202 LIGHT, CALORIC, ELECTRICITY AND MAGNETISM. 



4. But Faraday was the fortunate discoverer of a new and remarka- 

 ble relationship between light and electricity or magnetism. He proved 

 that a beam of polarized light may be caused to rotate between the poles 

 of a magnet and the converse. 



To explain this, let us suppose a beam of light to fall at an angle of 

 incidence of 56° 45' upon a plate of glass whose plane is placed at right 

 angles to the horizon. This beam will be reflected at the same angle 

 and proceed horizontally. If now it fall at the same angle of incidence 

 upon a second plate of glass placed so that its plane of reflection will 

 be vertical, instead of being reflected as ordinary light would be, it will 

 pass through this plate. The beam has evidently been changed by its 

 reflection from the first surface and is called polarized light. If now 

 the plane of reflection of the second surface be turned round 90°, so as 

 to be horizontal and parallel to the first plane, the light will suffer re- 

 flection, and not be transmitted. The same eflect without changing the 

 position of the second plane, is produced by the interposition of a num- 

 ber of solid and liquid substances between the two reflecting-planes, so 

 that the polarized beam may pass through them. The beam is by their 

 agency so clianged as to sufl^er reflection where it would otherwise have 

 been transmitted, and vice versa; the effect is the same as if the beam had 

 been turned round on its axis. Now let us suppose that, instead of caus- 

 ing the polarized beam to pass through those solid or liquid substances 

 alluded to, it were made to pass between the jaws of a powerful electro- 

 magnet, the beam would be made to rotate in like manner ; that is, as long 

 as the electric current flowed around the soft iron and made it a powerful 

 magnet, the beam of light would be reflected from the second plate of 

 glass, but as soon as the electric current ceased to flow and the soft iron 

 lost its magnetism, the beam would become incapable of reflection. This 

 is substantially the discovery of Faraday, and shows a wonderfully close 

 connexion between light and electricity. 



5. On the Magnetic condition of all Matter. Prof. Faraday has al- 

 so opened a new field of peculiar interest by his researches into the 

 magnetic condition of various kinds of matter. The poles of a power- 

 ful electro-magnet are placed, one on each side of a bar of the metal, 

 glass, or other substance whose magnetic condition is to be examined. 

 The bar may be two inches in length and half an inch each in depth and 

 breadtli. It is delicately suspended by means of a few fibres of silk 

 from a cocoon, and the whole protected from currents of air by being 

 coveted with a glass jar. If now the bar be inclined at an angle of 

 about 45° to the line joining the poles of the magnet, and the magnet- 

 ism excited by means of battery connection, the bar will move from 



