646 



SCIENCE. 



[N. S. Vol. XV1]1. No. 464. 



series of states in reverse order if the ex- 

 ternal influences are changed slowly so as 

 to make the pressure and volume of the 

 fluid pass through the same series of values 

 in reverse order. 



The characteristics of a reversible pro- 

 cess are therefore as follows: 



(a) A substance which undergoes a re- 

 versible process must be under varying 

 external influence. A closed system can 

 not perform a reversible process. 



(b) A substance as it undergoes a re- 

 versible process is at each instant in a state 

 of thermal equilibrium; and if, at a given 

 instant during a reversible process, the 

 external influences should cease to change, 

 causing a sudden cessation of the doing of 

 work on or by the substance, and of the 

 interchange of heat between the substance 

 and its surroundings, no commotion would 

 be left in the substance. 



(c) A reversible process must take place 

 slowly, strictly with infinite slowness. An 

 actual process, that is, a process which ac- 

 tually does proceed, can be only approxi- 

 mately reversible. Examples of reversible 

 processes are given in the article on trail- 

 ing sweeps, for it is important that it be 

 clearly recognized that a reversible process 

 is the limit which a trailing sweep ap- 

 proaches when it is performed more and 

 more slowly. 



3. SWEEPING OR IRREVERSIBLE PROCESSES. 



Wliile a substance is settling or tending 

 to settle to thermal equilibrium it may be 

 said to undergo a process. Such a process 

 can not, in general, be arrested and main- 

 tained at any stage short of complete 

 thermal equilibrium, but always and in- 

 evitably proceeds towards that state. Such 

 a process is, therefore, called a sweeping 

 process or simply a sweep. 



A sweeping process takes place in one 

 direction only, that is, if A and B are two 

 successive stages of a sweep, stage B fol- 

 lowing stage A, then stage B grows out 



of stage A inevitably, but stage A can not 

 be made to follow or grow out of stage B 

 by any means whatever. A sweeping pro- 

 cess, therefore, is irreversible. 



Molecular Conception of the Sweeping 

 Process.— Wliile a gas (and perhaps any 

 substance) is settling to thermal equilib- 

 rium, immediately after an explosion, for 

 example, the character of the molecular 

 motion at a given point changes rapidly 

 from instant to instant and the character 

 of the molecular motion at a given instant 

 varies greatly from point to point in the 

 gas; in other words, the gas is the seat of 

 more or less violent turbulence wliile it is 

 settling to thermal equilibrium. 



The effects of mutual collision among 

 the molecules, the effects of the collision 

 of the molecules against the walls of the 

 containing vessel and the effects of the 

 confused movement* of the gas molecules 

 from one part of the vessel to another part 

 are al-ways to even up the differences in the 

 character of molecular motion in different 

 parts of the vessel. On the other hand, 

 the external influences which can be 

 brought to bear on a substance act on all 

 the molecules in the same general way, so 

 that the tendency of a turbulent state of 

 a gas to die away on account of the in- 

 ternal actions just pointed out can not be 

 counteracted by external influences,! and, 



* If a great number of white and black balls are 

 placed in a box and shaken up, the conf\ised 

 motion tends to cause an even distribution of 

 white and black balls throughout the box, for the 

 reason that, of all possible arrangements of the 

 balls, approximately even distribution is the most 

 probable. 



t The maintenance of an unending state of 

 turbulence in a trailing sweep because of rapidly 

 changing external influence is by no means a 

 case in which the tendency of a turbulent state 

 to die away is counteracted by external action, 

 b>it rather a case in which the goal, namely the 

 final state of thermal equilibrium, is made to 

 recede continuouslv. 



