John Herapath (1790-1868)

A mathematical inquiry into the causes, laws and principal phenomena of heat, gases, gravitation, etc.

Thus between hope and despair, between unceasing attempts and mortifying failures, I continued until May 1814, at which time my ideas of heat underwent a complete revolution. Previous to this time I had conceived heat to be the effect of an elastic fluid; and on this supposition had repeatedly attempted to reduce its laws to mathematical calculation; but uniform disappointment at length induced me to give this hypothesis a careful investigation, by comparing it with general and particular phenomena. The result of this investigation convinced me that heat could not be the consequence of an elastic fluid. At the time I was making the comparison, I took every opportunity of examining how far the other hypothesis (which until now I had forgot was sanctioned by the names of Newton and Davy) agreed with phenomena, and so well pleased with its simplicity, and the easy, natural manner in which the different phenomena seemed to flow from it, that I regretted having neglected it so long, and determined to consider it more attentively. A difficulty, however, soon appeared in the application of this theory of heat to gaseous bodies, which I had some trouble to conquer; for as I still adhered to the hypothesis of gases being composed of particles endued with the power of mutually repelling one another, I could by no means imagine how any intestine motion could augment or diminish this power. Here then I was involved in another dilemma; but after I had revolved the subject a few times in my mind, it struck me that if gases, instead of having their particles endued with repulsive forces, subject to so curious a limitation as Newton proposed, were made up of particles, or atoms, mutually impinging on one another, and the sides of the vessel containing them, such a constitution of aeriform bodies would not only be more simple than repulsive powers, but, as far as I could perceive, would be consistent with phenomena in other respects, and would admit of an easy application of the theory of heat by intestine motion. Such bodies I easily saw possessed several of the properties of gases; for instance, they would expand, and, if the particles be vastly small, contract almost indefinitely; their elastic force would increase by an increase of motion or temperature, and diminish by a diminution; they would conceive heat rapidly, and conduct it slowly; would generate heat by sudden compression, and destroy it by sudden rarefaction; and any two, having ever so small a communication, would quickly and equally intermix.

Postulata

1. Let it be granted that matter is composed of inert, massy, perfectly hard, indestructible atoms, incapable of receiving any change or impression in their original figure and nature.
2. Let it be granted that all solid and fluid bodies have their smaller parts composed of these atoms, which may be of different sizes and figures, and variously associated, according to the manner which the constitution and nature of the bodies require.
3. Let it be granted that gaseous or aeriform bodies, consist of atoms, or particles, moving about, and among one another, with perfect freedom.
4. Let it be granted that what we call heat arises from an intestine motion of the atoms, or particles, as is proportional to their individual momentum.
5. Let it be granted that a gaseous body of very great tenuity in its parts fills all space, and extends to its utmost limits.