MAGNETISM. 145 



given are definitely deflected, but cannot produce them except 

 when in motion : motion, therefore, is to be regarded in this 

 case as the initiative force. Magnetism will, however, di- 

 rectly affect the other forces light, heat, and chemical affin- 

 ity, and change their direction or mode of action, or, at all 

 events, will so affect matter subjected to these forces, that 

 their direction is changed. Since these lectures were deliv- 

 ered, Faraday has discovered a remarkable effect of the mag- 

 netic force in occasioning the deflection of a ray of polarised 

 light. 



If a ray of polarised light pass through water, or through 

 any transparent liquid or solid which does not alter or turn 

 aside the plane of polarisation, and the column, say of water, 

 through which it passes be subjected to the action of a pow- 

 erful magnet, the line of magnetic force, or that which would 

 unite the poles of the magnet, being in the same direction as 

 the ray of polarised light, the water acquires, with reference 

 to the light, similar, though not quite identical, properties to 

 oil of turpentine the plane of polarisation is rotated, and 

 the direction of this rotation is changed by changing the di- 

 rection of the magnetic force : thus, if we suppose a polar- 

 ised ray to pass first in its course the north pole of the mag- 

 net, then between that and the south pole it will be deflected, 

 or curved to the right ; while if it meets the south pole first 

 in its course, it will, in its journey between that and the north 

 pole, be turned to the left. If the substance through which 

 the ray is transmitted be of itself capable of deflecting the 

 plane of polarisation, as, for instance, oil of turpentine, then 

 the magnetic influence will increase or diminish this rotation, 

 according to its direction. A similar effect to this is observed 

 with polarised heat when the medium through which it is 

 transmitted is subjected to magnetic influence. 



Whether this effect of magnetism is rightly termed an ef- 

 fect upon light and heat, or is a molecular change of the mat- 

 ter transmitting the light and heat, is a question the resolu- 

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