234 THE POPULAR SCIENCE MONTHLY 



degeneration applies to the effects of turbulence which always plays a 

 certain havoc in a system. Thus a certain degeneration is associated 

 with the turbulence which is produced when a hot iron is dipped into 

 water, a certain degeneration is associated with the escape of a com- 

 pressed gas through an orifice, a certain degeneration is associated 

 with the flow of heat from a region of high temperature to a region 

 of low temperature, a certain degeneration is associated with the con- 

 version of work into heat by the rubbing of a coin on a board, and so on. 



Beversible Processes 



A substance in thermal equilibrium is generally under the influence 

 of external agencies. Thus surrounding substances confine a given 

 substance to a certain region of space, and they exert upon the given 

 substance a definite constant pressure; surrounding substances are at 

 the same temperature as the given substance and according to the 

 atomic theory the molecules of the given substance rebound from the 

 surrounding substance with their motion on the average unchanged; 

 surrounding substances may exert constant magnetic or electric in- 

 fluences on the given substance; and so on. If the external influences 

 which act upon a fluid in thermal equilibrium are made to change 

 very slowly, causing the pressure, volume and temperature of the fluid 

 to pass very slowly through a continuous series of values and in general 

 involving the doing of work upon or by the fluid and the giving of 

 heat to or taking of heat from the fluid, then the fluid will pass slowly 

 through a process consisting of a continuous series of states of thermal 

 equilibrium. Such a process is called a reversible process, for the 

 reason that the fluid will pass through the same series of states in 

 reverse order if the external influences are changed slowly in reverse 

 sense. 



Irreversible Processes 



When a substance is settling or tending to settle to thermal equi- 

 librium it may be said to undergo a process. Such a process can not 

 be arrested or held at any stage short of complete thermal equilibrium, 

 but it always and inevitably proceeds towards that state. Such a 

 process may, therefore, be called a sweeping process or simply a sweep. 

 The settling of a closed system to thermal equilibrium may be called 

 a simple sweep. For example, the equilibrium of a mixture of oxygen 

 and hydrogen in a closed vessel may be disturbed by a minute spark, 

 and the explosion of the gases together with the subsequent settling 

 of the water vapor to a quiescent state constitutes a simple sweep. The 

 equilibrium of a gas confined under high pressure in one half of a 

 two-chambered vessel may be disturbed by opening a cock which con- 

 nects the two chambers, and the rush of gas into the empty chamber 



