AND MODEUK PHYSICS. 219 



of space becomes determinate, and the forces acting 

 on the conductors can be obtained. Moreover, the 

 total change of displacement on entering or leaving 

 a conductor can be calculated, and this gives the 

 quantity which is known as the total electrical charge 

 on the conductor. The forces obtained by the above 

 method are exactly the same as those which would 

 exist if we supposed each conductor to be charged in 

 the ordinary sense with the quantities just found, and 

 to attract or repel according to the ordinary laws. 



If, then, we define electric displacement as that 

 change which takes place in a dielectric when it 

 becomes the seat of electrostatic energy, and if, 

 further, we suppose that the change, whatever it be 

 mechanically, satisfies certain well-known laws, and 

 that in consequence certain pressures and tensions 

 exist in the dielectric, electrostatic problems can be 

 solved without reference to a charge of electricity 

 residing on the conductors. 



Something such as this, it appears to me, is Max- 

 well's theory of electricity as applied to electrostatics. 

 It is not necessary, in order to understand it, to know 

 what change in the ether constitutes electric displace- 

 ment, or what is an electric charge, though, of course, 

 such knowledge would render our views more definite, 

 and would make the theory a mechanical one. 



When we turn to magnetism and electro-mag- 

 netism, Maxwell's theory develops itself naturally. 

 Experiment proves that magnetic induction is con- 

 nected with the rate -of change of electric displace- 

 ment, according to the laws already given. If, then, 

 we knew the nature of the change to which the name 



