356 



ELEMENTARY LESSONS ON [CHAP. ix. 



analogy is an important point in the theory, and 

 immediately suggests the question whether the respective 

 rates of propagation are the same. Now the velocity 

 of propagation of electromagnetic induction is that 

 velocity "77" which was shown (Art. 365) to represent 

 the ratio between the electrostatic and the electro- 

 magnetic units, and which (in air) is believed to be 



2-9857 x io 10 centimetres per second. 

 And the velocity of light (in air) has been repeatedly 

 measured (by Fizeau, Cornu, Michelson, and others) 

 giving as the approximate value 



2-9992 x io 10 centimetres per second. 

 The close agreement of these figures is at least re- 

 markable. Amongst other mathematical deductions 

 from the theory may be mentioned the following : (i.) 

 all true conductors of electricity must be opaque 1 to light; 

 (ii.) for transparent media the specific inductive capacity 

 ought to be equal to the square of the index of refraction. 

 Experiments by Gordon, Boltzmann, and others, show 

 this to be approximately true for waves of very great 

 wave-length. The values are shown below. For gases 

 the agreement is even closer. 



1 The author of these Lessons has found that in some crystalline bodies 

 which conduct electricity better in one direction than in another, the opacity 

 to light differs correspondingly. Coloured crystals of Tourmaline conduct 

 electricity better across the long axis of the crystal than along that axis. 

 Such crystals are much more opaque to light passing along the axis than 

 to light passing across it. And, in the case of rays traversing the crystal 

 across the axis, the vibrations across the axis are more completely absorbed 

 than those parallel to the axis : whence it follows that the transmitted light 

 will be polarized. 



