The Chemistry of Imponderable Substances

~~An Extract~~

7. Theories concerning the Nature of Heat.

Two theories concerning the nature of heat have been most prevalent amongst philosophers. 1. It has been supposed to be a peculiar ethereal fluid. 2. It has been conjectured to be a property of common matter; a specific motion of the particles of bodies.

The arguments in favour of the first of these theories, have been chiefly deduced from the phænomena {391} of latent heat, of radiant heat, and of change of capacity; whilst the last of them has been supported by experiments on the excitation of heat by friction, in cases in which there existed no perceptible source, from which, considered as a substance, it could possible be derived.

The late experiments of Dr. Herschel have demonstrated, that radiant heat must be constituted by the motions of a peculiar substance. And these motions may be conceived to be either rectilinear projections, or undulations.

It has been lately supposed that they are undulations. And on this theory it has been assumed, 1. That an elastic ethereal medium exists in space. 2. That this medium is diffused through the pores of different ponderable substances, in different states of density. 3. That radiant heat is constituted by particular undulations of it, when in a free state. 4. The sensible heat is occasioned by particular undulations of it, in its states of diffusion through the pores of ponderable substances. 5. That certain peculiar vibratory motions of the particles of ponderable substances are capable of producing the undulations in the ethereal medium which constitute heat. 6. And reciprocally that those undulatory motions of the ethereal medium are capable of producing peculiar vibrations of the particles of ponderable substances.

These propositions are evidently countenanced by the experiments of Count Rumford and Professor Pictet, on the heat produced by friction. They are rendered more conclusive by the analogy between the laws of the motions of radiant heat, and those of sound. And they, in some measure, reconcile the two different theories.

~~Extracts from A Syllabus of a Course of Lectures on Chemistry, delivered at the Royal Institution of Great Britain

The Collected Works of Sir Humphry Davy, ed. John Davy (London: Smith, Elder and Co., 1839), II, 386-409.
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