68i 



LECTURE LV. 



ON MAGNETISM. 



.llIE theory of magnetism bears a very strong resemblance to that of elec- 

 tricity, and it must therefore be placed near it in a system of natural philoso- 

 phy. We have seen the electric fluid not only exerting attractions and re- 

 pulsions, 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 

 eft'ects, in its passage. The attraction and repulsion, and the peculiar distri- 

 bution of the neighbouring fluid, are found in tlie phenomena of magnetism; 

 but we do not perceive that there is ever any actual excitation, or any per- 

 ceptible 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 particles of a 

 peculiar fluid to repel each other, and to attract the particles of metallic iron 

 with equal forces, diminishing as the square of the distance increasqs; and 

 the particles of such iron must also be imagined to repel each other, in a si- 

 milar 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 pe- 

 culiar force, which happens to be precisely a balance to the attraction of the 

 magnetic fluid for iron. This is obviously improbable; but tlie hypotlieses 



VOL. I. 4 Q 



