressive motion, and only extend to small intervals from the bodies whence they emanate.