102 Mr. J. Lirmor on Wiener's Localization of the 



the phenomena of electricity as well as those of light*, and 

 in such an account is founded one of its chief claims. 



A development of the electric theory has recently been 

 essayed by von Helmholtzt, on the basis of the formal equa- 

 tions of Heaviside and Hertz, in which the free aether is still 

 supposed to be an elastic medium of excessively small density 

 in which the dense atoms ore imbedded. If such a view 

 should turn out to be the basis of a consistent body of theory, 

 the considerations given above with respect to the intensities 

 of molecular tractions would have a bearing on it also. 



Let ns now consider more particularly the explanation that 

 would be offered by the electric theory of light. The difference 

 between a material medium and a vacuum consists in an altered 

 effective dielectric coefficient. This difference is simply and 

 naturally explained by the hypothesis that the material mole- 

 cules are polar owing to their associated atoms having atomic 

 charges equal in amount but opposite in sign, and that they 

 therefore possess electric moments just as the molecules of a 

 magnet possess magnetic moments. An electric force thus 

 tends to pull the two constituents of a molecule asunder ; and 

 its full intensity is exerted in this manner, not merely its dif- 

 ferential intensity over the range of the molecular volume. But 

 a magnetic force has no such tendency even when Ave take the 

 molecule to be magnetically polarized, because the two poles 

 of a magnetic element cannot be dissociated from each other ; 

 the magnetic moment is thus directly associated with the 

 atom, not with the molecule. Jn the case of the stationary 

 light-waves the antinodes of the electric force are therefore 

 places where alternating disturbances of a kind suitable to 

 produce decomposition of the molecules are maintained, and 

 may produce strong effects through sympathetic molecular 

 vibration or otherwise ; but at the intermediate antinodes of 

 the magnetic force the individual ultimate atoms may be 

 disturbed by the alternating magnetic force, but there is no 

 tendency to separation of the constituents of the molecule. 

 On the electric theory, therefore, there is abundant justifica- 

 tion both for the magnitude of the effect produced, and for 

 its localization as determined by Wiener's experimental in- 

 vestigation. 



The theory, noticed first it seems by Weber, which ascribes 

 molecular magnetism to the orbital rotation round each other 

 of ionic charges, and which has \evy strong recommendations 

 from the point of view of the dynamics of the aether, may 

 form a partial exception to this statement. It leaves the 



* Cf. "A Dynamical Theory . . . ," Phil. Trans. 1834, §§ 122-124. 

 t "Wied. Ann. 1&94. 



