336 POLARITY INDUCTION. SECT. XXXIV, 



other of these laws. Many bodies are paramagnetic besides iron, 

 as the loadstone, which consists of the peroxide and protoxide of 

 iron mixed with small portions of silica and alumina ; also some 

 of the gems and metals, as cobalt, nickel, &c. A substance is 

 often paramagnetic if it contains only the 130,000th part of its 

 weight of iron ; but by far the greater number are diamagnetic, as 

 all animal and vegetable matter, acids, oils, sugar, starch, bread, 

 &c., and all the gases except oxygen, which is highly paramag- 

 netic ; and its force increases with its density : but notwithstand- 

 ing the predominance of diamagnetic matter at the surface, the 

 terrestrial globe is paramagnetic in fact it is a powerful magnet. 



Besides the substances which are paramagnetic naturally, that 

 property may be imparted by a variety of methods, as by friction 

 with magnets or even juxtaposition with them ; and a bar of hard 

 steel held at the angleof the dip will become a magnet on re- 

 ceiving a few strokes with a hammer on its upper end. 



Polarity is one of the most distinguishing characters of mag- 

 netism : it is the property which a magnet possesses when freely 

 suspended of resting spontaneously in the magnetic meridian, or 

 nearly north and south, and always returning to that position 

 when disturbed in consequence of the mean magnetic attraction 

 of the earth ; yet the magnet has no tendency to move to the 

 north or south even when floating on water, because the same 

 pole that attracts one end repels the other. Both poles of a 

 magnet attract iron, which in return attracts either pole of the 

 magnet with an equal and contrary force. The action of a magnet 

 on unmagnetised iron is confined to attraction, whereas the reci- 

 procal agency of magnets is characterised by a repulsive as well as 

 by an attractive force ; for a north pole repels a north pole, and a 

 south pole repels a south pole ; but a north and south pole mutu- 

 ally attract one another which proves that paramagnetism is a 

 dual power in which the conservation of force is perfectly main- 

 tained, for the force of attraction is exactly equal to the force of 

 repulsion. One kind of polarity cannot exist without the other : 

 they are absolutely simultaneous, dependent, and of equal inten- 

 sity. 



Induction is the power which a magnet possesses of exciting 

 temporary or permanent paramagnetism in such bodies in its 

 vicinity as are capable of receiving it. By this property the 

 mere approach of a magnet renders iron and steel paramagnetic, 



