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Nov. 1845.] Magnetic Rotation of Light. 315 



the power of rotating the light, oniy those whose planes of 

 rotation are more or less perpendicular to the ray affect it; 

 and that it is the resultant or sum of forces in any one direc- 

 tion which is active in producing rotation. But even then a 

 striking difference remains, because the resultant in the same 

 plane is not absolute in direction, but relative to the course 

 of the ray, being in the one case as the circle c, and in the 

 other as the circle d, fig. 3 ; whereas the resultant of the mag- 

 netic or electric induction is absolute, and not changing with 

 the course of the ray, being always either as expressed by c 

 or else as indicated by d. 



2234. All these differences, however, will doubtless disap- 

 pear or come into harmony as these investigations are ex- 

 tended ; and their very existence opens so many paths, by 

 which we may pursue our inquiries, more and more deeply, 

 into the powers and constitution of matter. 



2235. Bodies having rotating power of themselves, do not 

 seem by that to have a greater or a less tendency to assume a 

 further degree of the same force under the influence of mag- 

 netic or electric power. 



2236. Were it not for these and other differences, we might 

 see an analogy between these bodies, which possess at all times 

 the rotating power, as a specimen of quartz which rotates only 

 in one plane, and those to which the power is given by the 

 induction of other forces, as a prism of heavy glass in a helix, 

 on the one hand ; and, on the other, a natural magnet and a 

 helix through which the current is passing. The natural 

 condition of the magnet and quartz, and the constrained con- 

 dition of the helix and heavy glass, form the link of the ana- 

 logy in one direction ; whilst the supposition of currents ex- 

 isting in the magnet and helix, and only a tendency or tension 

 to currents existing in the quartz and heavy glass, supplies 

 the link in the transverse direction. 



2237. As to those bodies which seem as yet to give no indi- 

 cation of the power over light, and therefore none of the as- 

 sumption of the new magnetic conditions ; these may be di- 

 vided into two classes, the one including air, gases and vapours, 

 and the other rock crystal, Iceland spar, and certain other 

 crystalline bodies. As regards the latter class, I shall give, 

 in the next series of these researches, proofs drawn from phe- 

 nomena of an entirely different kind, that they do acquire the 

 new magnetic condition ; and these being so disposed of for 

 the moment, I am inclined to believe that even air and gases 

 have the power to assume the peculiar state, and even to affect 

 light, but in a degree so small that as yet it has not been made 

 sensible. Still the gaseous state is such a remarkable condi- 



