ACTION AT A DISTANCE. 321 



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 pregnant discoveries, that of the magnetic 

 rotation of polarised light, enables us to proceed a step farther. The phe- 

 nomenon, when analysed into its simplest elements, may be described thus : 

 Of two circularly polarised rays of light, precisely similar in configuration, but 

 rotating in opposite directions, that ray is propagated with the greater velocity 

 which rotates in the same direction as the electricity of the magnetizing 

 current. 



It follows from this, as Sir W. Thomson has shewn 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 mag- 

 netic 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 which 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 progagation 

 of small magnetic disturbances in the supposed electro-magnetic medium. 



This velocity is very great, from 288 to 314 millions of metres per second, 

 according to different experiments. Now the velocity of light, according to 



VOL. II. 41 



