THE CONSERVATION OF ENERGY 



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 

 positively be asserted, therefore, is that all the known 

 forms of radiant energy heat, light, electro-magnet- 

 ism travel through space at the same rate of speed, 

 and consist of traverse vibrations ''lateral quivers," 

 as Fresnel said of light known to differ in length, 

 and not positively known to differ otherwise. It has, 

 indeed, been suggested that the newest form of radiant 

 energy, the famous X-ray of Professor Roentgen's dis- 

 ry, 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 "imponder- 

 able fluids" of our forebears, and reduce them all to 

 manifestations of motion among particles of matter. 

 At first glimpse that seems an enormous change of 

 view. And yet, when closely considered, that change 

 in thought is not so radical as the change in phrase 

 might seem to imply. For the nineteenth - century 

 physicist, in displacing the "imponderable fluids" of 

 many kinds one each for light, heat, electricity, mag- 

 ^m has been obliged to substitute for them one 

 all-pervading fluid, whose various quivers, waves, rip- 

 ples, whirls or strains produce the manifestations 

 h in popular parlance an> termed forms of force. 

 This all -pervading fluid the physicist terms the ether, 

 and he thinks of it as having no weight. In effect, 



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