ON PHYSICAL LINES OF FORCE. 507 



The relation (E) between the amount of rotation and the size of the 

 vortices shews that different substances may differ in rotating power inde- 

 pendently of any observable difference in other respects. We know nothing 

 of the absolute size of the vortices ; and on our hypothesis the optical phenomena 

 are probably the only data for determining their relative size in different sub- 

 stances. 



On our theory, the direction of the rotation of the plane of polarization 

 depends on that of the mean moment of momenta, or angular momentum, of the 

 molecular vortices ; and since M. Verdet has discovered that magnetic substances 

 have an effect on light opposite to that of diamagnetic substances, it follows that 

 the molecular rotation must be opposite in the two classes of substances. 



We can no longer, therefore, consider diamagnetic /bodies as being those 

 whose coefficient of magnetic induction is less than that of space empty of 

 gross matter. We must admit the diamagnetic state to be the opposite of the 

 paramagnetic ; and that the vortices, or at least the influential majority of them, 

 in diamagnetic substances, revolve in the direction in which positive electricity 

 revolves in the magnetizing bobbin, while in paramagnetic substances they 

 revolve in the opposite direction. 



This result agrees so far with that part of the theory of M. Weber* 

 which refers to the paramagnetic and diamagnetic conditions. M. Weber sup- 

 poses the electricity in paramagnetic bodies to revolve the same way as the 

 surrounding helix, while in diamagnetic bodies it revolves the opposite way. 

 Now if we regard negative or resinous electricity as a substance the absence 

 of which constitutes positive or vitreous electricity, the results will be those 

 actually observed. This will be true independently of any other hypothesis 

 than that of M. Weber about magnetism and diamagnetism, and does not 

 require us to admit either M. Weber's theory of the mutual action of electric 

 particles in motion, or our theory of cells and cell-walls. 



I am inclined to believe that iron differs from other substances in the 

 manner of its action as well as in the intensity of its magnetism ; and I think 

 its behaviour may be explained on our hypothesis of molecular vortices, by 

 supposing that the particles of the iron itself are set in rotation by the tan- 

 gential action of the vortices, in an opposite direction to their own. These 

 large heavy particles would thus be revolving exactly as we have supposed the 



* Taylor's Scientific Memoirs, Vol. v. p. 477. 



642 



