578 THE POPULAR SCIENCE MONTHLY 



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 concep- 

 tions ends in a reductio ad ahsurdum. 



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 pres- 

 sure 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 sur- 

 faces 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 Mc- 

 Cullagh 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 remem- 

 bered 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 bom- 

 bardment, they are not necessarily interdependent. The fact that there 



