XIII 



ON PABAMAGNETIO AND DIAMAGNETIC FORCES* 



THE notion of an attractive force, which draws bodies 

 toward the centre of the earth, was entertained by 

 Anaxagoras and his pupils, by Democritus, Pythag- 

 oras, and Epicurus; and the conjectures of these ancients 

 were renewed by Galileo, Huyghens, and others, who 

 stated that bodies attract each other as a magnet attracts 

 iron. Kepler applied the notion to bodies beyond the 

 surface of the earth, and affirmed the extension of this 

 force to the most distant stars. Thus it would appear 

 that vn the attraction of iron by a magnet originated the 

 conception of the force of gravitation. Nevertheless, if 

 we look closely at the matter, it will be seen that the 

 magnetic force possesses characters strikingly distinct from 

 those ol the force which holds the universe together. The 

 theory of gravitation is, that every particle of matter at- 

 tracts every other particle; in magnetism also we have 

 attraction, but we have always, at the same time, repul- 

 sion, the final effect being due to the difference of these 

 two forces. A body may be intensely acted on by a 

 magnet, and still no motion of translation will follow, if 

 the repulsion be equal to the attraction. Previous to mag- 



* Abstract of a discourse delivered in the Royal Institution of Great Britain, 

 Itebruary 1, 1866. 



(338) 



