Proceedings of Learned Societies. 



461 



plained. "When a current of electricity is sent in a coil around the 

 iron bar, «, only the upper end of which is shown in the diagram, it is 

 lengthened to a very minute degree, consequently it presses up- 

 wards the brass rod, b, and this acting on the lever raises' the free 

 end, uncoiling the wire round the axis, and bringing the mirror, c, to 

 ajposition more nearly approaching the perpendicular. This action 

 is so slight that to render it visible to the audience, a horizontal 

 ray of light, shown in the diagram by a dotted line, was reflected 

 from the mirror on to a screen at some distance, when the slightest 

 movement of the mirror was rendered evident by the alteration in 

 the position of the ray, d. On magnetizing the iron, the reflected 

 ray was depressed, and on breaking the current the ray returned 

 to its original position. 



So exceedingly delicate was the entire apparatus, that the ejec- 

 tion of .a few drops of warm water from a pipette upon the iron bar, 



A 



produced an immediate depression of the reflected ray. The proba- 

 ble explanation of the lengthening of the iron bar under the influence 

 of the electric coil is, that the particles have a tendency to arrange 

 themselves along the lines of magnetic force, in the direction of the 

 bar. This explanation is supported by a beautiful experiment of 

 Mr. Grove's, which was also shown for the first time at Professor 

 Tyndall's lecture. A cylinder, with glass ends, was filled with a 

 mixture of magnetic oxide of iron and water. This formed a muddy 

 liquid, through which a ray of light could hardly pass. On 

 placing this in the centre of an electric coil, it was found that on 

 making the current the particles, being free to move, arranged 

 themselves in the direction of the axis of the cylinder, and the ray 

 of light passed through with less obstruction. On breaking the 

 current in the coil, the liquid again became muddy and opaque. 



