time; the refreshing river 



way character of time. Only in reversed cinematograph films, but 

 never in nature, does water flow uphill of itself, or the smoke and 

 gases of an exploding bomb recompress themselves, like the djinn 

 of fable, into the reassembling case with its explosive content. 



This description approaches the simple examples which are given 

 by all elementary expositions of statistical mechanics.^ If a number 

 of atoms are introduced at one corner of a room, they will in a short 

 space of time be found equally distributed within it (assuming that 

 their kinetic energy is great enough to overcome their mutual attrac- 

 tions). In other words, it is impossible for a gas in a vacuum to occupy 

 anything less than the whole of the space available. In the same way, 

 if "hot" molecules are introduced at one corner of the room, and 

 "cold" molecules at another, their collisions will soon ensure that all 

 the molecules have the same velocity and that the temperature of the 

 room is uniformly warm. 



The significance of the second law of thermodynamics is, therefore, 

 that all particles, when left to themselves, tend to become disarranged 

 with respect to one another. Now such a process is similar to the 

 shuffling of a pack of cards, or the random distribution of a quantity 

 of black and white balls when continuously shaken together. "Shuff- 

 ling," wrote Eddington, "is the only thing nature can never undo." 

 High probability, therefore, is associated with randomness, low ■ 

 probability with the opposite, whatever you like to call it — perhaps 

 arrangement or order. Hence the definition that entropy is the sum 

 of the logarithms of the probabilities of the "complexions" of the 

 parts of a system. A complexion or micro-state is simply an assembly 

 of particles having the same velocity, rotational energy, rotational 

 axis, etc. The more complexions there are in the system the less 

 disordered it is. The presence of many complexions having very 

 high or very low energies different from those in their vicinity is the 

 condition under which useful work can be obtained from the system. 

 This relatively unusual or improbable state constitutes what the 

 physicist calls "order" or "arrangement." The corresponding "dis- 

 order" is measured by the logarithm of the probability. 



Thus in an isolated system the net increase of entropy implies a 

 net decrease of "order" and a net increase of "disorder." In such an 

 isolated system a decrease of entropy can only occur in one part 

 provided it is over-compensated by a simultaneous and greater 



^ e.g. A System of Physical Chemistry by W, C. McC. Lewis, 3 vols., vol. iii (London 

 1919). 



208 



