ELECTROMAGNETIC THEORY OF LIGHT. 609 



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M. Lorenz has shown that by adding, to the equations given by 

 Kirchhoff for electrical currents, suitably chosen terms, which do 

 not at all affect any experimental conclusion, we get a new series 

 of equations which indicate an action from layer to layer in the 

 medium, and a phenomenon of undulation travelling with the 

 velocity of light. He thus arrives at results similar to those which 

 Maxwell had deduced from an entirely different theory. 



M. Edlund has attempted to show that electrical phenomena, 

 both statical and dynamical, may be explained by the aid of a single 

 fluid, which in all probability is nothing but the ether. 



M. Edlund assumes that all bodies in the neutral state contain a 

 normal quantity of ether, and that a positive or negative electrifi- 

 cation corresponds to a share of ether, greater or less than that 

 of the normal charge. It is easy to deduce from this that the 

 action of two bodies is proportional to the excess of their 

 respective charges over the normal charges. 



An electrical current is then only a transport of ether in a given 

 direction ; if we assume that the action of the two masses only 

 depends on their velocity and on their relative acceleration along 

 the right line joining them, then by reasoning analogous to that of 

 Weber, and determining certain coefficients by the identification of 

 the formulae with the results of experiment, we arrive at an expla- 

 nation of Ampere's laws and the phenomena of induction. 



All the preceding theories imply the existence of an intermediate 

 medium ; for if any mechanical effect, force, or potential, is trans- 

 mitted with a finite velocity from one particle to another, it follows 

 that a medium of a suitable structure must have been the seat of this 

 action while this effect had quitted the first particle and had not 

 yet reached the second. Maxwell has taken the properties of this 

 medium into account, and has thus established remarkable numerical 

 relations between the phenomena of electricity and of light, which 

 are supported by experiment. 



627. ELECTROMAGNETIC THEORY OF LIGHT. We have seen 

 on several occasions how favourable the various phenomena of 

 electricity and magnetism are to Faraday's conception, which consists 

 in giving up the idea of actions at a distance, and considering forces 

 as transmitted by the elastic reaction of an intermediate medium. 

 This is a hypothesis which at the present day forms the basis of the 

 physical theory of light, but it would be contrary to the spirit of 

 science to assume that there are as many different media as there are 

 phenomena to explain, as was formerly done by the distinct hypotheses 

 of calorific fluid, of electrical fluids, and of magnetic fluids. 



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