6 PHYSICAL SCIENCE 



it expressly disclaims such hypotheses, for, as 

 Maxwell showed, the chance collisions of the 

 individual molecules in a gas will lead to differ- 

 ing molecular velocities, and to a concentration 

 of energy in the fast-moving molecules. If we 

 could follow the motions of the individual mole- 

 cules, and separate the fast from the slow, we 

 could use this energy. The principle of dissipa- 

 tion, therefore, only holds while we are obliged, as 

 always as yet in practice, to deal with ordinary 

 molecules statistically and in the aggregate. 



The principles thus applied to isolated systems 

 have been extended to the visible universe. 

 Predictions have been made that ultimately the 

 energy of the universe will become completely 

 unavailable, and will settle down into the energy 

 of heat, uniformly distributed. But this final 

 sleep of the universe depends on the assumptions 

 that the universe is an isolated system, finite in 

 extent, and that no process of molecular concen- 

 tration of energy, such as was imagined by 

 Maxwell, is going on anywhere throughout the 

 depths of time and space. 



A more restricted, though more fruitful, 

 application of the dissipation principle enabled 

 Helmholtz, and, in a much more general manner, 

 Willard Gibbs, to place on a firm footing the 

 theory of non-isolated but isothermal systems — 

 systems, that is, maintained at a uniform and 

 constant temperature by the gain or loss of 

 outside heat. The external work which such a 

 system can perform, by means of a reversible 

 change at constant temperature, tends to a 

 minimum, and the system is in permanent 

 equilibrium when, and when only, this available 



