1893.] the Electric and Lumimferous Medium. 439 



planation, by comparison of the intangible primordial medium with 

 other dynamical systems of which we can directly observe the 

 phenomena. 



The chief representative of exact physical speculation of the 

 second of these types has been Lord Kelvin. In the older attempts 

 of this kind the dynamical basis of theories of the constitution of the 

 aether consisted usually in a play of forces, acting at a distance, 

 between ultimate elements of molecules of the medium ; from this we 

 must, however, except the speculations of Greek philosophy and the 

 continuous vortical theories of the school of Descartes, which were 

 of necessity purely descriptive and imaginative, not built in a con- 

 nected manner on any rational foundation. It has been in particular 

 the aim of Lord Kelvin to deduce material phenomena from the play 

 of inertia involved in the motion of a structureless primordial fluid ; 

 if this were achieved it would reduce the duality, rather the many- 

 sidedness, of physical phenomena to a simple unity of scheme ; it 

 would be the ultimate conceivable simplification. The celebrated 

 vortex theory of matter makes the indestructible material atoms 

 consist in vortex rings in a primordial fluid medium, structureless, 

 homogeneous, and frictionless, and makes the forces between the 

 atoms which form the groundwork of less fundamental theories 

 consist in the actions excited by these vortices on one another 

 through the inertia of the fluid which is their basis actions which are 

 instantaneously transmitted if the fluid is supposed to be absolutely 

 incompressible. 



In case this foundation proves insufficient, there is another idea of 

 Lord Kelvin's by which it may be supplemented. The characteristic 

 properties of radiation, which forms so prominent an element in 

 actual phenomena, can be explained by the existence of an elastic 

 medium for its transmission at a finite, though very great, speed ; 

 such a medium renders an excellent account of all its relations, if we 

 assume it to possess inertia and to be endowed with some elastic 

 quality of resistance to disturbance roughly analogous to what we can 

 observe and study in ordinary elastic solids of the relatively incom- 

 pressible kind, such as india-rubber and jellies. Lord Kelvin has been 

 the promoter and developer of a view by which the elastic forces 

 between parts of such a medium may be to some extent got rid of as 

 ultimate elements, and be explained by the inertia of a spinning 

 motion of a dynamically permanent kind, which is distributed 

 throughout its volume. If we imagine very minute rapidly- spinning 

 fly-wheels or gyrostats spread through the medium, they will retain 

 their motion for ever, in the absence of friction on their axles, and 

 they will thus form a concrete dynamical illustration of a type of 

 elasticity which arises solely from inertia ; and this illustration will 

 be of great use in realising some of the peculiarities of a related 



