MAGNETIC AND DIAMAGNETIC SUBSTANCES. 281 



Some artificial magnets retain this new property when the cause 

 which produced it has ceased to act ; these are permanent magnets. 

 Tempered steel is the body best suited for preparing permanent mag- 

 nets, and is ordinarly employed in the form of long rods or bars. 



Different kinds of cast and of wrought iron may also be power- 

 fully magnetised by natural or by artificial magnets, but they lose 

 most of their properties when the magnetising agent has ceased to 

 act. In this way temporary magnets are obtained ; and the term 

 residual magnetism is applied to the comparatively feeble mag- 

 netisation which persists, at any rate for some time, in bodies 

 which have been temporarily magnetised. 



289. MAGNETIC AND DIAMAGNETIC SUBSTANCES. Until the 

 present century, iron was the only substance which was known to 

 be attracted by magnets ; it was afterwards found that certain metals, 

 such as nickel and cobalt, whose chemical analogies with iron are so 

 remarkable, also possess these properties, but to a smaller extent. 

 It has further been found, by means of very powerful magnets, that 

 a great number of substances are also attracted by magnets, but the 

 actions are incomparably weaker. The term magnetic is applied to 

 all bodies which can be attracted by a magnet, and the term mag- 

 netism is applied to the whole of the phenomena to which magnets 

 give rise, and, by extension, to the cause of these phenomena. 



In 1778, Bruginans observed that a piece of bismuth is repelled 

 by a magnet. The importance of this observation was disregarded, 

 until Faraday's discovery that a certain number of other substances 

 also possess this property. From the special way in which the 

 experiment was made, Faraday applied the term diamagnetic to 

 bodies which are repelled by magnets. 



We may say, in short, that all bodies in nature are more or less 

 susceptible of the action of magnets. They have been divided into 

 two groups magnetic, paramagnetic, or positive substances, which are 

 attracted, like iron ; and diamagnelic or negative substances, which are 

 repelled by magnets, like bismuth. 



290. DISTRIBUTION OF MAGNETISM IN MAGNETS. POLES. 

 We may obtain a bar magnetised regularly a needle, for example 

 by rubbing it several times, and always in the same direction, with 

 a piece of natural magnet, or with the same end of any artificial 

 magnet. 



When such a needle is placed in iron filings, the particles attach 

 themselves more particularly to the ends of the needle, to a certain 

 distance from them, and they stick end to end to each other, forming 

 more or less abundant tufts. 



