58 BEPORT — 1891. 



The object of the paper is to draw attention to the advan- 

 tages that are to be derived, especially by the student, from 

 taking as hypothesis, not, as is usual, the law of the inverse 

 square, but the existence of this elastic medium. The hypo- 

 thesis may not be exactly true, but it is as nearly true, and in 

 the same manner true, as that a wave of sound may be repre- 

 sented by a wavy line. We have as much right to deduce 

 theorems of electrostatics from consideration of the behaviour 

 of this medium under stress, as to deduce the law of inter- 

 ference of sound from the summation of waves upon a diagram. 

 In both cases the whole truth is not presented, but sufficient 

 to insure the accuracy of the deduction. 



Suppose, then, with Dr. Lodge, the existence of an incom- 

 pressible perfect fluid filling all space and all bodies. Suppose 

 that in some bodies — silver, copper, and conductors generally 

 — the fluid finds exceedingly little opposition to motion, but 

 that in others — air, glass, silk, and insulators generally — the 

 particles of the body are so entangled or embedded in the 

 fluid that the fluid cannot move without carrying along the 

 particles with it, and that whenever displacement of the body 

 takes place there is called into play a force of restitution pro- 

 portional to the displacement, to the amount of the moved 

 particles, and to a quantity depending on the nature of the 

 body. This last quantity is Maxwell's coefficient of electric 

 elasticity. 



In making these suppositions we are not laying upon that 

 maid-of-all-work the sether any burden other than those she 

 has for other purposes been already taught to bear. 



Further, suppose that a charge of positive electricity is 

 really an increase of the amount of fluid^ — eether — -contained by 

 a body, a negative charge the reverse of this. 



If we make these suppositions it is easy to show that 

 phenomena must occur similar to those ordinarily ascribed to 

 the attractions and repulsions of electrified particles. 



1. Since the aether is incompressible and fills all space, if 

 aether be pumped into one of the first class of bodies (conductors), 

 it must, as Dr. Lodge points out, be drawn from somewhere 

 else, and, since it cannot be got out of bodies of the second class 

 (dielectrics), it must be drawn from bodies of the first class. 

 In other words, if the dielectric surrounding some conductor 

 be pushed back to any extent, it will encroach to an exactly 

 equal extent on some other conductor or conductors. In ordi- 

 nary language, if any electrical charge be imparted to a con- 

 ductor, an equal and opposite charge will be induced on some 

 other conductor or conductors. 



2. Let us consider the strain phenomena resulting from the 

 simplest case of " charging," i.e., pumping in or drawing out 



