Magnetic Characters of Metalsy ^r. 75 



partly by electro-magnets, which have afforded different 

 modes of assailing those substances that were the objects of 

 investigation. 



15. In all cases where delicate magnetic needles are em- 

 ployed, especially when the suspension is by means of a fibre, 

 and tfie system astatic, the experiments are exceedingly te- 

 dious, and much time is required to allow of the system's 

 repose from its agitations before an attempt can be made to 

 approach it with the specimen to be examined. When, how- 

 ever, a single needle is employed, whose support is a finely 

 pointed pivot, the experiments are less subject to delay than 

 by the other mode, though much caution and some dexterity 

 are still required to enable the experimenter to arrive at 

 satisfactory results. But in whichever way the magnetic 

 needle may be suspended in these delicate investigations, the 

 bodies under examination must either be held immediately 

 in the hand, or indirectly, by means of some other body pre- 

 viously ascertained to have no influence on the needle. If 

 held in the hand, thermo-electric currents are to be suspected ; 

 and if attached to the end of a wooden rod, by means of 

 sealing wax, or resinous cement, other electric actions may 

 interfere with the results ; or may, indeed, be the sole cause 

 of any motions that may happen to be observed. Moreover, 

 a delicate magnetic needle does not possess a sufficient de- 

 gree of power to bring into play the minute portions of mag- 

 netism that lie dormant in many bodies. These exiguous 

 sleeping forces can never be roused into a state of activity, 

 and, consequently, can never be discovered by merely pre- 

 senting the bodies in which they reside to the pole of a feeble 

 magnetic needle. To accomplish their discovery a compa- 

 ratively powerful magnetic action is absolutely required ; 

 for, when thus assailed, their polarization is more easily en- 

 forced, and their detection almost certain. The magnetic 

 needle, however, may be usefully employed in cases where 

 the suspected magnetism of a body is of some easily detected 

 amount ; and it may be resorted to with advantage, in preli- 

 minary trials, under all circumstances, because of the possi- 

 bility of the specimen under examination possessing a suffi- 

 cient amount of magnetism to be detected by it, and tho 



