100 Mechanical Energy of the Universe. 



Heat, moreover, tends to diffuse itself uniformly by con- 

 duction and radiation, until all matter shall have acquired 

 the same temperature. 



There is, consequently, Professor Thomson concludes, so 

 far as we understand the present condition of the universe, 

 a tendency towards a state in which all physical energy will 

 be in the state of heat, and that heat so diffused, that all 

 matter will be at the same temperature ; so that there will 

 be an end of all physical phenomena. Vast as this specula- 

 tion may seem, it appears to be soundly based on experi- 

 mental data, and to represent truly the present condition of 

 the universe, so far as we know it. 



My object now is to point out how it is conceivable that, 

 at some indefinitely distant period, an opposite condition of 

 the world may take place, in which the energy which is now 

 being diffused may be concentrated into foci, and stores of 

 chemical power again produced from the inert compounds 

 which are now being continually formed. 



There must exist between the atmospheres of the heavenly 

 bodies a material medium capable of transmitting light and 

 heat ; and it may be regarded as almost certain, that this 

 interstellar medium is perfectly transparent and diatherman- 

 ous ; that is to say, that it is incapable of converting heat, or 

 light (which is a species of heat), from the radiant into the 

 fixed or conductible form. 



If this be the case, the interstellar medium must be incap- 

 able of acquiring any temperature whatever; and all heat 

 which arrives in the conductible form at the limits of the 

 atmosphere of a star or planet, will there be totally con- 

 verted, partly into ordinary motion, by the expansion of the 

 atmosphere, and partly into the radiant form. The ordinary 

 motion will again be converted into heat, so that radiant heat 

 is the ultimate form to which all physical energy tends ; and 

 in this form it is, in the present condition of the world, dif- 

 fusing itself from the heavenly bodies through the interstellar 

 medium. 



Let it now be supposed, that, in all directions round the 

 visible world, the interstellar medium has bounds, beyond 

 which there is empty space. 



