. . , ATTRACTION. 



hewed that the transmission of electric and magnetic forces is accompanied by 

 l r1T | occurring in every part of the intervening medium. He traced the 

 Unas of tmse through the medium; and he ascribed to them a tendency to 

 ^^ m themselves and to separate from their neighbours, thus introducing the 

 idea of ftnm in the medium in a different form from that suggested by Newton ; 

 for. whereas Newton's stress was a hydrostatic pressure in every direction, 

 Faradav's is a tension along the lines of force, combined with a pressure in 

 all normal directions. By shewing that the plane of polarisation of a ray of 

 light passing through a transparent medium in the direction of the magnetic 

 force is made to rotate, Faraday not only demonstrated the action of magnetism 

 on light, but by using light to reveal the state of magnetisation of the medium, 

 he " illuminated," to use his own phrase, " the lines of magnetic force." 



From this phenomenon Thomson afterwards proved, by strict dynamical 

 reasoning, that the transmission of magnetic force is associated with a rotatory 

 motion of the small parts of the medium. He shewed, at the same time, how 

 the centrifugal force due to this motion would account for magnetic attraction. 



A theory of this kind is worked out in greater detail in Clerk Maxwell's 

 Treatise on Electricity and Magnetism. It is there shewn that, if we assume 

 that the medium is in a state of stress, consisting of tension along the lines 

 of force and pressure in all directions at right angles to the lines of force, 

 the tension and the pressure being equal in numerical value and proportional 

 to the square of the intensity of the field at the given point, the observed 

 electrostatic and electromagnetic forces will be completely accounted for. 



The next step is to account for this state of stress in the medium. In 

 the case of electromagnetic force we avail ourselves of Thomson's deduction 

 from Faraday's discovery stated above. We assume that the small parts of the 

 medium are rotating about axes parallel to the lines of force. The centrifugal 

 force due to this rotation produces the excess of pressure perpendicular to the 

 lines of force. The explanation of electrostatic stress is less satisfactory, but 

 there can be no doubt that a path is now open by which we may trace to 

 the action of a medium all forces which, like the electric and magnetic forces, 

 vary inversely as the square of the distance, and are attractive between bodies 

 of different names, and repulsive between bodies of the same names. 



The force of gravitation is also inversely as the square of the distance, 

 but it differs from the electric and magnetic forces in this respect, that the bodies 

 between which it acts cannot be divided into two opposite kinds, one positive 



