CONTRIBUTIONS TO SCIENCE. 549 



rotating power independently 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 substances. 



Now, independently of the action of a magnetic field on 

 polarised light, all the phenomena of diamagnetism can be 

 accounted for on the hypothesis that the magnetic perme- 

 ability of diamagnetic substances is less than that of a 

 vacuum, so that they behave like a paramagnetic substance 

 immersed in a medium more magnetic than itself. But Max- 

 well has pointed out that " 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 

 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 sub- 

 stances, revolve in the direction in which positive electricity 

 revolves in the magnetising bobbin, while in paramagnetic sub- 

 stances they revolve in the opposite direction. 



Perhaps we cannot conclude this account of the hypo- 

 thesis of molecular vortices better than by quoting Max- 

 well's own words : l 



I think we have good evidence for the opinion that some 

 phenomenon of rotation is going on in the magnetic field ; that 

 this rotation is performed by a great number of very small 

 portions of matter, each rotating on its own axis, this axis being 

 parallel to the direction of the magnetic force, and that the 

 rotations of these different vortices are made to depend on one 

 another by means of some kind of mechanism connecting them. 



The attempt which I [have] made to imagine a working 

 model of this mechanism must be taken for no more than it 

 really is, a demonstration that mechanism may be imagined 

 capable of producing a connection mechanically equivalent to 



1 Electricity and Magnetism, vol. ii. Art. 831 (1st ed.) 



