122 THE LIMITATIONS OF SCIENCE 



ter concerned appears to be at rest or in motion. Since 

 attractive forces exist between all bodies, we find that 

 the idea of work is involved whenever the distance 

 between bodies is altered and we denote this potential 

 energy as the force of attraction times the distance be- 

 tween them. I accept frankly the idea of action at a 

 distance, not because it seems reasonable, for my ex- 

 perience teaches me that to cause motion I must con- 

 struct a link between two bodies ; but the fact remains, 

 that both gravitational and electrical attractions seem 

 to act at a distance and all attempts made to explain 

 them have so confused the mind that the occult phe- 

 nomenon is less perplexing than the occult explanation. 

 In the majority of such problems we can go no further ; 

 but in certain cases, as for instance the theory of 

 gases, a portion of the potential energy due to the pres- 

 sure of gases may be ascribed to the impact of small 

 moving masses of gases. Even here, a portion of the 

 energy of the gas must be considered as potential and 

 incapable of further explanation. Kinetic energy is 

 the power of work due to the motion of bodies. Its 

 measure is one half the moving mass into the square of 

 its velocity. These two kinds account for all the energy 

 of material bodies; they are mutually interchangeable 

 and their sum is a constant. 



A third class of energy remains to be discussed. It 

 is called radiant energy; its difference lies in the fact 

 that it is not associated with material bodies. Cer- 



