substances due to heat, conceived the hypothesis of molecular vortices. He worked out his theory, but owing to the want of experimental data, did not publish immediately. In 1845 Joule brought to a successful result a series of experimental investigations designed to measure the exact mechanical equiva- lent of a given amount of heat. In 1849 William Thomson, professor of physics at Glasgow, gave an account to the Royal Society of Edinburgh of Carnot's theory, and the problem then was, "How must the theory of the heat-engine be modi- fied, supposing that heat is not a substance, but a mode of motion?" Rankine reduced his results to order, and contributed them to the Royal Society of Edinburgh in two papers entitled "On the mechanical action of heat, especially in gases and vapors" and "The centrifugal theory of elasticity as applied to gases and vapors." He was elected a fellow, and read his papers early in 1850. That same year the British Association met in Edinburgh. Rankine was Secretary of Section A, and he had ready an elaborate paper "On the laws of the elasticity of solid bodies," in which the same hypothesis of molecular vortices is the guiding idea.
Rankine was not content to suppose the heat of a body to be the energy of the molecules due to some kind of motion. He supposed, like the other pioneers in thermodynamics, that the invisibly small parts of bodies apparently at rest are in a state of motion, the velocity of which, whether linear or angular, is very high. But he went further; he imagined the motion to be like that of very small vortices each whirling about its own axis; from which it would follow that the elasticity of a gas is due to the centrifugal force of this motion; an increase of angular velocity would mean an increase of centrifugal force. His own statement of the hypothesis is as follows: "The hypothesis of molecular vortices may be defined to be that which assumes that each atom of matter consists of a nucleus or central point enveloped by an elastic atmosphere, which is retained in its position by attractive forces, and that the elasticity due to heat arises from the centrifugal force of those atmospheres, revolving or oscillating about their under or central points." Rankine's
