232 PHYSICAL SCIENCE 



linked to the nucleus by tubes of force. A dis- 

 turbance may throw one of the tubes into a loop 

 which may possibly close into an anchor-ring 

 and be cast out into space attended by a train of 

 electro-magnetic waves, as a quantum of radiation. 

 We shall consider the quantum theory more fully 

 below. It forms the most surprising development 

 of modern physical research. 



From the time of Maxwell onwards, electro- 

 magnetic considerations have formed an essential 

 part of any theory of the aether. It is certain 

 that luminous and electro-magnetic radiations 

 are essentially the same in kind, and only differ 

 in the length of the waves. We, of course, might 

 have ceased to try to represent the properties of 

 the aether by means of any imaginary mechanical 

 model, and, regarding light as a system of electro- 

 magnetic waves, have pushed the inquiry no 

 further, but, besides the difficulty of explaining 

 the facts of radiation at least two considerations 

 prevented our resting content with a mere series 

 of electro-magnetic equations as a final explana- 

 tion. While some phenomena maybe co-ordinated 

 successfully, no conception is thus given of the 

 natui^e of a static electric charge, or of an ordinary 

 electric current, and there seems, on this mode 

 of representation, no means of attacking the 

 problem of the nature of gravitation, which, it 

 was thought, must some day be explained in 

 terms of the universal medium, if that medium 

 was to survive as a permanent conception in 

 physical science. 



Attempts were, therefore, often made to 

 describe ideal models which should represent the 



