42 THE MECHANISM OF LIFE 



temperature heat, but the latter is unavailable energy, and we 

 cannot transform it. 

 Thus we get our second principle : 



In all transformations of energy some of the latter becomes 



' unavailable. Thus the capacity for doing work continually 



diminishes . . . . . . . . . . . . (2) 



Now consider another notion which is really involved in our 

 second principle, but which it is useful to discuss separately; 

 that is, the notion of — 



Reversible and Irreversible Transformations. — Take a suitable 

 form of dynamo and cause it to revolve, thus generating a current 

 of, say, K kilowatts, by the expenditure of, say, H horse-power. 

 Then take K kilowatts of current, and supply it to the dynamo, 

 when the latter will begin to revolve and will act as a motor. 

 Thus the same machine is reversible; when current is supplied 

 i it will generate mechanical energy, and when mechanical energy 

 is supplied it will generate electricity. 



But notice particularly that we supplied K kilowatts of current, 

 and got H horse-power. Now if we supply H horse-power, we 

 only get about 95 per cent. K of current. Therefore our rever- 

 sibility is not quite perfect; if it were we should have got K 

 kilowatts of current, instead of 95 per cent. K. 



Take the case of a steam engine and consider its theory. We 

 have a " working substance," the water in the boiler. Now let 

 this working substance be heated; it expands enormously, does 

 work by moving the pistons, and finally escapes into the con- 

 denser, where it heats up the circulating water. Therefore, the 

 working substance takes heat from the steam boiler, converts 

 some of this heat into mechanical work, and gives up some to the 

 condenser. Now let the water in the boilfer cool down, and try to 

 reverse the process by actuating the engine in the opposite 

 direction. If the mechanism were reversible, heat would be taken 

 from the condenser, the mechanical work done on the engine from 

 outside would transform into heat, and these two quantities of 

 heat would be transferred to the water in the boiler, heating up 

 the latter to boiling-point and beyond, and so establishing steam 

 • pressure. This cannot be done (in practice), and so the steam 

 engine is an irreversible mechanism. 



Take a very famous experiment and try to reverse it: 



Joule, in 1843, caused a paddle-wheel to revolve in water cor^- 



