358 FKAGMENTS OF SCIENCE. 



a symbol, moreover, which is incompetent to cover all 

 the facts,* but which does good practical service whilst 

 we are waiting for the actual truth. 



The state of excitement into which iron is thrown 

 by the influence of a magnet, is sometimes called 

 ' magnetisation by influence.' More commonly, how- 

 ever, the magnetism is said to be ' induced ' in the 

 iron, and hence this mode of magnetising is called 

 ( magnetic induction/ Now, there is nothing theo- 

 retically perfect in Nature: there is no iron so soft as 

 not to possess a certain amount of coercive force, and 

 no steel so hard as not to be capable, in some degree, 

 of magnetic induction. The quality of steel is in some 

 measure possessed by iron, and the quality of iron is 

 shared in some degree by steel. It is in virtue of this 

 latter fact that the unmagnetised darning-needle was 

 attracted in your first experiment; and from this you 

 may at once deduce the consequence that, after the 

 steel has been magnetised, the repulsive action of a 

 magnet must be always less than its attractive action. 

 For the repulsion is opposed by the inductive action of 

 the magnet on the steel, while the attraction is assisted 

 by the same inductive action. Make this clear to your 

 minds, and verify it by your experiments. In some 

 cases you can actually make the attraction due to the 

 temporary magnetism overbalance the repulsion due to 

 ihe permanent magnetism, and thus cause two poles of 

 the same kind apparently to attract each other. When, 

 however, good hard magnets act on each other from a 

 sufficient distance, the inductive action practically van- 



* This theory breaks down when applied to diamagnetic bodies 

 which are repelled by magnets. Like soft iron, such bodies are 

 thrown into a state of temporary excitement, in virtue of which 

 they are repelled ; but any attempt to explain such a repulsion 

 by the decomposition of a fluid will demonstrate its own futility. 



