SECT. XXXI. ROTATION OF LIGHT. 319 



with the poles of a powerful galvanic battery, and a polished 

 prism of heavy glass, or silicated borate of lead, was placed in the 

 axis of the spiral as a core, through the length or axis of which 

 a ray of polarized light was sent. This ray, viewed through a 

 piece of tourmaline or a Nichol's eye-piece, vanished and reappeared 

 as usual at each quarter revolution of the eye-piece ; but when a 

 current of electricity was sent through the spiral at the time the 

 ray had vanished, it instantly reappeared, and remained as long as 

 the electric current continued to flow ; but the instant the electri- 

 city ceased the light vanished, and as often as the electric current 

 flowed through the spiral, or was interrupted, so often did the 

 polarized ray appear and vanish. 



The character of the force thus impressed on the heavy glass 

 is that of rotation, for the stopping and renewing of the electric 

 current had the same effect as the revolving motion of the eye- 

 piece in making the light alternately appear and vanish. Accord- 

 ingly, Dr. Faraday found that, when the electricity flowed 

 through the spiral in one direction, the rotation of the plane of 

 polarization was right-handed ; and when it flowed in the other 

 direction, the rotation of the plane of polarization was left-handed, 

 the rotation increasing with the length of the prism and the 

 intensity of the electricity. The same phenomena were produced 

 by a very powerful magnet when a ray of polarized light was 

 sent through the heavy glass parallel to the line of magnetic 

 force. 



Heavy glass or silico-borate of lead has the property more than 

 any other substance of making light rotate under electric and 

 magnetic influence ; but many substances have the property 

 more or less, as flint and crown glass, rock salt, all the fixed and 

 essential oils, water, and many other liquids, but none of the 

 gases possess it. In those substances that have the power of 

 circular polarization naturally, the magnetic and electric influences 

 increase or diminish the rotation according to its direction. 



Polarized heat is made to revolve in the same manner, when 

 the medium through which it passes is subject to magnetic 

 influence. 



Mr. Grove observes that if light and heat be merely modes of 

 force, which there is every reason to believe that they are, it 

 may be fairly stated that in these experiments magnetism affects 

 these forces directly ; for light and heat being, in that view, 



