ON MAGNETISM. 533 



had nickel been as dense as platina, or as light as cork, we could not have 

 supposed that it contained any considerable quantity of iron, but in fact 

 the specific gravity of these metals is very nearly the same, and nickel is 

 never found in nature but in the neighbourhood of iron ; we may therefore 

 suspect, with some reason, that the hypothesis of the existence of iron in 

 nickel may be even chemically true. The aurora borealis is certainly in 

 some measure a magnetical phenomenon, and if iron were the only sub- 

 stance capable of exhibiting magnetic effects, it would follow that some 

 ferruginous particles must exist in the upper regions of the atmosphere. 

 The light usually attending this magnetical meteor may possibly be 

 derived from electricity, which may be the immediate cause of a change 

 of the distribution of the magnetic fluid, contained in the ferruginous 

 vapours, that are imagined to float in the air. 



We are still less capable of distinguishing with certainty in magnetism, 

 than in electricity, a positive from a negative state, or a real redundancy of 

 the fluid from a deficiency. The north pole of a magnet may be con- 

 sidered as the part in which the magnetic fluid is either redundant or de- 

 ficient, provided that the south pole be understood in a contrary sense : 

 thus, if the north pole of a magnet be supposed to be positively charged, 

 the south pole must be imagined to be negative ; and in hard iron or steel 

 these poles may be considered as unchangeable. 



A north pole, therefore, always repels a north pole, and attracts a south 

 pole. And in a neutral piece of soft iron, near to the north pole of a 

 magnet, the fluid becomes so distributed by induction, as to form a tem- 

 porary south pole next to the magnet, and the whole piece is of course 

 attracted, from the greater proximity of the attracting pole. If the bar is 

 sufficiently soft, and not too long, the remoter end becomes a north pole, 

 and the whole bar a perfect temporary magnet. But when the bar is of 

 hard steel, the state of induction is imperfect, from the resistance opposed 

 to the motion of the fluid ; hence the attraction is less powerful, and an 

 opposite pole is formed, at a certain distance, within the bar ; and beyond 

 this another pole, similar to the first ; the alternation being sometimes re 

 peated more than once. The distribution of the fluid within the magnet is 

 also affected by the neighbourhood of a piece of soft iron, the north pole 

 becoming more powerful by the vicinity of the new south pole, and the 

 south pole being consequently strengthened in a certain degree ; so that the 

 attractive power of the whole magnet is increased by the proximity of the 

 iron. A weak magnet is capable of receiving a temporary induction of a 

 contrary magnetism from the action of a more powerful one, its north pole 

 becoming a south pole on the approach of a stronger north pole ; but the 

 original south pole still retains its situation at the opposite end, and 

 restores the magnet nearly to its original condition, after the removal of 

 the disturbing cause. 



The polarity of magnets, or their disposition to assume a certain direc- 

 tion, is of still greater importance than their attractive power. If a small 

 magnet, or simply a soft wire, be poised on a centre, it will arrange itself 

 in such a direction, as will produce an equilibrium of the attractions and 

 repulsions of the poles of a larger magnet ; being a tangent to a certain 



