implicitly in the different agents which have played 

 so prominent a part in this philosophy. What is 

 electricity ? It is no single agent : it is a name for 

 many agents in one heat, light, magnetism, and 

 others. What is magnetism ? Nothing apart from 

 electricity. What is artificial light? Like natural 

 light, it goes hand in hand with heat, and it has the 

 same power of working chemical wonders upon the 

 magic screen of the photographic camera. What is 

 heat? It is one of the signs of luminous, and electri- 

 cal, and chemical action. What is chemical power ? 

 A power which bursts into light and heat in flame, 

 and which changes into electricity and magnetism in 

 the galvanic trough. What are the attractive forces 

 which are associated with electricity and magnetism, 

 and which play so important a part in chemical 

 changes ? Nothing is known about them, and, after 

 all, they may prove to be only varying aspects of 

 that force of attraction which is supposed to be 

 neither electrical, nor magnetical, nor chemical, 

 even the force of gravity. Indeed, so intimate and 

 inseparable is the connexion between these agents, 

 that it is more easy to look upon them as signs of 

 action than as agents. 



Nor is this the only change which is necessitated 

 in this part of the creed of science. It is commonly 

 held that electricity and the companion phenomena 

 are not only agents, but imponderable agents, 

 agents, that is to say, which are quite beyond the 

 scope of physical inquiry. This opinion, However, 

 is as groundless as the other ; for, on careful inquiry, 

 these so-called agents appear to be signs of a certain 



