The Three Secretaries 149 



writes Preece, "have definitely and conclusively proved the 

 existence of one medium throughout all space, called the 

 ether, through which waves of energy, called radiations, are 

 propagated with the same velocity, but in different forms and 

 with different frequencies, although all of the same charac- 

 ter. At one end of the scale we have actinic disturbances 

 producing photographic impressions ; at the other end of the 

 scale electric waves producing electro-magnetic disturbances, 

 while the intermediate radiations give light and heat." 1 



Compare now the summary of present opinions just quoted, 

 omitting only the words within the brackets (which I have 

 myself added), with what Henry wrote nearly half a century 

 before : 



"We cannot avoid the conclusion [that] all the phenomena 

 of the imponderables result from the different actions of one 

 all-pervading principle. . . . An iron rod, rapidly hammered, 

 becomes red hot, or, in other words, emits heat and light. 

 The same rod, insulated by a non-conductor, exhibits electri- 

 cal attraction and repulsion. Again, if this rod be struck with 

 a hammer while in a vertical position, it becomes magnetic. 

 We have here the evolution of the four classes of phenomena 

 by a simple agitation of the atoms. We cannot, in accor- 

 dance with the known simplicity of the operations of nature, 

 for a moment imagine that these different results are to be 

 referred to as many different and independent principles." 2 



So far as theory goes, it would seem that Clerk Maxwell's 

 proposition in 1865, that light is an electro-magnetic distur- 

 bance, was simply a variation of the previous proposition of 

 Henry, and that Henry's utterance was an indication of the 

 deep insight into the inevitable future course of experimental 

 research in this direction. The facts brought out by Max- 



1 " Electric Signalling without Wires," 2 Proceedings of the American Association 



Journal of the Society of Arts, Volume XLH, for the Advancement of Science, 1851, Vol- 

 pages 274, 275, February 23, 1894. ume n, pages 84-91. 



