308 LAW OF MAGNETIC INTENSITY. SECT. XXX. 



by the same needle, during the same time, at different 

 distances from a magnet, gives the law of magnetic in- 

 tensity, which follows the inverse ratio of the squares of 

 the distances, a law that is not affected by the inter- 

 vention of any substance whatever between the magnet 

 and the needle, provided that substance be not itself 

 susceptible of magnetism. Induction and the reciprocal 

 action of magnets are therefore subject to the laws of 

 mechanics ; but the composition and resolution of the 

 forces are complicated, in consequence of four forces 

 being constantly in activity, two in each magnet. 



Mr. Were Fox, who has paid much attention to this 

 branch of the science, has lately discovered that the law 

 of the magnetic force changes from the inverse squares 

 of the distances, to the simple inverse ratio, when the 

 distance between two magnets is as small as from the 

 fourth to the eighth of an inch, or even as much as half 

 an inch when the magnets are large. He found, that 

 in the case of repulsion, the change takes place at a still 

 greater distance, especially when the two magnets differ 

 materially in intensity. 



There can hardly be a doubt but that all the phenom- 

 ena of magnetism, like those of electricity, may be ex- 

 plained on the hypothesis of one ethereal fluid, which is 

 condensed or redundant in the positive pole, and deficient 

 in the negative ; a theory that accords best with the sim- 

 plicity and general nature of the laws of creation ; never- 

 theless, Baron Poisson has adopted the hypothesis of 

 two extremely rare fluids pervading all the particles of 

 iron, and incapable of leaving them. Whether the par- 

 ticles of these fluids are coincident with the molecules 

 of the iron, or that they only fill the interstices between 

 them, is unknown and immaterial. But it is certain that 

 the sum of all the magnetic molecules, added to the sum 

 of all the spaces between them, whether occupied by 

 matter or not, must be equal to the whole volume of the 

 magnetic body. When the two fluids in question are 

 combined they are inert, so that the substances contain- 

 ing them show no signs of magnetism ; but when sepa- 

 rate they are active, the molecules of each of the fluids 

 attracting those of the opposite kind, and repelling those 

 of the same kind. The decomposition of the united 



