648 



SCIENCE. 



[N. S. Vol. XVIII. No. 4G4. 



dent, for any one can see that a sudden 

 stoppage of the piston would leave some 

 slight turbulence in the gas, which would 

 not be the case if the gas were in equilib- 

 rium at the instant the piston is stopped. 

 When the piston is moved more and more 

 slowly, the departure of the gas from strict 

 thermal equilibrium at each stage of the 

 expansion or compression becomes less and 

 less, and the expansion or compression ap- 

 proaches more and more nearly to a revers- 

 ible process. 



The rapid heating (or cooling) of a gas 

 in a closed vessel is a trailing sweep. So 

 long as heat is given to the gas at a per- 

 ceptible rate there will be perceptible dif- 

 ferences of temperature in different parts 

 of the gas ; the gas in its tendency to settle 

 to thermal equilibrium never catches up 

 with the increasing temperature of the 

 walls of the containing vessel. 



When the gas is heated (or cooled) more 

 and more slowly, that is, when heat is given 

 to the gas at a rate which becomes more 

 and more nearly imperceptible, then the 

 departure of the gas from strict thermal 

 equilibrium at each stage of the heating 

 process becomes less and less, and the heat- 

 ing (or cooling) approaches more and more 

 nearly to a reversible process. 



6. STEADY SWEEPS. 



A substance may be subjected to external 

 action which, although unvarying, is in- 

 compatible 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 is called a steady 

 sweep. 



Examples.— The two faces of a slab or 

 the two ends of a wire may be kept per- 

 manently at different temperatures. 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, never from the region 

 of low temperature to the region of high 

 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 kept per- 

 manentl.y at different electric pressures, for 

 example, by connecting the wire to the 

 terminals of a battery or dynamo. When 

 this is done a steady electric current flows 

 along the wire, the battery does work 

 steadily on the wire, and this work reap- 

 pears steadily as heat in the wire. Re- 

 versal of the current does not reverse this 

 process and cause heat energy to disappear 

 in the wire (cooling the wire) and reap- 

 pear as work done on the battery by the 

 wire, but the process is irreversible and it 

 constitutes a steady sweep. 



The notion of steady sweeps is of the 

 utmost importance in thermodynamics 

 inasmuch as thermodynamics treats di- 

 rectly of states of thermal equilibrium and 

 of steady sweeps only. 



The notion of entropy is involved in 

 the notion of a steady sweep; and the 

 notion of temperature is involved in the 

 notion of thermal equilibrium. 



7. THERMODYNAMIC DEGENERATION AND 

 REGENERATION. 



A sweeping process always plays a cer- 

 tain havoc, or effects a certain degeneration 

 in a system. Thus, there is a certain de- 

 generation associated with the escape of 

 a compressed gas through an orifice ; there 

 is a certain degeneration associated with 

 the flow of heat from a region of high 

 temperature to a region of low tempera- 

 ture; there is a certain degeneration asso- 

 ciated with the direct conversion of work 

 into heat, and so on. 



In a simple sweep the degeneration 

 lies wholly in the relation between the 



