280 PRELIMINARY. 



PART III. MAGNETISM. 

 CHAPTER I. 



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PRELIMINARY. 



287. ON MAGNETS. From the earliest times the name of load- 

 stone has been given to certain natural ores which have the property 

 of attracting iron filings ; they consist of an oxide of iron whose 

 chemical formula is Fe 3 O 4 . The various parts of a loadstone possess 

 these properties of attraction to unequal extents : the filings attach 

 themselves in preference to certain parts of the surface in the form 

 of tufts. 



These phenomena have an evident resemblance to those of 

 statical electricity. The analogy, however, is not complete, and 

 observation indicates essential differences between them; thus, the 

 loadstone does not act indiscriminately upon all substances ; the 

 filings when attracted are not repelled after contact, and when 

 once detached are not found to possess any new property. At 

 each step in the course of this new study, we shall have to point 

 out analogies and differences of this kind between the two orders 

 of phenomena. 



The loadstone, by mere rubbing, can magnetise steel that is to 

 say, can impart to it the property of attracting iron, and this without 

 losing any of its own power. As steel bars, magnetised artificially, 

 have a more regular form than loadstones, they are more convenient 

 for investigation ; experiment shows, moreover, that the phenomena 

 are of exactly the same nature in the two cases. 



288. MAGNETS NATURAL AND ARTIFICIAL, PERMANENT AND 

 TEMPORARY. The general term magnet is given to all substances 

 which have the property of attracting iron filings. Natural magnets 

 are the pieces of magnetic ore found in nature; artificial magnets 

 are pieces of steel, or specimens of iron more or less pure, to which 

 the same properties have been imparted. 



