THE SOURCE AND NATURE OF VITAL ENERGY 295 



into its position, and when the stone fell it only redeveloped 

 the energy that was originally required to lift it to its position. 

 The amount of energy required to lift the stone to its position 

 is exactly the same as that which is developed by the stone 

 when it falls to the ground, and the lifting of the stone and 

 its falling illustrates the conversion of active into potential 

 energy and reconversion of potential energy into an equivalent 

 amount of active energy. It would seem, however, that when 

 the stone reaches the ground the energy disappears. But if 

 we examine the fallen stone carefully, and the earth under- 

 neath it, we find that both have been warmed. The moment 

 that the motion of the stone ceased, heat appeared. Heat is 

 a form of energy, and thus, when the stone comes to rest on 

 the ground, the motion of the stone is converted into that 

 form of energy which is called heat. This heat is soon dissi- 

 pated from the stone and from the earth, for they presently 

 resume their former temperature. The heat has simply gone 

 off into the air; it is not destroyed but has simply distributed 

 itself, and slightly raised the temperature of the air. Nowhere 

 in this series of changes has there been any loss of energy, but 

 simply the conversion of one form into another. Some 5000 

 years ago the Egyptians lifted a large number of stones and 

 placed them one on top of another so as to make the pyramids, 

 exerting a large amount of energy; the energy used in placing 

 the stones in position was stored away in the pyramids in the 

 form of potential energy and is there still. If at any time the 

 pyramids should topple over and the stones fall to the ground, 

 there would be redeveloped an amount of motion exactly equal 

 to the amount used to lift them in position. Thus energy 

 may be stored away and remain in a potential form for ages; 

 but at any future time the energy originally stored away may 

 reappear in the form of active energy. 



The energy present in a dormant form in coal requires a 

 little more explanation. Chemists have shown that the small- 

 est particles of matter which we can see are themselves made of 



