36 Natural Magnets. [Book I. 



in fpecific gravity, and feem to contain, feveral fub- 

 ftances befides iron in their compofition, fuch as argil- 

 laceous and filiceous earth. Mr. Kirwan is of opi- 

 nion that fulphur enters into their fubftance, as the ore 

 fmells of it when heated red hot : probably, alfo, they 

 may fometimes contain nickel, as that metal, when 

 purified to a certain degree, acquires the properties of 

 the magnet. 



Natural magnets are found more or lefs in almofl 

 every iron-mine, of different mapes, and of different 

 iizes. Some old writers mention magnets that would 

 fwim on water *; thefe were probably fome light, 

 fpongy, volcanic fubftances, impregnated with iron. 

 In Virginia there is a magnetic fand, which contains 

 about one half iron. Thofe magnets which have the 

 fineft grain poffefs the magnetic virtue in the higheft 

 perfection, and retain it longer than any other f. 



The great and well-known properties of the mag- 

 :net are, ifl, its attractive power; 2dly, its polarity, 

 or difpofition to conform to the plane of the meridian j 

 ^dly, the property of dipping or inclining to the earth; 

 and laftly, the power of communicating the magnetic 

 virtues to other ferrugineous bodies. 



I. Magnets attract clear iron more forcibly than 

 any other ferrugineous body. The iron ores are at- 

 tracted more or lefs forcibly, in proportion as they are 

 impregnated with the metal, and in proportion as it 

 exifts in a metallic date. The force of the attraction 

 between a magnet and iron will^depend in a great 

 meafure ort the weight and form of the iron j but art 

 and obfervation have furnilhed us with no invariable 

 rule in thefe refpects. 



They are in general about feven limes heavier ilun water. 

 f Kirwan and Cavallo. 



On* 



