242 HEAT AND MAGNETISM. 



natural cause which certainly exercises a very remark- 

 able power over matter, and we have advanced so far in 

 our investigations as to have learnt the secret of convert- 

 ing one form of force into another, or of giving to a 

 principle, produced by one agency, a new character 

 under new conditions ; of changing, in fact, electricity 

 into magnetism, and from magnetism again evolving 

 many of the effects of electrical currents. 



If a magnetic bar- is freely suspended above the 

 earth, it takes, in virtue of some terrestrial power, a 

 given direction, which is an indication of the earth's 

 magnetic force. Whether this is the consequence of 

 the currents of electricity, which Ampere supposes to 

 circulate around the globe, from east to west, or the 

 result of points of attraction in the earth itself, the 

 phenomenon is equally wonderful. To whatever cause 

 we may refer the visible effects, it appears certain that 

 this earth is composed of particles in a magnetic state, 

 the character varying with physical conditions, and that 

 terrestrial magnetic force is the collective action of all 

 the atoms of this planetary mass.* 



* " The foundation of our researches is the assumption that the 

 terrestrial magnetic force is the collective action of all the magne- 

 tised particles of the earth's mass. We represent to ourselves mag- 

 netisation as the separation of the magnetic fluids. Admitting 

 the representation, the mode of action of the fluids (repulsion of 

 similar, and attraction of dissimilar, particles inversely as the 

 square of the distance) belongs to the number of established 

 truths. No alteration in the results would be caused by changing 

 this mode of representation for that of Ampere, whereby, instead 

 of magnetic fluids, magnetism is held to consist in constant gal- 

 vanic currents in the miniitest particles of bodies. Nor would it 

 occasion a difference if the terrestrial magnetism were ascribed to 

 a mixed origin, as proceeding partly from the separation of the 

 magnetic fluids in the earth, and partly from galvanic currents, 

 in the same ; inasmuch as it is known that for each galvanic 

 current may be substituted such a given distribution of the mag- 

 netic fluids' in a surface bounded by the current, as would exercise 

 in each point of external space precisely the same magnetic action 

 as would be produced by the galvanic current itself" General 



