320 ROTATION OF LIGHT. SECT. XXXI. 



motions of ordinary matter, magnetism in affecting these move- 

 ments affects the forces which occasion them. If, however, this 

 effect of magnetism be a molecular change of the matter trans- 

 mitting the light and heat, then it follows that the light and heat 

 are indirectly affected by the electricity or magnetism. Dr. 

 Faraday says that the magnetic forces do not act on the ray 

 of light directly, without the intervention of matter, but 

 through the mediation of the substance in which they and the 

 ray have a simultaneous existence ; the substances and the forces 

 giving to and receiving from each other the power of acting on 

 the light. Dr. Thomson has shown, by a mathematical investi- 

 gation of the subject, that Dr. Faraday's discovery seems to prove 

 the truth of Ampere's explanation of the ultimate nature of 

 magnetism. However, in Ampere's theory, the current of elec- 

 tricity flowing round the iron makes it a permanent magnet, but 

 it does not make the heavy glass or the other bodies, which have 

 the same property, either temporary magnets when the light is 

 rotating within them, or permanent magnets when the inductive 

 action of the current of electricity ceases. Hence the molecular 

 condition of the substances, when the light is rotating in them, 

 must be specifically distinct from that of magnetised iron : it 

 must therefore be a new magnetic condition, and the force which 

 the matter in this state possesses must be a new magnetic force. 

 After describing his admirable experiment, Dr. Faraday ob- 

 serves that " it has established for the first time a true, direct 

 relation and dependence between light and the magnetic and 

 electric forces ; and thus a great addition is made to the facts 

 and considerations which tend to prove that all natural forces are 

 tied together, and have one common origin. It is no doubt diffi- 

 cult, in the present state of our knowledge, to express our expec- 

 tations in exact terms ; and though I have said that another of 

 the powers of nature is in these experiments directly related to the 

 rest, I ought perhaps rather to say that another form of the 

 great power is distinctly and directly related to the other forms ; 

 or that the great power manifested by particular phenomena in 

 particular forms is here further identified and recognised by the 

 direct relation of its form of light to its forms of electricity and 

 magnetism. The relation existing between polarized light and 

 magnetism and electricity is even more interesting than if it had 

 teen shown to exist with common light only. It cannot but 



