368 THE PHILOSOPHY OF BIOLOGY 



munication with the source of heat, and so it does not 

 cool, but draws heat from the source and remains at 

 a constant temperature, T 2 . The expansion is there- 

 fore an isothermal one. Work is done by the gas. 



This completes the cycle. But the gas is heated, 

 and when the piston is at position II, the valve is 

 turned so as to close the cylinder by the non-conduct- 

 ing plug 0. The heat already contained in the gas 

 continues to expand, the latter doing more work, but 

 this expansion causes the temperature to fall from 

 T z to TV*. This is the operation with which the 

 cycle commenced. 



Summarising the positive Carnot cycle, we see that 

 the engine takes heat from a source (+) and gives up 

 part of this to a refrigerator ( - ) , (in an actual steam- 

 engine heat is taken from the boiler and given up to 

 the condenser water). If we measure the quantity of 

 heat taken from the boiler in the steam which enters 

 the cylinders we shall find that this quantity of heat is 

 greater than the quantity which is given up to the 

 condenser water. What becomes of the balance ? It 

 is converted into the mechanical work of the engine. 

 The Carnot engine therefore takes a quantity of heat, 

 Q 2 , from the source and gives up another quantity of 

 heat, Q lt to the refrigerator. We find that Q 2 is 

 greater than Q lt and the balance, Q 3 - Q l is represented 

 by the work done by the engine. Heat-energy falls 

 from a state of high, to a state of low potential, and is 

 partly transformed into mechanical work. 



THE CARNOT NEGATIVE CYCLE 



This is simply the positive cycle reversed. The 

 reader should puzzle it out for himself if he is not 

 already familiar with it. It consists of an adiabatic 



