on the Mechanical Function of an JEther. 423 



by mopping up part of the energy of cells or regions of fresh 

 aether over which it pcasses ; a process which delivers the aether 

 from stress without involving it in any yielding motion, and 

 without relaxing the stress in contiguous regions. That such 

 must he the kind of process in aether we know, because the 

 stress between attracting bodies is by no means lessened by 

 their approach, as it is when a crude complex substance like 

 indiarubber is the agent ; on the contrary, a body may be 

 urged with increasing speed into regions of greater and greater 

 intrinsic energy ; nevertheless the stress in the region behind 

 it has decreased, and therefore the potential energy as a whole 

 is diminished by the motion *. (2) If the body's speed is de- 

 creasing, it is losing kinetic and the aether is gaining potential 

 energy (by means of the stress caused and left in the aether it is 

 sweeping over or through) . In both these cases an unbalanced 

 force is acting on the boundary of the energetic aether and the 

 matter, and the activity is Ft', being the rate at which the 

 body gains or loses kinetic energy. (3) If its speed is con- 

 stant, the matter is merely transmitting a stress which is the 

 same in front and behind it ; in that case it is acted on by 

 balanced forces, the aetherial energy passes fore and aft through 

 the atom, assuming the kinetic form for the moment, and is 

 left behind unchanged. 



Referring to the phrase " unbalanced force " used above, 

 the force is only unbalanced with regard to the single piece 

 of matter under consideration, the reaction-force is exerted on 

 another piece of matter — the reacting body — at the other end 

 of the stress. The quiescent connecting medium, the vehicle of 

 the stress, sustains no part of the reaction ; it merely transmits 

 a static stress, and only at its boundaries is there any motion. 



(There is nothing here to prevent aetherial energy from 

 being ultimately hydrodynamically kinetic, but considered 

 from the practical material point of view aetherial energy is 

 what it has been agreed to call " potential ; " i. e. it displays 

 itself as a force to which matter can yield, rather than 

 as a motion which something in space can resist.) 



But now return to the consideration of Maxwell's stress 

 due to radiation. It exists in any region filled with waves of 

 any kind, and acts in the direction of the rays on the bodies 

 which bound that region at either end. It repels a target 

 upon which light falls, and it repels the source whence the 

 light originates ; it is a definite stress producing on perfectly 

 opaque bodies a normal pressure equal to the energy of the 

 radiation per unit volume of the space filled with it. So long- 

 as the light extends all the way from source to sink there is 

 nothing exceptional about this stress, and the third law is 

 * As an illustration of a field of energy bsing discharged without 

 normal yield we may think of a sharp blade moving through a forest of 

 stretched elastics. 



