Page:Popular Science Monthly Volume 76.djvu/239

Rh constitutes a simple sweep. When external influences change continuously a substance in its tendency to settle to thermal equilibrium never catches up with the changing conditions, but trails along behind them, and we have what may be called a trailing sweep. Thus the rapid heating or cooling of a gas in a vessel is a trailing sweep. So long as heat is given to or taken from the gas at a perceptible rate there will be perceptible differences of temperature in different parts of the gas; and the gas in its tendency to settle to thermal equilibrium never catches up with the increasing or decreasing temperature of the walls of the containing vessel.

A substance may be subjected to external action which, although permanent or unvarying, is incompatible with thermal equilibrium. When such is the case the substance settles to a permanent or unvarying state which is not a state of thermal equilibrium. Such a state of a substance may be called a steady sweep. For example, the two faces of a slab or the two ends of a wire may be kept permanently at different temperatures, and when this is done the slab or wire settles to an unvarying state which is by no means a state of thermal equilibrium. Heat flows through the slab or along the wire from the region of high temperature to the region of low temperature. This flow of heat through the slab or along the wire is an irreversible process and it constitutes a steady sweep. The ends of a wire may be connected to a battery or dynamo so that a constant electric current flows through the wire and the heat which is generated in the wire by the current may be steadily carried away by a stream of water or air. Under these conditions the wire settles to an unvarying state which is by no means a state of thermal equilibrium, the battery or dynamo does work on the wire and this work reappears steadily as heat in the wire.

Every one must admit that the impetuous character of a sweeping process suggests a certain havoc, a certain degeneration in the substance or system in which the sweep takes place. Consider, for example, a charge of gun-powder which has been exploded; if it is exploded in a large empty vessel, everything is there after the explosion, all of the energy is there and all of the material substance is there, but it can not be exploded a second time! The man on the street has heard much during recent years of the conservation of energy and of the conservation of matter, and the old proverb that "you can't eat your cake and have it" presents to his mind a question which in its less familiar forms, as relating to engines for example, he tries in vain to rationalize in terms of these principles of conservation. Nearly all of the intuitive