153,154] The State of the Medium in the Electrostatic Field 139 



action through a medium. The new conductor is a collection of positive and negative 

 charges : the phenomena under question are produced by these charges being acted upon 

 by the other charges in the field, but whether this action is action at a distance or action 

 through a medium cannot be told. 



Indeed, it will be seen that, viewed in the light of the electron-theory and of Faraday's 

 theory of dielectric polarisation, electrical action stands on just the same level as 

 gravitational action. In each case the system of forces to be explained may be regarded 

 as a system of forces between indestructible centres, whether of electricity or of matter, 

 and the law of force is the law of the inverse square, independently of the state of the 

 space between the centres. And although scientists may be said to be agreed that 

 gravitational action, as well as electrical action, is in point of fact propagated through 

 a medium, yet a consideration of the case of gravitational forces will shew that there is 

 no obvious a priori argument which can be used to disprove action at a distance. 



Failing an a priori argument, an attempt may be made to disprove action at a distance, 

 or rather to make it improbable, by an appeal to experience. It may be argued that as 

 all the forces of which we have experience in every-day life are forces between substances 

 in contact, therefore it follows by analogy that forces of gravitation, electricity and 

 magnetism, must ultimately reduce to forces between substances in contact i.e. must be 

 transmitted through a medium. Upon analysis, however, it will be seen that this argument 

 divides all forces into two classes : 



(a) Forces of gravitation, electricity and magnetism, which appear to act at a 

 distance. 



(/3) Forces of pressure and impact between solid bodies, hydrostatic pressure, etc. 

 which appear to act through a medium. 



The argument is now seen to be that because class (/3) appear to act through a medium, 

 therefore class (a) must in reality act through a medium. The argument could, with equal 

 logical force, be used in the exactly opposite direction : indeed it has been so used by the 

 followers of Boscovitch. The Newtonian discovery of gravitation, and of apparent action 

 at a distance, so occupied the attention of scientists at the time of Boscovitch that it 

 seemed natural to regard action at a distance as the ultimate basis of force, and to 

 try to interpret action through a medium in terms of action at a distance. The reversion 

 from this view came, as has been said, with Faraday. 



Hertz's subsequent discovery of the finite velocity of propagation of electric action, 

 which had previously been predicted by Maxwell's theory, came to the support of Faraday's 

 view. To see exactly what is meant by this finite velocity of propagation, let us imagine 

 that we place two uncharged conductors A, B at a distance r from one another. By 

 charging J, and so performing work at A, we can induce charges on conductor B, and 

 when this has been done, there will be an attraction between conductors A and B. We 

 can suppose that conductor A is held fast, and that conductor B is allowed to move 

 towards A, work being performed by the attraction from conductor A. We are now 

 recovering from B work which was originally performed at A. The experiments of Hertz 

 shew that a finite time is required before any of the work spent at A becomes available 

 at B. A natural explanation is to suppose that work spent on A assumes the form of 

 energy which spreads itself out through the whole of space, and that the finite time 

 observed before energy becomes available at B is the time required for the first part of 

 the advancing energy to travel from A to B. This explanation involves regarding energy 

 as a definite physical entity, capable of being localised in space. It ought to be noticed 

 that our senses give us no knowledge of energy as a physical entity : we experience force, 

 not energy. And the fact that energy appears to be propagated through space with finite 



