AND OF THE SECOND LAW 31 



from its own. There is here flow of heat from the hotter to the 

 colder body and the process is an irreversible one. 



(b) The body experiences resistance from friction which 

 develops heat; it is not possible to effect completely the opposite 

 operation of restoring the whole system to its initial state. 



(c) The body expands without at the same time developing 

 an amount of external energy which is exactly equal to the work 

 of its own elastic forces. For example, this occurs when the 

 pressure which a body has to overcome is essentially (i.e., finitely) 

 less than the body's own internal tension. In such a case it is 

 not possible to bring the whole system (of which the body is a 

 part) completely back into its initial state. Illustrations 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) Two gases at the same pressure and temperature are 

 separated by a partition. When this is suddenly removed, the 

 two gases mix or diffuse. This too is an essentially irreversible 

 process. 



Outside of chemical phenomena, we may instance still other 

 examples of irreversible processes: flow of electricity in conductors 

 of finite resistance, emission of heat and light radiation, and 

 decomposition of the atoms of radio-active substances. 



" Numerous reversible processes can at least be imagined, 

 as, for instance, those consisting throughout of a succession of 

 states of equilibrium, and therefore directly reversible in all their 

 parts. Further, all perfectly periodic processes, e.g., an ideal 

 pendulum or planetary motion, are reversible, for, at the end of 

 every period the initial state is completely restored. Also, all 

 mechanical processes with absolutely rigid bodies and incom- 

 pressible liquids, as far as friction can be avoided, are reversible. 

 By the introduction of suitable machines with absolutely unyield- 

 ing connecting-rods, frictionless joints, and bearings, inextensible 

 belts, etc., it is always possible to work the machine in such a 



