SECT. XXX. BARON POISSON'S HYPOTHESIS. 309 



fluids is accomplished by the inductive influence of either 

 of the separate fluids ; that is to say, a ferruginous body 

 acquires polarity by the approach of either the south or 

 north pole of the magnet. The magnetic fluids pervade 

 each molecule of the mass of bodies, and in all proba- 

 bility the electric fluid does the same, though it appears 

 to be confined to the surface ; if so, a compensation must 

 take place among the internal forces. The electric 

 fluid has a perpetual tendency to escape, and does es- 

 cape, when not prevented by the coercive power of the 

 surrounding air and other non-conducting bodies. Such 

 a tendency does not exist in the magnetic fluids, which 

 never quit the substance that contains them under any 

 circumstances whatever ; nor is any sensible quantity of 

 either kind of polarity ever transferred from one part to 

 another of the same piece of steel. It appears that the 

 two magnetic fluids, when decomposed by the influence 

 of magnetizing forces, only undergo a displacement to 

 an insensible degree within the body. The action of all 

 the particles so displaced upon a particle of the magnetic 

 fluid in any particular situation, compose a resultant 

 force, the intensity and direction of which it is the prov- 

 ince of the analyst to determine. In this manner M. 

 Poisson has proved that the result of the action of all 

 the magnetic elements of a magnetized body, is a force 

 equivalent to the action of a very thin stratum covering 

 the whole surface of a body, and consisting of the two 

 fluids the austral and the boreal, occupying different 

 parts of it ; in other words, the attractions and repul- 

 sions externally exerted by a magnet, are exactly the 

 same as if they proceeded from a very thin stratum of 

 each fluid occupying the surface only, both fluids being 

 in equal quantities, and so distributed that their total 

 action upon all the points in the interior of the body is 

 equal to nothing. Since the resulting force is the differ- 

 ence of the two polarities, its intensity must be greatly 

 inferior to that of either. 



In addition to the forces already mentioned, there 

 must be some coercive force analogous to friction, which 

 arrests the particles of both fluids, so as first to oppose 

 their separation, and then to prevent their reunion. In 

 soft iron the coercive force is either wanting or ex- 



