94 ELEMENTARY LESSONS ON [CHAP. n. 



discovered that a ray of polarised light passing through 

 certain substances in a powerful magnetic field has the 

 direction of its vibrations changed. This phenomenon, 

 which is sometimes called "The Magnetisation of Light," 

 is better described as " The Rotation of the Plane of 

 Polarisation of Light by Magnetism." The amount of 

 rotation differs in different media, and varies with the 

 magnetising force. More recently Kerr has shown that 

 a ray of polarised light is also rotated by reflection at 

 the end or side of a powerful magnet. Further mention 

 is made of these discoveries in the Chapter on Electro- 

 optics, Lesson XXXV. 



115. Physical Theory of Magnetism. All these 

 various phenomena point to a theory of magnetism very 

 different from the old notion of fluids. It appears that 

 every particle of a magnet is itself a magnet, and that 

 the magnet only becomes a magnet as a whole by the 

 particles being so turned as to point /one way. This 

 conclusion is supported by the observation that if a glass 

 tube full of iron filings is magnetised, the filings can be 

 seen to set themselves endways, and that, when thus once 

 set, they act as a magnet until shaken up. It appears 

 to be harder to turn the individual molecules of solid 

 steel, but when so once set, they remain end-on unless 

 violently struck or heated. l The optical phenomena 

 led Clerk Maxwell to the further conclusion that these 

 longitudinally-set molecules are rotating round their long 

 axes, and that in the " aether " of space there is also a 

 vortical motion along the lines of magnetic induction ; 

 this motion, if occurring in a perfect medium (as the 

 " aether " may be considered), producing tensions along 

 the lines and pressures at right angles to them, would 

 afford a satisfactory explanation of the magnetic attrac- 



1 It follows from this theory that when all the particles were turned end- 

 on, the limits of possible magnetisation would have been attained. Some 

 careful experiments of Beetz entirely confirm this conclusion, and add weight 

 to the theory. 



