AND OF THE SECOND LAW 73 



forces. For example this occurs when the pressure which a 

 body has to overcome is essentially (that is, finitely) less than 

 the body's own internal tension. In such a case it is not possible 

 to bring said body back to its initial state by a completely opposite 

 procedure. Examples of this group are: steam escaping from 

 a high-pressure boiler, compressed air flowing into a vacuum tank 

 and a spring suddenly released from its state of high tension. 



(d) Suppose two gases existing at the same pressure and 

 temperature are on opposite sides of a partition; when the par- 

 tition is quickly removed the two gases will diffuse or mix. These 

 gases will not unmix of themselves and the diffusion process is 

 an irreversible one and is somewhat like the process considered 

 under (c). 



The foregoing facts and propositions have in the main already 

 been stated in this presentation and it will be profitable to make 

 comparisons with the definition of irreversible and reversible 

 events given on p. 30 and with the examples on pp. 31, 32. 



HEAT CONDUCTION 



The group under head (a) represents the irreversible processes 

 which perhaps occur most often, namely, the direct passage of 

 heat, by conduction or radiation, from a hot body to a cold 

 body, here say from a hot gas to a cold gas. The former loses 

 in heat energy what the latter gains. As radiation phenomena 

 have very special features of their own and for the present may 

 be said to be outside of our selected province, we will confine 

 our attention to heat conduction alone. Moreover, for our present 

 purpose, we will suppose said flow or change to take place without 

 alteration of volume of either the hot or the cold gas. Then will the 

 hot gas experience a drop in temperature and the cold one a rise 

 in temperature. We have already treated such isometric changes 

 and know that the number of complexions is thereby diminished 

 in the originally hotter body and increased in the originally colder 



