THE STORY OF NINETEENTH-CENTURY SCIENCE 



have shown that a condition of electrical strain may be 

 developed into a wave system by recurrent interruptions 

 of the electric state in the generator, and that such 

 waves travel through the ether with the rapidity of 

 light. Since then the electro-magnetic theory of light 

 has been enthusiastically referred to as the greatest gen- 

 eralization of the century ; but the sober thinker must 

 see that it is really only what Hertz himself called it- 

 one pier beneath the great arch of conservation. It is 

 an interesting detail of the architecture, but the part 

 cannot equal the size of the whole. 



More than that, this particular pier is as yet by no 

 means a very firm one. It has, indeed, been demon- 

 strated that waves of electro-magnetism pass through 

 space with the speed of light, but as yet no one has de- 

 veloped electric waves even remotely approximating the 

 shortness of the visual rays. The most that can posi- 

 tively be asserted, therefore, is that all the known forms 

 of radiant energy heat, light, electro - magnetism- 

 travel through space at the same rate of speed, and con- 

 sist of traverse vibrations " lateral quivers," as Fresnel 

 said of light known to differ in length, and not posi- 

 tively known to differ otherwise. It has, indeed, been 

 suggested that the newest form of radiant energy, the 

 famous X ray of Professor Rontgen's discovery, is a 

 longitudinal vibration, but this is a mere surmise. Be 

 that as it may, there is no one now to question that all 

 forms of radiant energy, whatever their exact affinities, 

 consist essentially of undulatory motions of one uniform 

 medium. 



A full century of experiment, calculation, and con- 

 troversy has thus sufficed to correlate the " impondera- 

 ble fluids " of our forebears, and reduce them all to man- 



