32 THE PHYSICAL SIGNIFICANCE OF ENTROPY 



way as to bring the system completely into its initial state without 

 leaving any change in or out of the machines, for the machines 

 of themselves do not perform any work." 



Other examples of such reversible processes are: Free fall in 

 a vacuum, propagation of light and sound waves without absorp- 

 tion and reflection and unchecked electrical oscillations. All the 

 latter processes are either naturally periodic, or they can be made 

 completely reversible by suitable devices so that no sort of change 

 in Nature remains behind; for example, the free fall of a bpdy 

 by utilizing the velocity acquired to bring the body back to its 

 original height, light and sound waves by suitably reflecting them 

 from perfect mirrors. 



(2) Character of Process Decided by the Limiting States 



" Since the decision as to whether a particular process is irre- 

 versible or reversible depends only on whether the process can 

 in any manner whatsoever be. completely reversed or not, the 

 nature of the initial and final states, and not the intermediate 

 steps of the process, entirely settle it, The question is, whether 

 or not it is possible, starting from the final state, to reach the 

 initial one in any way without any other change. . . . The final 

 state of an irreversible process is evidently in some way discrimi- 

 nate from the initial state, while in reversible processes the two 

 states are in certain respects equivalent. ... To discriminate 

 between the two states they must be fully characterized. Besides 

 the chemical constitution of the systems in question, the physical 

 conditions, viz., the state of aggregation, temperature, and pressure 

 in both states, must be known, as is necessary for the application 

 of the First Law." 



" Let us consider any process whatsoever occurring in Nature. 

 This conducts all participating bodies from a particular initial 

 condition A to a certain final condition B. The process is 

 either reversible or irreversible, any third possibility being 



