130 CORRELATION OF PHYSICAL FORCES. 



through which it passes be subjected to the action of a 

 powerful 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, pro- 

 perties to oil of turpentine the plane of polarisation is ro- 

 tated, and the direction of this rotation is changed by changing 

 the direction of the magnetic force : thus, if \ve suppose a 

 polarised ray to pass first in its course the north pole of the 

 magnet, then between that and the south pole, it will be de- 

 flected, or curved to the right ; while if it meet 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 rota- 

 tion, 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 

 effect upon light and heat, or is a molecular change of the 

 matter transmitting the light and heat, is a question the reso- 

 lution of which must be left to the future ; at present, the 

 answer to it would depend upon the theory we adopt. If the 

 view of light and heat which I have stated be adopted, then 

 we may fairly say that magnetism, in these experiments, 

 directly affects the other forces ; for light and heat being, 

 according to that view, motions of ordinary matter, magnetism, 

 in affecting these movements, affects the forces which occa- 

 sion them. If, however, the other theories be adhered to, it 

 would be more consistent with the facts to view these results 

 as exhibiting an action upon the matter itself, and the heat 

 and light as secondarily affected. 



When substances are undergoing chemical changes, and a 

 magnet is brought near them, the. direction or lines of action 

 of the chemical force will be changed. There are many old 

 experiments which probably depended on this effect, but 

 which were erroneously considered to prove that permanent 

 magnetism could produce or increase chemical action : these 



