AND OF THE SECOND LAW 45 



the latter case is a purely ideal one, which is really never realized 

 in nature and is only a convenient and fruitful fiction in theoretical 

 demonstrations. 



(3) M. PLANCK. " It is not possible to construct a periodically 

 functioning motor which effects nothing more that the lifting 

 of a load and the cooling of a heat reservoir." 



The proof of this is purely experimental and cumulative 

 and in this respect is exactly like that for the First Law, the Con- 

 servation of Energy, and has exactly the same sort of validity. 



(4) Perpetual motion of an isolated system, such as a mechan- 

 ism with friction, is impossible and not even approximately real- 

 izable. 



This refers to perpetual motion of the second class, a clear 

 illustration of which is given on p. 8 of Goodenough's Notes on 

 Thermodynamics: "A mechanism with friction is inclosed in 

 a case through which no energy passes. Let the mechanism be 

 started in motion. Because of friction work is converted into 

 heat which remains in the system, since no energy passes through 

 the case. Suppose that the heat thus produced could be completely 

 transformed into work; then this work would be used again to 

 overcome friction and the heat thus produced would be again 

 transformed into work. We should then have perpetual motion 

 in a mechanism with friction without the addition of energy 

 from an external source." This can be shown to be equivalent 

 but not identical with the " perpetual motion of the second kind," 

 touched upon in p. 30; the latter does confessedly draw on 

 external energy and furnishes a surplus of power for use, say, in 

 technical service. 



Nominally, such a machine is a case of perpetual motion, but 

 not in the usually accepted sense, for it furnishes no surplus of 

 power; it is the getting of something for nothing, of getting cost- 

 free power, which has always been the attractive feature of so-called 

 perpetual motion. Still this machine is as much at variance 

 with experience as PLANCK'S perpetually working motor of the 



