270 ELECTRICITY. 



originates in polarization, and like electricity, communi- 

 cates its powers with an undulatory motion. One zone 

 of polarity must acquire its maximum of expansion, 

 and thus give rise to another. This is confirmed by ex- 

 perience ; for in magnetising a very fine steel wire, it ac- 

 quires the alternate poles of north and south, in its whole 

 length. 



To understand the propagation of magnetism, we must 

 reflect a little upon its production ; and examine what 

 effect is produced by drawing a magnet over a bar of 

 steel : the two poles are impelled forward, so that the 

 part which was -J- ?» becomes — m; like a wave of the 

 sea, which fills up a furrow before it, whilst it leaves 

 another behind. 

 AU the opera- This_ mechanism in the action of undulatory propaga- 

 ^r^'^'th^ °^'"^^ *^°"' *^ doubtless general in all the operations of nature ; 

 pagated. but it is very difficult to shew it. It has long ago been 



observed, that the compression of a small portion" of air 

 is succeeded by its expansion, whereby contiguous por- 

 tions must be compressed, and these, by expanding in 

 their turn, compress others, &c. It is thus that the com- 

 munication of sound through the air has been accounted 

 for ; but this mode of communication has not yet been 

 suspected to take place through solid bodies. 

 Chladnl'sexpc- The majority of philosophers oppose the discovery of 

 rimcnts. the celebrated Chladni*, of the tremulous motion of the 



particles of bodies in the production of sound. But no- 

 thing is more easy to demonstrate both from the nature of 

 the thing, and by experiments the necessity and the exist- 

 ence of this tremor. We need not much insist upon the 

 theory, because the same proofs which are adduced to 

 demonstrate the imdulatory communication of sound 

 through the air, may be applied to all other bodies ; be- 

 sides as motion cannot be communicated without employ- 

 ing some portion of time, all the particles cannot be simi- 

 larly affected at the same moment. 

 Experiment This is manifested in the following experiment : cover 

 with a wireand qj^^ ^^f ^j^^ extremities of a steel wire with powder of Ly- 

 copodium, and then strike it with a sharp and moderately 



♦ Inserted in the Journal de Physique, Vol 47i ?• 390. 



hard 



