04 THE PROGRESS OF SCIENCE n 



hammering of the ultra-mundane corpuscles on 

 the bob confers its kinetic energy, on the one 

 hand, and takes it away on the other ; and the 

 state of potential energy means the condition 

 of the bob during the instant at which the 

 energy, conferred by the hammering during the 

 one half-arc, has just been exhausted by the 

 hammering during the other half-arc. It seems 

 safe to look forward to the time when the concep- 

 tion of attractive and repulsive forces, having 

 served its purpose as a useful piece of scientific 

 scaffolding, will be replaced by the deduction of 

 the phenomena known as attraction and repulsion, 

 from the general laws of motion. 



The doctrine of the conservation of energy 

 which I have endeavoured to illustrate is thus 

 defined by the late Clerk Maxwell : 



" The total energy of any body or system of 

 bodies is a quantity which can neither be in- 

 creased nor diminished by any mutual action of 

 such bodies, though it may be transformed into 

 any one of the forms of which energy is suscep- 

 tible." It follows that energy, like matter, is 

 indestructible and ingenerable in nature. The 

 phenomenal world, so far as it is material, ex- 

 presses the evolution and involution of energy, 1 

 its passage from the kinetic to the potential 

 condition and back again. Wherever motion of 

 matter takes place, that motion is effected at 

 the expense of part of the total store of energy. 



