262 



Dr. S. A. Shorter on the Constitution 



Suppose now that the engine goes through a reversible 

 isothermal cycle. According to. the first law of thermo- 

 dynamics, the total work done by the system is equal to the 



Fig. 1. 



total heat absorbed by it. According to the second law 

 the total heat absorbed is zero. Hence the total work done 



is zero. 



Let the cycle consist of the following four stages : — 



(1) The surface-tension being maintained at the con- 

 stant value t, let the volume v of the system be changed 

 till the surface increases from a to cr+8a. During this 

 change the pressure p will remain constant. 



(2) The area of the surface being maintained con- 

 stant, let the pressure p be changed till the surface- 

 tension changes from t to t + St. 



(3) The surface tension being maintained constant, 

 let the volume be changed till the area of the surface 

 changes from a -{-So- to cr. 



(4) The area of the surface being maintained con- 

 stant, let the pressure be changed till the surface- 

 tension changes from r 4- Br to r. 



