286 The Outline of Science 



settle down to the temperature of surrounding bodies. As it 

 does so, where does its previous energy go ? In some measure it 

 may pass to other bodies in contact with the piece of iron, but 

 ultimately the heat becomes radiated away in space where we 

 cannot follow it. It has been added to the vast reservoir of 

 unavailable heat energy of uniform temperature. It is sufficient 

 here to say that if all bodies had a uniform temperature we should 

 experience no such thing as heat, because heat only travels from 

 one body to another, having the effect of cooling the one and 

 warming the other. In time the two bodies acquire the same 

 temperature. The sum-total of the heat in any body is measured 

 in terms of the kinetic energy of its moving molecules. 



There must come a time, so far as we can see at present, 

 when, even if all the heat energy of the universe is not radiated 

 away into empty infinite space, yet a uniform temperature will 

 prevail. If one body is hotter than another it radiates heat to 

 that body until both are at the same temperature. Each body 

 may still possess a considerable quantity of heat energy, which 

 it has absorbed, but that energy, so far as reactions between those 

 two bodies are concerned, is now unavailable. The same principle 

 applies whatever number of bodies we consider. Before heat 

 energy can be utilised we must have bodies with different tem- 

 perature. If the whole universe were at some uniform tem- 

 perature, then, although it might possess an enormous amount 

 of heat energy, this energy would be unavailable. 



What a Uniform Temperature would mean 



And what does this imply? It implies a great deal: for 

 if all the energy in the world became unavailable, the universe, 

 as it now is, would cease to be. It is possible that, by the constant 

 interchange of heat radiations, the whole universe is tending to 

 some uniform temperature, in which case, although all molecular 

 motion would not have ceased, it would have become unavailable. 

 In this sense it may be said that the universe is running down. 



