i8o SCIENCE AND THE HUMAN MIND 



materially the progress of modern physics and chemistry 

 in many other directions. They led to the theory of an 

 absolute scale of temperature, and pointed the way 

 to that series of researches at low temperatures which 

 eventually liquefied all known gases and completed 

 the proof of the continuity of all types of matter in its 

 three states. 



For the gain of useful work from a supply 

 of heat, a temperature inequality is necessary. 

 But, in nature, temperature inequalities are con- 

 tinually being diminished by conduction of heat and 

 in other ways. Hence, in a closed system, the heat 

 energy tends steadily to become less and less available 

 for the performance of useful work. When the avail- 

 able energy becomes a minimum, no further work 

 can be done, and thus we determine the necessary 

 conditions of equilibrium of the system. In this way 

 the mathematical theory of chemical and physical 

 equilibrium has been built up by Kelvin, Helmholtz 

 and Willard Gibbs. 



Similar ideas, extended to the whole solar system, 

 indicate that mechanical energy is continually wasting 

 into heat by friction, and heat energy continually 

 becoming less available by the reduction of inequalities 

 of temperature. Hence we are led to contemplate a 

 distant future in which all our stores of energy shall have 

 been converted into heat uniformly distributed through 

 matter in mechanical equilibrium, and all further 

 change becomes for ever impossible. But it should 

 be noted that this conclusion rests on several unproved 

 assumptions. It supposes that the solar system, or 

 the Universe if it be extended thereto, may be treated 

 as an isolated system into which no energy is entering ; 



