40 LECTURES TO SCIENCE TEACHERS. 



same way. But I want to point out to you that we can 

 alter the volume of a gas in different ways. In the case I 

 am speaking of we may increase the pressure so gradually 

 that it has not time to get warm, but if I were to compress 

 the gas very suddenly it would generate a certain amount 

 of heat by the mechanical work I was doing, and con- 

 sequently, this heat would tend to expand the gas and 

 prevent a given pressure from reducing the volume so 

 much as in the previous case. Thus if I compress the gas 

 without letting any heat escape, the increase in pressure 

 corresponding to a given diminution in volume will be less. 

 Such a state is represented by two curves which I have 

 called adiabatic curves. If I increase the pressure from C, 

 keeping the temperature constant, the curve goes up, but 

 if I do it so suddenly that no heat can escape, then the 

 pressure is greater than would be indicated by that line. 

 In order that you may see that this is really the case, 

 that we can generate a considerable amount of heat by the 

 application of pressure to a gas, I will take a little cotton- 

 wool moistened in bi-sulphide of carbon, and by intro- 

 ducing it into this glass syringe and taking it out again 

 that introduces a certain amount of vapour of bi-sulphide 

 of carbon. Now if you watch carefully as I compress it, 

 you will see a flash indicating the amount of heat generated 

 by my compression. If I had done this more gently, and 

 had compressed it so slowly that the heat was able to 

 escape, and it was kept at a uniform temperature, then the 

 pressure when I had reduced this volume to a certain 

 amount would not be so great as in the case I have just 

 shown you. In other words, if we do it slow enough we 

 are following one of these isothermal curves, but if we 

 do it very rapidly we follow an adiabatic curve. 



Now in the consideration of the second law of thermo- 

 dynamics there is one thing we have continually to speak 

 of, and that is the employment of a reversible engine. 

 The idea of a reversible engine was introduced by Carnot 

 in 1824, and has proved of enormous service in the principles 

 of thermo-dynamics. I will show you an example of a 

 certain engine whose action can be reversed, but neverthe- 

 less it is not what Carnot calls a reversible engine. Here 

 I have a Gramme machine, for generating electricity. 

 By rotation by a bell over the left-hand axle I cause coils 



