348 POLARITY AND INDUCTION. [SEC*, xxx. 



dip of the magnetic needle was first noticed by Robert Norman, 

 in the year 1576. 



Very delicate experiments have shown that a variety of 

 substances are more or less susceptible of magnetism. Many 

 of the gems give signs of it ; cobalt and nickel always pos- 

 sess the properties of attraction and repulsion. But the 

 magnetic agency is most powerfully developed in iron, and 

 in that particular ore of iron called the loadstone, which 

 consists of the protoxide and the peroxide of iron, together 

 with small portions of silica and alumina. A metal is often 

 susceptible of magnetism if it only contains the 130,000th 

 part of its weight of iron, a quantity too small to be detected 

 by any chemical test. 



The bodies in question are naturally magnetic, but that 

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

 friction with magnetic bodies, or juxtaposition to them ; 

 but none is more simple than percussion. A bar of hard 

 steel, held in the direction of the dip, will become a mag- 

 net on receiving a few smart blows with a hammer on its 

 upper extremity ; and M. Hansteen has ascertained that 

 every substance has magnetic poles when held in that posi- 

 tion, whatever the materials may be of which it is composed. 



One of the most distinguishing marks of magnetism is 

 polarity, or the property a magnet possesses, when freely 

 suspended, of spontaneously pointing nearly north and south, 

 and always returning to that position when disturbed. 

 Another property of a magnet is the attraction of unmag- 

 netised iron. Both poles of a magnet attract iron, which in 

 return attracts either pole of the magnet with an equal and 

 contrary force. The magnetic intensity is most powerful 

 at the poles, as may easily be seen by dipping the magnet 

 into iron filings, which will adhere abundantly to each pole, 

 while scarcely any attach themselves to the intermediate 

 parts. The action of the magnet on unmagnetised iron is 

 confined to attraction, whereas the reciprocal agency of 



