262 CONNECTION OF ELECTRICITY AND MAGNETISM. 



a plienomeuoii of the same kiud as that action at a distance whicli is 

 exerted by means of the tension of ropes and the pressure of rods. 

 When the muscles of oar bodies are excited by that stimulus which we 

 are able in some unknown way to apply to them, the fibers tend to 

 shorten themselv^es and at the same time to expand laterally. A state 

 of stress is produced in the muscle, and the limb moves. This explana- 

 tion of muscular action is by no means comi^lete. It gives no account 

 of the cause of the excitement of the state of stress, nor does it even 

 investigate those forces of cohesion which enable the muscles to support 

 this stress. Nevertheless, the simple fact that it substitutes a kind of 

 action which extends continuously along a material substance for one 

 of which we know only a cause and an effect at a distance from each 

 other, induces us to accept it as a real addition to our knowledge of 

 animal mechanics. 



For similar reasons we may regard Faraday's conception of a state 

 of stress in the electro-magnetic field as a method of explaining action 

 at a distance by means of the continuous transmission of force, even 

 though we do not know how the state of stress is produced. 



But one of Faraday's most x)regnant discoveries, that of the magnetic 

 rotation of polarized light, enables us to ijroceed a step farther. The 

 phenomenon, when analyzed into its simplest elements, may be described 

 thus: Of two circularly-polarized rays of light, precisely similar 4n con- 

 figuration, but rotating in opposite directions, that ray is propagated 

 with the greater velocity which rotates in the same direction as the elec- 

 tricity of the magnetizing current. 



It follows from this, as Sir W. Thomson has shown by strict dynamical 

 reasoning, that the medium, when under the action of magnetic force, 

 must be in a state of rotation ; that is to say, that small portions of the 

 medium, which we may call molecular vortices, are rotating each on its 

 own axis, the direction of this axis being that of the magnetic force. 



Here, then, we have an explanation of the tendency of the lines of 

 magnetic force to spread out laterally and to shorten themselves. It 

 arises from the centrifugal force of the molecular vortices. 



The mode in which electromotive force acts in starting and stopping 

 the vortices is more abstruse, though it is, of course, consistent with 

 dynamical principles. 



We have thus found that there are several different kinds of work to 

 be done by the electro-magnetic medium if it exists. We have also seen 

 that magnetism has an intimate relation to light, and we know that 

 there is a theory of light w^hich supposes it to consist of the vibrations 

 of a medium. How is this luminiferous medium related to our electro- 

 magnetic medium? 



It fortunately happens that electro-magnetic measurements have been 

 made, from which we can calculate, by dynamical principles, the velocity 

 of propagation of small magnetic disturbances in the supposed electro- 

 magnetic medium. 



