AND MODERN PHYSIC a 175 



in a wave of polarised light travelling parallel to the 

 linos of force in a magnetic field is rotated by its 

 passage through the field. The numerical laws of 

 this relation had been investigated by Verdet, and 

 Maxwell showed how his hypothesis of molecular 

 vortices led to laws which agree in the main with 

 those found by Yerdet 



Ho points out that tho connection between 

 magnetism and electricity has tho same mathe- 

 matical form as that between certain other pairs of 

 phenomena, ono of which has a linear and the other 

 a rotatory character; and, further, that an analogy 

 may be worked out assuming cither the linear 

 character for magnetism and the rotatory character 

 for electricity, or tho reverse. Ho alludes to Prof. 

 Challis* theory, according to which magnetism is to 

 consist in currents in a fluid whoso directions corre- 

 spond with the lines of magnetic force, while electric 

 currents are supposed to be accompanied by, if not 

 dependent upon, a rotatory motion of tho fluid about 

 tho axis of tho current; and to Von Hclmholtz's 

 theory of a somewhat similar character. He then 

 gives his own reasons agreeing with those of Sir 

 W. Thomson (Lord Kelvin) for supposing that there 

 must bo a real rotation going on in a magnetic field 

 in order to account for tho rotation, of the plane of 

 polarisation, and, accepting these reasons as valid, he 

 develops tho consequences of his theory with the 

 results stated above. 



His own verdict on tho theory is given in tho 

 "Electricity and Magnetism" (voL ii., 831, first 

 edition, p. 410) : 



