ELECTRICAL CONFERENCE AT PHILADELPHIA 631 



the same manner. In obtaining this result Maxwell calculated the 

 forces acting on the medium at every point, and compares these with 

 imaginary stresses in a medium at the given point. Hence, the energy 

 stored up can be represented either as due to the mutual attraction of 

 the electricity at a distance, or to the stresses in the medium at every 

 point, and thus, as Thomson has shown, by a volume integral of the 

 square of the force at every point. Hence, we are at liberty to deny 

 the existence of all action at a distance, and attribute it to the inter- 

 vening medium, which, to be logical, we must assume to be continuous 

 and not molecular in constitution. 



Thomson has pointed out that magnetism must be of the nature of ro- 

 tation, such as possibly vortex motion in a fluid, and Maxwell has done 

 something toward making a mechanical model of such a medium. Thom- 

 son's wonderful address at Montreal has also given us much to think of 

 in the same direction. 



But here we have reached the limit of our science, and even that serv- 

 ant of our reason, imagination, fails us. We are yet unable to picture 

 to ourselves what takes place in a medium subject to electrostatic ac- 

 tion. We are face to face with the great problem of nature, and the 

 questions, What is matter? What is electricity? evoke no answer from 

 the wisest among us. Our mathematics has guided us safely up to a 

 certain point and will guide us still further; science will advance and 

 we shall know more. But, for the present, this is the limit which we 

 have yet attained in this direction. However, the idea of a medium is 

 still serviceable in other portions of our science. 



We have seen that the medium explains the electrical and magnetic 

 attraction of bodies at rest. The question then comes up as to what 

 happens in the medium when these bodies move. Are the imaginary 

 stresses in the medium transmitted from place to place instantaneously 

 or do they require time? Mathematics in the hands of the immortal 

 Maxwell has answered this question, and we now know that any mag- 

 netic or electric distrubance is propagated through space with a velocity 

 equal to the ratio of the electro-magnetic to the electrostatic unit of 

 electricity. This great physical constant has now been found by experi- 

 ment to be equal to the velocity of light, and thus has arisen that great 

 modern theory, Maxwell's electro-magnetic theory of light. Indeed, at 

 the present day, so perfectly does this theory agree with experiment that 

 we can almost regard it as a certainty. The velocity of light and the 

 ratio of the units agree far within the limits of experimental error. The 

 fact that bodies having a true (not electrolytic) electric conduction are 



