ROTATION OF LIGHT BY MAGNETIC AND ELECTRIC FORCES. 
13 
water, solution of sulphate of soda, solution of tartaric acid, alcohol, ether, and oil 
of turpentine ; all of which were affected, and acted on light exactly in the manner 
described in relation to magnetic action (2173.). 
2212. I submitted air to the influence of these helices carefully and anxiously, but 
could not discover any trace of action on the polarized ray of light. I put the long 
helix (2190.) into the other two (2191. 2192.), and combined them all into one con- 
sistent series, so as to accumulate power, but could not observe any effect of them on 
light passing through air. 
2213. In the use of helices, it is necessary to be aware of one effect, which might 
otherwise cause confusion and trouble. At first, the wire of the long helix (2190.) 
was wound directly upon the thin glass tube which served to contain the fluid. 
When the electric current passed through the helix it raised the temperature of the 
metal, and that gradually raised the temperature of the glass and the film of water in 
contact with it, and so the cylinder of water, warmer at its surface than its axis, 
acted as a lens, gathering and sending rays of light to the eye, and continuing to act 
for a time after the current was stopped. By separating the tube of water from the 
helix, and by other precautions, this source of confusion is easily avoided. 
2214. Another point of which the experimenter should be aware, is the difficulty, 
and almost impossibility, of obtaining a piece of glass which, especially after it is cut, 
does not depolarize light. When it does depolarize, difference of position makes 
an immense difference in the appearance. By always referring to the parts that do 
not depolarize, as the black cross, for instance, and by bringing the eye as near as 
may be to the glass, this difficulty is more or less overcome. 
2215. For the sake of supplying a general indication of the amount of this induced 
rotating force in two or three bodies, and without any pretence of offering correct 
numbers, I will give, generally, the result of a few attempts to measure the force, and 
compare it with the natural power of a specimen of oil of turpentine. A very power- 
ful electro-magnet was employed, with a constant distance between its poles of 2b 
inches. In this space was placed different substances ; the amount of rotation of the 
eye-piece observed several times and the average taken, as expressing the rotation for 
the ray length of substance used. But as the substances were of different dimen- 
sions, the ray lengths were, by calculation, corrected to one standard length, upon the 
assumption that the power was proportionate to this length (2163.). The oil of tur- 
pentine was of course observed in its natural state, i. e. without magnetic action. 
Making water 1, the numbers were as follows: — 
Oil of turpentine . . . . 118 
Heavy glass (2151.) ... 60 
Flint-glass 2’8 
Rock salt 22 
