ROTATION OF A RAY OF LIGHT BY MAGNETISM. 
/ 
other substances ; 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 exceedingly different to each 
other in chemical, physical, and mechanical properties, develope this effect ; for solids 
and liquids, acids, alkalies, oils, water, alcohol, ether, 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, independent 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 pur- 
pose of exhibiting the phenomena. I regret that it should be the best, since it is not 
likely to be in the possession of many, and few will be induced to take the trouble 
of preparing it. If made, it should be well annealed, for otherwise the pieces will 
have considerable power of depolarizing light, and then the particular phenomena 
under consideration are much less strikingly observed. The borate of lead, however, 
is a substance much more fusible, 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 assumption of 
the rotating state. Rock-salt and fluor-spar gave evidence of the power in a slight 
degree ; and I think that a crystal of alum did the same, but its ray length in the 
transparent part was so small that I could not ascertain the fact decisively. Two 
specimens of transparent fluor, lent me by Mr. Tennant, gave the effect. 
21 78. Rock-crystal, four inches across, gave no indications of action on the ray, 
neither did smaller crystals, nor cubes about three-fourths of an inch in the side, 
which were so cut as to have two of their faces perpendicular to the axis of the 
crystal (1692, 1693.), though they were examined in every direction. 
2179. Iceland spar exhibited no signs of effect, either in the form of rhomboids, 
or of cubes like those just described (1695.). 
2180. Sulphate of baryta, sulphate of lime, and carbonate of soda, were also without 
action on the light. 
2181. A piece of fine clear ice gave me no effect. I cannot however say there is 
none, for the effect of water in the same mass would be very small, and the irregu- 
larity of the flattened surface from the fusion of the ice and flow of water, made 
the observation very difficult. 
