DEFECT OF POISSON'S THEORY. 157 



positions, and indication of the wants of a new 

 theory. 



It will be remarked in Poisson's theory (and also in 

 the theory of Article 63) that it is assumed that the 

 magnetism of both kinds originally attached to one 

 molecule is never moved beyond the region of that 

 molecule ; and it is also implied in Poisson's assump- 

 tions, that the disturbance of this molecular magnetism 

 is produced by the actions of other molecules or masses, 

 without regard to the question whether those molecules 

 or masses are in the same continuous substance. It 

 follows from this that, according to the fundamental 

 suppositions of both theories, if a given mass of iron be 

 divided into any number of parts, the state of its mag- 

 netism and its action upon a compass-needle will not 

 be altered. At the dividing planes there may be 

 (theoretically) on one surface a peculia state of mag- 

 netism, but this will be accompanied by equal magnet- 

 ism of the opposite kind on the other surface, and the 

 proposition will still hold. The truth of the theories is 

 easily tested by such experiments as the following : 



Provide a bar of iron, 6 inches long, and also four 

 bars whose section is the same, but each 1J inch long. 

 Ascertain (by applying each end of each bar centrally 

 to the E or W side of the compass) that they possess no 

 permanent magnetism. Now apply the long bar 

 endways at azimuth 45, at any distance at which 

 it will produce a quadrantal deviation of several 

 degrees. Remove the long bar, and put the four 



