CHAP, ii.] ELECTRICITY AND MAGNETISM. 81 



coercive force; a much better term, due to Lament, 

 is retentivity. The retentivity of hard-tempered steel 

 is great ; that of soft wrought iron is very small. The 

 harder the steel, the greater its retentivity. 



91. Theories of Magnetism. The student will 

 not have failed to observe the striking analogies between 

 the phenomena of attraction, repulsion, induction, etc., 

 of magnetism and those of electricity. Yet the two sets 

 of phenomena are quite distinct. A positively electrified 

 body does not attract either the North -pointing or the 

 South -pointing pole of the magnet as such; in fact, it 

 attracts either pole quite irrespective of its magnetism, 

 just as it will attract any other body. There does 

 exist, indeed, a direct relation between magnets and 

 currents of electricity, as will be later explained. There 

 is none known, however, between magnets and stationary 

 charges of electricity. 



No theory as to the nature of magnetism has yet 

 been placed before the reader, who has thus been told 

 the fundamental facts without bias. In many treatises 

 it is the fashion to speak of a magnetic fluid or fluids ; 

 it is, however, absohitely certain that magnetism is not 

 a fluid, whatever else it may be. The term, which is a 

 relic of bygone times, is only tolerated because, under 

 certain circumstances, magnetism distributes itself in 

 magnetic bodies in the same manner as an elastic 

 fluid would do. Yet the reasons against its being a 

 fluid are even more conclusive than in the case of 

 electricity. An electrified body when touched against 

 another conductor, electrifies the conductor by giving 

 up a part of its electricity to it. But a magnet when 

 rubbed upon a piece of steel magnetises it without 

 giving lip or losing any of its own magnetism. A fluid 

 cannot possibly propagate itself indefinitely without loss. 

 The arguments to be derived from the behaviour of 

 a magnet on breaking, and from other experiments 

 narrated in Lesson X., are even stronger. No theory 

 G 



