L. Page — A Century's Progress in Physics. 335 



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- 

 able fact that the equations describing the motion of 

 Kelvin's quasi-labile ether are of exactly the same form 

 as the electromagnetic equations. Electric displacement 

 is represented by an actual displacement of the ether, 

 magnetic intensity by a rotation. Hence everything 

 which can be explained by the electrodynamic equations 

 finds an analogue in terms of Kelvin's ether. Still 

 another type of dynamic ether which fits the known facts 

 was proposed by McCullagh and perfected by Larmor. 

 In this ether a rotational elasticity is premised, such as 

 would exist if each particle of the medium consisted of 

 three rigidly connected gyrostats with mutually perpen- 

 dicular axes. In this ether electrical displacements cor- 

 respond to rotations, and magnetic strains to etherial dis- 

 placement. 



A Neiv Point of View. — While the dynamical school 

 was still dominant in England, another point of view 

 was developing on the continent. Kirchhoff denied 

 that it was the province of science to provide mechanical 

 explanations of the ether and electrodynamic phenomena 

 such as Kelvin conceived to be necessary in order to make 

 these- phenomena intelligible. Kirchhoff 's contention 

 was that the object of science is purely descriptive, — 

 phenomena must be observed, classified, and mutual con- 

 nections described by the fewest number of differential 

 equations possible. Mach expressed the same idea 

 somewhat more concisely when he asserted that the aim 

 of science is "economy of thought." For instance, in 



