Nov. 1845.] Rotation of a Ray of Light by Magnetism. 301 



2172. Using water, alcohol, mercury, and other fluids con- 

 tained in very large delicate thermometer-shaped vessels, I 

 could not discover that any difference in volume occurred 

 when the magnetic curves passed through them. 



2173. It is time that I should pass to a consideration of 

 this power of magnetism over light as exercised, not only in 

 the silicated borate of lead (2151.), but in many other sub- 

 stances ; and here we perceive, in the first place, that if all 

 transparent bodies possess the power of exhibiting the action, 

 they have it in very different degrees, and that up to this time 

 there are some that have not shown it at all. 



2174. Next, we may observe, that bodies that are exceed- 

 ingly different to each other in chemical, physical, and me- 

 chanical properties, develope this effect; for solids and liquids, 

 acids, alkalies, oils, water, alcohol, aether, all possess the power. 



2175. And lastly, we may observe, that in all of them, 

 though the degree of action may differ, still it is always the 

 same in kind, being a rotative power over the ray of light ; and 

 further, the direction of the rotation is, in every case, inde- 

 pendent of the nature or state of the substance, and dependent 

 upon the direction of the magnetic line of force, according to 

 the law before laid down (2160.). 



2176. Amongst the substances in which this power of action 

 is found, I have already distinguished the silico-borate of Lead 

 (2151.) as eminently fitted for the purpose of exhibiting the 

 phaenomena. I regret that it should be the best, since it is 

 not likely to be in the possession of many, and few will be in- 

 duced to take the trouble of preparing it. If made, it should 

 be well-annealed, for otherwise the pieces will have consider- 

 able power of depolarizing light, and then the particular phae- 

 nomena under consideration are much less strikingly observed. 

 The borate of lead, however, is a substance much more fusi- 

 ble, softening at the heat of boiling oil, and therefore far 

 more easily prepared in the form of glass plates and annealed; 

 and it possesses as much magneto-rotative power over light 

 as the silico-borate itself. Flint-glass exhibits the property, 

 but in a less degree than the substances above. Crown-glass 

 shows it, but in a still smaller degree. 



2177. Whilst employing crystalline bodies as diamagnetics, 

 I generally gave them that position in which they did not 

 affect the polarized ray, and then induced the magnetic curves 

 through them. As a class, they seemed to resist the assump- 

 tion of the rotating state. Rock-salt and fluor-spar gave evi- 

 dence of the power in a slight degree ; and I think that a cry- 

 stal of alum did the same, but its ray length in the transparent 



