247, 248] ELECTROMAGNETIC WAVES. 517 



determinations of the velocity of light, so that many German 

 authors are accustomed to speak of A as the reciprocal of the 

 velocity of light. It seems preferable, however, to keep the 

 definition of A and v purely electrical, as we have given it in 212. 



A further confirmation of the electromagnetic theory of light 

 was sought in the fact that the index of refraction, being inversely 

 proportional to the velocity, should in non-magnetic bodies, for 

 which ft = l, be proportional to the square root of the electric 

 inductivity. This relation was experimentally verified for a 

 sufficient number of transparent dielectrics to make it appear that 

 the agreement was not accidental, although many exceptions were 

 found. 



Nevertheless, although these considerations made the electro- 

 magnetic nature of light very probable, the theory of propagation 

 of actual electrical disturbances with finite velocity remained 

 unverified by experiment until 1887, when Hertz began the 

 publication of his remarkable researches*, which have since carried 

 conviction of the truth of Maxwell's theory of electricity and 

 magnetism to the most conservative parts of the scientific world. 

 For an account of them the reader is referred to Hertz's collected 

 papers on "Die Ausbreitung der elektrischen Kraft," or to the 

 English translation by D. E. Jones. 



248. Transfer of Energy. Poyn ting's Theorem. We 



shall now form the equation of activity for any portion T of the 

 field. If E= W+ T be the total energy, H the dissipativity, we 

 have 



(i) + H = ~ t 



+ HJ(Xu +Yv + Zw) dr. 

 Since e and JJL do not vary with the time we have, by (C), (D), 



9* i Y^3P 



-*- +x 



* "Ueber die Ausbreitungsgeschwindigkeit der elektrodynamisohen Wirkun- 

 gen." Wied. Ann. 34, p. 551, 1888, trans, p. 107. 



