SECOND LAW OF THERMODYNAMICS 229 



changes it is important to understand that our study is not to be 

 concerned with thermal and chemical actions themselves, but with their 

 results. The actions themselves are as a rule extremely complicated. 

 Thus the details of behavior of the coal and air in a furnace are hope- 

 lessly complicated ! The important practical thing, 3 however, is the 

 amount of steam that can be produced by a pound of coal, and this 

 depends upon (1) the condition of the water from which the steam is 

 made, that is, whether the water is hot or cold to start with, (2) the 

 condition of the air and of the coal which are to combine in the furnace, 

 (3) the pressure and temperature of the steam which is to be produced, 

 and (4) the condition of the flue gases as they enter the chimney. That 

 is to say, the only things which it is necessary to consider are things 

 which relate to quiescent substances. A quiescent substance may be 

 said to be in a standing condition or state and the whole subject of 

 heat (thermoclynamics) may be said to refer to changes of state, that 

 is, to changes from one quiescent condition to another quiescent condi- 

 tion without regard to the details of action which lead from one 

 quiescent condition to the other. 



Mechanical Energy and Heat Energy 



In studying thermal and chemical changes we have to do with a 

 new kind of energy. The gravitational energy of an elevated store of 

 water can be wholly converted into mechanical work, the energy of two 

 electrically charged bodies can be wholly converted into mechanical 

 work (for example, by allowing the charged bodies to move towards 

 each other), the kinetic energy of a moving car can be wholly converted 

 into mechanical work, and so on. On the other hand, the energy of 

 the hot steam which enters a steam engine from a boiler can not be 

 wholly converted into mechanical work. Any store of energy which 

 can be wholly converted into mechanical work may be called mechanical 

 energy. The energy of the hot steam which enters a steam engine 

 from a boiler is called heat energy. 4 



In the attempt to exclude all thermal changes from the purely 

 mechanical discussion of energy one is confronted by the fact that 

 friction (with its accompanying thermal changes) is always in evidence 



3 This practical matter is exactly of the same character as any purely scien- 

 tific matter involving the application of thermodynamics. It is always a ques- 

 tion concerning the correlation of measurable and sensible things. 



4 The important difference between mechanical energy and heat energy, 

 namely, that one can be wholly converted into mechanical work whereas the 

 other can not, may be clearly understood in terms of the atomic theory. Every 

 particle of a moving car travels in the same direction and all of the particles 

 work together to produce mechanical effect when the car is stopped; the 

 molecules of hot steam, however, fly to and fro in every direction, and no 

 method can be devised whereby the whole of the energy of the molecules of 

 hot steam can be used to produce mechanical effect. 



