THE ETHER OF SPACE 297 



matter moves, in the sense that it has the property of loco- 

 motion and can effect impact and bombardment; while 

 ether is strained, and has the property of exerting stress 

 and recoil. All potential energy exists in the ether. It 

 may vibrate, and it may rotate, but as regards locomotion 

 it is stationary the most stationary body we know abso- 

 lutely stationary, so to speak; our standard of rest. 



All that we ourselves can effect, in the material universe, 

 is to alter the motion and configuration of masses of matter ; 

 we can move matter, by our muscles, and that is all we can 

 do directly: everything else is indirect. This is worth 

 thinking over by those who have not already realized the 

 fact. 



But now comes the question, how is it possible for matter 

 to be composed of ether? How is it possible for a solid to 

 be made out of fluid ? A solid possesses the properties of 

 rigidity, impenetrability, elasticity, and such like ; how can 

 these be imitated by a perfect fluid such as the ether must 

 be? The answer is, they can be imitated by a fluid in 

 motion; a statement which we make with confidence as 

 the result of a great part of Lord Kelvin's work. 



It may be illustrated by a few experiments. 



A wheel of spokes, transparent or permeable to matter 

 when stationary, becomes opaque when revolving, so that a 

 ball thrown against it does not go through, but rebounds. 

 The motion only affects permeability to matter; trans- 

 parency to light is unaffected. 



A silk cord hanging from a pulley becomes rigid and 

 viscous when put into rapid motion ; and pulses or waves 

 which may be generated on the cord travel along it with 

 a speed equal to its own velocity, whatever that velocity 

 may be, so that they appear very nearly to stand still. 

 This is a case of kinetic rigidity; and the fact that the 

 wave transmission velocity is equal to the rotatory speed of 

 the material, is typical and important, for in all cases of 

 kinetic elasticity these two velocities are of the same order 

 of magnitude. 



