《Natural History（10 自然史 第10卷）》：

　　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.

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