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CHAP. III. 



MAGNETISM. 



1 HE theory of magetism bears a very strong resemblance to that 

 of electricity, and it must therefore be placed near it in a system of 

 natural philosophy. We have seen the electric fluid not only exert- 

 ing attractions and repulsions, and causing a peculiar distribution of' 

 neighbouring portions of a fluid similar to itself, but also excited in 

 one body, and transferred to another, in such a manner as to be 

 perceptible to the senses, or at least to cause sensible effects, in 

 its passage. The attraction and repulsion, and the peculiar distribu- 

 tion of the neighbouring fluid, are found in the phenomena of mag. 

 netism ; but we do not perceive that there is any actual excitation, 

 or any perceptible transfer of the magnetic fluid from one body to 

 another distinct body; and it has also this striking peculiarity, that, 

 metallic iron is very nearly, if not absolutely, the only substance 

 capable of exhibiting any indications of its presence or activity. 



For explaining the phenomena of magnetism, we suppose the par. 

 tides of a peculiar fluid to repel each other, and to attract the par. 

 tides of metallic iron with equal forces, diminishing as the square of 

 the distance increases ; and the particles of such iron must also be 

 imagined to repel each other, in a similar manner. Iron and steel, 

 when soft, are conductors of the magnetic fluid, and become less 

 and less pervious to it as their hardness increases. The ground 

 work of this theory is due to Mr. Aepinus, but the forces have been 

 more particularly investigated by Coulomb, and others. There are 

 the same objections to these hypotheses as to those which constitute 

 the theory of electricity, if considered as original and fundamental 

 properties of matter : and it is additionally difficult to imagine, 

 why iron, and iron only, whether apparently magnetic or not, should 

 repel similar particles of iron with a peculiar force, which happens 

 to be precisely a balance to the attraction of the magnetic fluid for 

 iron. This is obviously improbable ; but the hypotheses are still of 

 great utility in assisting us to generalise, and to retain in memory 

 a number of particular facts which would otherwise be insulated. 

 The doctrine of the circulation of streams of the magnetic fluid hue 

 been justly aud universally abandoned -, and some other theories, 

 much more ingenious, and more probable, for instance that of Mr. 



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