1824.] new Phenomenon of Electromagnetism. 25 



cury, and the phenomena of rotation by the magnet ; and it was 

 found by direct experiment, that the conducting powers of the 

 tin, at and just before its point of fusion, did not perceptibly 

 differ, and that they were much higher than those of mercury. 

 Lastly, the communication was made from the battery by two 

 tubes having nearly the same diameter as the wires filled with 

 mercury, so that the electricity, for some inches before it entered 

 the basin, passed through mercury ; and still the appearances 

 continued the same. 



From the rapidity of the undulations round the points of the 

 cones, I thought they would put in motion any light bodies 

 placed above the mercury ; but I could not produce the slightest 

 motion in a very light wheel hung on an axle ; and when fine 

 powders of any kind were strewed upon the surface, they merely 

 underwent undulations, without any other change of place; and 

 fine iron filings strewed on the top of the cone, arranged them- 

 selves in right lines at right angles to the line joining the two 

 wires, and remained stationary, even on the centre of the cone. 

 The effect, therefore, is of a novel kind, and in one respect 

 seems analogous to that of the tides. It would appear as if the 

 passage of the electricity diminished the action of gravity on 

 the mercury. And that there is no change of volume of the 

 whole mass of the mercury appears from the experiment, p. 24 ; 

 and this was shown likewise by inclosing the apparatus in a kind 

 of manometer, terminating in a fine tube containing air inclosed 

 by oil ; and which, by its expansion or contraction, would have 

 shown the slightest change of volume in the mercury : none 

 however took place when the contacts were alternately made 

 and broken, unless the circuit was uninterrupted for a sufficient 

 time to communicate sensible heat to the mercury. 



This phenomenon, in which the same effects are produced at 

 the two opposite poles, seems strongly opposed to the idea of 

 the electromagnetic results being produced by the transition 

 currents or motions of a single imponderable fluid. 



On the conjectural part of the subject I shall not however 

 enter, for the reasons stated in the beginning of this paper; but 

 I cannot with propriety conclude, without mentioning a circum- 

 stance in the history of the progress of electromagnetism, 

 which, though well known to many Fellows of this Society, has, 

 I believe, never been made public, namely, that we owe to the 

 sagacity of Dr. Wollaston, the first idea of the possibility of the 

 rotations of the electromagnetic wire round its axis, by the 

 approach of a magnet; and 1 witnessed, early in 1821, an 

 unsuccessful experiment which he made to produce the effect 

 in the laboratory of the Royal Institution. 



