Foundations of the Universe 285 



The exhaustion of our coal deposits would mean, so far as our 

 knowledge extends at present, the end of the world's civilisation. 

 There are other known sources of energy, it is true. There is the 

 energy of falling water; the great falls of Niagara are used to 

 supply the energy of huge electric power stations. Perhaps, also, 

 something could be done to utilise the energy of the tides an- 

 other instance of the energy of moving water. And attempts 

 have been made to utilise directly the energy of the sun's rays. 

 But all these sources of energy are small compared with the 

 energy of coal. A suggestion was made at a recent British Asso- 

 ciation meeting that deep borings might be sunk in order to 

 utilise the internal heat of the earth, but this is not, perhaps, a 

 very practical proposal. By far the most effective substitutes 

 for coal would be found in the interior energy of the atom, a 

 source of energy which, as we have seen, is practically illimitable. 

 If the immense electrical energy in the interior of the atom can 

 ever be liberated and controlled, then our steadily decreasing coal 

 supply will no longer be the bugbear it now is to all thoughtful 

 men. 



The stored-up energy of the great coal-fields cari be used up, 

 but we cannot replace it or create fresh supplies. As we have 

 seen, energy cannot be destroyed, but it can become unavailable. 

 Let us consider what this important fact means. 



14 



Dissipation of Energy 



Energy may become dissipated. Where does it go? since 

 if it is indestructible it must still exist. It is easier to ask the 

 question than to give a final answer, and it is not possible in this 

 OUTLINE, where an advanced knowledge of physics is not as- 

 sumed on the part of the reader, to go fully into the somewhat 

 difficult theories put forward by physicists and chemists. We 

 may raise the temperature, say, of iron, until it is white-hot. If 

 we stop the process the temperature of the iron will gradually 



