Foundations of the Universe 283 



matter, was constant in amount, which could not be created nor 

 destroyed, was one of the great scientific acquisitions of the past 

 century. 



It is not possible to enter deeply into this subject here. It 

 is sufficient if we briefly outline its salient aspects. Energy is 

 recognised in two forms, kinetic and potential. The form of 

 energy which is most apparent to us is the energy of motion; for 

 example, a rolling stone, running water, a falling body, and so 

 on. We call the energy of motion kinetic energy. Potential 

 energy is the energy a body has in virtue of its position it is its 

 capacity, in other words, to acquire kinetic energy, as in the case 

 of a stone resting on the edge of a cliff. 



Energy may assume different forms; one kind of energy 

 may be converted directly or indirectly into some other form. 

 The energy of burning coal, for example, is converted into heat, 

 and from heat energy we have mechanical energy, such as that 

 manifested by the steam-engine. In this way we can transfer 

 energy from one body to another. There is the energy of the 

 great waterfalls of Niagara, for instance, which are used to sup- 

 ply the energy of huge electric power stations. 



What Heat is 



An important fact about energy is, that all energy tends to 

 take the form of heat energy. The impact of a falling stone 

 generates heat ; a waterfall is hotter at the bottom than at the top 

 the falling particles of water, on striking the ground, generate 

 heat; and most chemical changes are attended by heat changes. 

 Energy may remain latent indefinitely in a lump of wood, but 

 in combustion it is liberated, and we have heat as a result. The 

 atom of radium or of any other radio-active substance, as it dis- 

 integrates, generates heat. "Every hour radium generates suffi- 

 cient heat to raise the temperature of its own weight of water, 

 from the freezing point to the boiling point." And what is heat? 

 Heatis molecular motion. The molecules of every substance, as 



