92 PHYSIOLOGICAL PHYSICS. [Chap. ix. 



Two fl Beads. As in electricity there ha,ve been 

 supposed to be two subtle imponderable fluids, posi- 

 tive and negative, pervading all objects, so there have 

 been supposed to be two magnetic fluids, which attract 

 one another. In unmagnetised bodies these fluids 

 neutralise one another j in magnetised bodies they are 

 separated. 



Magnetic induction. Just as a conductor 

 charged with electricity, when brought near an un- 

 charged body, was supposed to decompose the neutral 

 fluid of the uncharged body, attracting one of the 

 electricities to the end near it, and repelling the other 

 electricity to the other end (page 6), so a magnet, when 

 brought into contact with a substance capable of being 

 attracted by it, was supposed by induction to separate 

 the magnetic fluids of the attracted body, attracting 

 the one to one end and repelling the other. Thus, 

 when a piece of soft iron is touched, say by the north 

 pole of a magnet, the iron adheres to the magnet, and 

 becomes for the time also a magnet, having a north and 

 south pole, the south being the one in contact with 

 the north of the original magnet. As soon, however, 

 as the piece of soft iron is removed from the magnet 

 it loses all its magnetism. Iron that has been rendered 

 brittle, or hard steel, are not so easily affected by a 

 magnet as soft iron ; but when at length they are 

 affected, the magnetism developed in them is more 

 permanent. Well-tempered steel, especially, suffers 

 little attraction by a magnet, and is magnetised with 

 difficulty, rubbing with the magnet requiring to be re- 

 sorted to, but it then becomes a PERMANENT MAGNET. 

 The force which makes tempered steel resist the in- 

 fluences, and, when it has been affected, causes the mag- 

 netism to be retained, is called COERCIVE FORCE. 



Permanent magnetisation is effected in 

 various ways : (1) by single touch, i.e. by laying on a 

 table the bar to be magnetised and stroking it several 



