SECOND LAW OF THERMODYNAMICS 235 



constitutes a simple sweep. When external influences change con- 

 tinuously 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. 



Steady Sweeps 



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 unvary- 

 ing 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. 



Thermodynamic Degeneration 



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 conserva- 

 tion 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 



