368 A CENTURY OF SCIENCE 



logues to illustrate the phenomena of electromagnetism, 

 he was never enthusiastic over the speculations of the 

 advocates of a dynamical ether. The electrodynamic equa- 

 tions provided an accurate representation of the electric 

 and magnetic fields, and beyond that he felt it was need- 

 less to go. That Gibbs (23, 475, 1882) held the same 

 view is made evident by the closing paragraphs of a 

 paper in which he shows that the electromagnetic theory 

 of light accounts in minutest detail for the intricate phe- 

 nomena accompanying the passage of light through cir- 

 cularly polarizing media. He says : 



; "The laws of the propagation of light in plane waves, which 

 have thus been derived from the single hypothesis that the dis- 

 turbance by which light is transmitted consists of solenoidal 

 electrical fluxes, . . . are essentially those which are received 

 as embodying the results of experiment. In no particular, so 

 far as the writer is aware, do they conflict with the results of 

 experiment, or require the aid of auxiliary and forced hypotheses 

 to bring them into harmony therewith. 



In this respect the electromagnetic theory of light stands in 

 marked contrast with that theory in which the properties of an 

 elastic solid are attributed to the ether, a contrast which was 

 very distinct in Maxwell's derivation of Fresnel's laws from 

 electrical principles, but becomes more striking as we follow the 

 subject farther into its details, and take account of the want of 

 absolute homogeneity in the medium, so as to embrace the 

 phenomena of the dispersion of colors and circular and elliptical 

 polarization. ' ' 



Further Dynamical Theories. Kelvin, however, was 

 not satisfied with this type of ether. To him dynamics 

 was the foundation of all physical phenomena, and noth- 

 ing could be said to be explained until a mechanical model 

 was provided. So he returned to the elastic solid theory, 

 and developed the consequences of the assumption, 

 already made use of by Cauchy, that the ether has a nega- 

 tive volume elasticity of such a value as to make the 

 velocity of the compressional wave zero. In order to 

 prevent such an ether from collapsing it is necessary to 

 assume that it is rigidly attached at its boundaries and 

 that cavities cannot be formed at any point in its interior. 

 Now Gibbs (37, 129, 1889) has pointed out the remark- 



