156 WHAT IS SCIENCE? 



an electric current. For these symbols Maxwell sub- 



dX dY dZ 



stituted ; that substitution was roughly 

 dt dt dt 



equivalent to saying that an electric current was related 

 to the things represented by X, Y, Z, t (never mind what 

 they are) in a way nobody had ever thought of before ; 

 it was equivalent to saying that so long as X, Y, Z, t were 

 related in a certain way, there might be an electric 

 current in circumstances in which nobody had believed 

 that an electric current could flow. As a matter of 

 fact, such a current would be one flowing in an absolutely 

 empty space without any material conductor along 

 which it might flow, and such a current was previously 

 thought to be impossible. But Maxwell's feeling for 

 symbolism suggested to him that there might be such a 

 current, and when he worked out the consequences of 

 supposing that there were such currents (not currents 

 perceptible in the ordinary way, but theoretical currents, 

 as molecules are theoretical hard particles), he arrived 

 at the unexpected result that an alteration in an electric 

 current in one place would be reproduced at another 

 far distant from it by waves travelling from one to the 

 other through absolutely empty space between. Hertz 

 actually produced and detected such waves ; and 

 Marconi made them a commercial article. 



That is the best attempt I can make at explaining the 

 matter. It is one more illustration of the marvellous 

 power of pure thought, aiming only at the satisfaction 

 of intellectual desires, to control the external world. 

 Since Maxwell's time, there have been many equally 

 wonderful theories, the form of which is suggested by 

 nothing but the mathematician's sense for symbols. The 

 latest are those of Sommerfeld, based the ideas of Niels 

 Bohr, and of Einstein. Every one has heard of the latter, 

 but the former (which concerns the constitution of the 



