that this takes place without practically any friction. The truth of the matter is that whether we call it a perfect gas, an incompressible fluid, a jelly, or a solid possessing perfect elasticity, we are tying ourselves down to a material ether, and the acceptance of any one of these conceptions ends in a reductio ad absurdum.
As for dispersion, if it be assumed that light is given forth from the entire surface of the sun equally in all directions, the lateral pressure of dispersion at each spherical zone, and at any distance—if it exists—would be equal, and there would be perfect equilibrium between adjacent cones of light; the bases of these cones are equipotential surfaces and no lateral dispersion could be rationally admitted. When, however, the light waves strike an object such as the earth, a shadow is formed. Leaving out the refraction in our mantling atmosphere to which we owe our twilights, the shadow which the earth casts is practically absolute for at least the distance from the earth to the moon (240,000 miles); and it is therefore evident that the medium itself is of such a nature that it will transmit transversal vibrations without any appreciable dispersion. If dispersion in space were serious the light of the stars would be shown as a haze and not as individual points,
The theory of a solid or, to speak more accurately, a rigid ether does not, as we shall see later, appear to be a necessity, and it presents the great weakness of compelling us to rack our common-sense to try and explain the passage of bodies through it, from the lightest comet to the most massive star. Rigidity of rotation was first proposed by McCullagh and its nature will be considered more fully when the subject of vortex-atoms is reached, but, however plausible it may be for material atoms—and it is eminently so—it seems to be a superfluous hypothesis for a non-atomic ether. Rigidity and elasticity of rotation can be compared to the gyroscope which resists deflection and yields elastically, although it is not itself elastic, nor immersed in a medium which could be considered elastic when the gyroscope is at rest. This is an elasticity of motion, not of matter; it must, however, be remembered that, as Lord Kelvin has pointed out, elasticity itself may be but another mode of motion. With elasticity of rotation, one might have practically a fluid ether possessing high elasticity with its oscillatory power, instead of the viscosity of ordinary fluids, with its dispersive quality.
Perfect elasticity by no means implies a solid or semi-solid state; an atomic structure presents elasticity of volume, but equilibrium in a homogeneous, non-continuous medium, regardless of the spaces which may exist between the component incompressible corpuscles, will supply rigidity and elasticity of shape. Pressure in space does not imply elasticity. If elasticity is a rotational effect and pressure one of bombardment, they are not necessarily interdependent. The fact that there