6 THE PHYSICAL SIGNIFICANCE OF ENTROPY 



quently applies to its mechanical and thermal events the laws 

 of thermodynamics) will regard the procsss as a whole to be 

 an irreversible one in accordance with the Second Law. . . . 

 Now a particular change of state cannot at the same time be 

 both reversible and irreversible. But the one observer has a 

 different idea of "change of state" from the other; the micro- 

 observer's conception of " change of state" is different from 

 that of the macro-observer. What then is "change of state?" 

 The state of a physical system can probably not be rigorously 

 denned, otherwise than the conception, as a whole, of all those 

 physical magnitudes whose instantaneous values, under given 

 external conditions, also uniquely determine the sequence of these 

 changing values. 



BOLTZMANN'S statement is much more clear, namely, "The 

 state of a body is determined, (a) by the law of distribution of 

 the particles in space and (b) by the law of distribution of 

 the velocities of the particles; in other words, a body's condition 

 is determined (a) by the number of particles which lie in each 

 elementary realm of the space and (b) by a statement of the 

 number of particles which belong to each elementary velocity 

 group. These elementary realms are all equal and so are the ele- 

 mentary velocity groups equal among themselves. But it is further- 

 more assumed that each elementary realm and each elementary 

 velocity group contains very many particles." 



Now if we ask the aforesaid two observers what they under- 

 stand by the state of the atomic host or gas under consideration, 

 they will give entirely different answers. The micro-observer 

 will mention those magnitudes which determine the location and 

 the velocity condition of all the individual atoms. This would 

 mean in the simplest case, in which the atoms are regarded as 

 material points, that there would be six times as many magnitudes 

 as atoms present, namely, for each atom there would be three 

 co-ordinates of location and three of velocity components; in 

 the case of composite molecules there would be many more such 



