34 



MAGNETISM. 



rallel to one another. Franklin's inge- 

 nious theory of the two modes of elec- 

 tric agency, the one consisting in an 

 excess, the other in a deficiency of fluid, 

 and the happy explanation it afforded 

 of the opposite electrical states resulting 

 from induced electricity, and its accu- 

 mulation in the Leyden phial, were ap- 

 plied with great ingenuity by JEpinus to 

 the contrariety of magnetic polarities in 

 the opposite ends of a magnet, and the 

 induction of similar magnetic states in 

 an unmagnetized bar of iron. His sys- 

 tem of magnetism, when digested into a 

 series of propositions, may be stated as 

 follows : 



(134.) 1. There exists in all bodies 

 capable of acquiring magnetic proper- 

 ties, a subtile fluid, which may be called 

 the magnetic fluid. 



2. The particles of this fluid repel one 

 another with a force which decreases as 

 the distance increases. 



3. The particles of the magnetic fluid 

 attract, and are attracted by the parti- 

 cles of iron, with a force varying accord- 

 ing to the same law. 



4. The particles of iron repel one 

 another according to the same law. 



5. The magnetic fluid is ijicapable of 

 quitting the body in which it is contained, 

 but it is capable of moving within the 

 substance of pure iron and of soft steel 

 without any considerable obstruction. 

 It is more and more impeded in its mo- 

 tion as the steel is tempered harder; 

 and in very hard tempered steel, and in 

 some of the ores of iron, it moves with 

 the greatest difficulty. 



(135.) In order to judge of the degree 

 in which the theory is qualified to re- 

 present the facts, we must study the se- 

 veral consequences which flow from the 

 above suppositions, and then compare 

 them with the actual phenomena which 

 are presented to our observation. 



(136.) Each particle of iron, by the 

 hypothesis, attracts a particle of mag- 

 netic fluid, placed at any particular dis- 

 tance, with a certain force. We may 

 conceive that magnetic fluid is gradually 

 added to the particle, until the quantity 

 thus added is such that the force of re- 

 pulsion which the fluid exerts upon any 

 distant particle of magnetic fluid, ex- 

 actly balances the attractive force of 

 the iron for that same particle. This 

 quantity may be regarded as the natural 

 quantity of fluid belonging to that par- 

 ticle of iron. According to this definition, 

 therefore, a mass of iron, all the par- 

 ticles of which contain their natural 



quantity of fluid, must be neutral with 

 regard to its action on any other par- 

 ticle of fluid, and also on any other par- 

 ticle of iron. Such is the condition of 

 unmagnetical iron or steel. Its mag- 

 netism is neutral, or in a state of equi- 

 librium. 



(137.) But should, from any cause, 

 this state of equilibrium be destroyed, 

 and magnetic fluid be either accumu- 

 lated beyond its natural quantity as re- 

 lates to the iron, or reduced below that 

 proportion, the part where this excess 

 or this deficiency exists becomes active 

 that is, acquires the properties of 

 either a north or a south pole. As the 

 fluid can never pass beyond the surface 

 of the mass of iron in which it is con- 

 tained, the total quantity residing in that 

 mass must remain precisely the same, 

 whatever be its mode of distribution; 

 and therefore the excess of fluid in those 

 parts where it is accumulated or re- 

 dundant, must be exactly compensated 

 by the redundant iron, if we may so ex- 

 press it, in those parts where the fluid 

 is deficient. In all cases it will be only 

 the redundant fluid or the redundant 

 iron that constitutes the active parts of 

 the magnet. 



(138.) It follows as a direct conse- 

 quence of the second condition of the 

 hypothesis, that the pole of one magnet 

 in which the fluid is redundant will repel 

 the pole of another magnet in which it 

 is also redundant ; because the fluid in 

 each is mutually repulsive of the other. 



(139.) From the third condition of 

 the same hypothesis, it likewise follows 

 that the pole having an excess of fluid, 

 or the overcharged pole, as we may 

 call it, of one magnet, will attract and 

 be attracted by the pole in which the 

 fluid is deficient, or the undercharged 

 pole of the other ; and this action must 

 be reciprocal. 



(140.) It is also a necessary conse- 

 quence of the fourth condition that the 

 redundant iron in the undercharged pole 

 of one magnet repels every similarly con- 

 stituted pole in other magnets ; because, 

 by the hypothesis, iron repels iron. 



(141.) Hence we deduce the general 

 law that similar poles repel, and dissi- 

 milar poles attract one another, a law 

 identical with that we have already de- 

 duced from experiment. 



(142.) Let us next see what account 

 the theory gives us of the induction of 

 magnetism. If the overcharged pole of 

 a magnet be brought near the end of a 

 bar of iron in its natural or unmagneti- 



