326 



THE LIFE OF THE PLANT 



of work: a foot-pound, a kilogramme-metre will therefore be 

 units of mechanical work, with which we compare and by which 

 we measure all work. 



Thus I did work when I separated the two balls. I used a 

 certain amount of energy, which can be measured in terms of 

 work done. Let us see what happened to our balls. Their 

 relative position has changed, and they have acquired the 

 power to move, without the application of any external force. 

 I just remove my hands, thereby ceasing to exert any in- 

 fluence, and the balls are set in motion : they return to their 

 former position and knock against each other. Evidently 

 they possess energy in the new position into which I brought 

 them, and they had none in that from which I removed them. 



Fig. 8i. 



This energy is latent, but ready to manifest itself at any moment 

 in motion. 



This simple example clearly demonstrates the two different 

 states, the two types of energy, so to speak : the active, 

 apparent energy, manifested in the motion (of my hand, of 

 the balls), and the latent energy, accumulated or stored, like 

 the energy of a bent bow. We see famihar examples all 

 around us : the energy manifested by a falling weight and the 

 energy accumulated in a weight which may fall ; the energy 

 manifested by an unbending bow, and that which is accumulated 

 in a bent one. All these are cases of active and latent energy 

 respectively. I must add that the weight once fallen, or the 

 bow unbent, does not possess any energy whatever, either active, 

 or latent. 



It is evident, however, that these two types of energy are 

 essentially different. In the first case it is manifest, in the 



