APPENDIX 369 



contraction 2^-1, an isothermal contraction 1^4, an 

 adiabatic expansion 4^3, and an isothermal expansion 

 3->2. A quantity of heat, Q lt is taken from the refrig- 

 erator at a temperature TI, and another quantity, Q 2 , 

 is given up to the source at a temperature T 2 . But 

 Qz is greater than Q lf and the engine therefore gives up 

 more heat than it receives, while, further, heat flows 

 from a body at a low temperature to another body at 

 a higher temperature. Where does the engine get 

 this energy from ? It gets it because work is done 

 upon it by means of an outside agency, and all of 

 this work is converted into heat. 



REVERSIBILITY 



The Carnot engine and cycle are therefore perfectly 

 reversible. Not only can the engine turn heat into 

 work, but it can turn work into heat. This perfect, 

 quantitative reversibility is, however, a property of the 

 imaginary mechanism only, and it does not exist in 

 any actual engine. 



ENTROPY 



Let us consider the cycle more closely. In the 

 operation 4->i, which is an isothermal expansion, there 

 is a flow of heat-energy from the source and a trans- 

 formation of energy into work. The gas in the con- 

 dition represented by the point 4 had a certain pressure 

 and a certain volume. In the condition represented by 

 the point i, its pressure has decreased, its volume has 

 increased, and its temperature is the same. Its physical 

 condition has been changed, and to bring it back into its 

 former condition something must be done to it. Let, 

 then, the gas continue to expand without receiving 

 any more heat, or parting with any : that is, let it 



2 A 



