of Magnetic and Diamagnetic Bodies. 47 



I could not be entirely satisfied by that theoretical contemplation 

 of the nature of diamagnetism which you, and after you Messrs. 

 Reich, Poggendorif, Weber, and Pliicker have entertained*. 

 This theory requires the hypothesis, that in every molecule of a 

 magnetic substance subject to external induction the magnetism 

 is so distributed, that to the inducing south pole is directed an 

 induced north pole, and to the inducing north pole an induced 

 south pole ; but that in diamagnetic substances the distribution 

 takes place in such a manner, that in eveiy molecule, to the in- 

 ducing south pole an induced south pole, and to the inducing 

 north pole an induced north pole is directed; or what is the 

 same thing, that the currents of the theoiy of Ampere in magnetic 

 substances are in the contrary direction to those in diamagnetic 

 substances. I tried rather to explain to myself the phfenomena 

 by an hypothesis which M. van Reesf had advanced ; conse- 

 quently I suppose that in magnetic, as in diamagnetic sub- 

 stances, the polarity of the molecules have the same direction ; 

 80 that all the north poles are turned to the south pole, and all 

 the south poles to the north pole of the inducing magnet ; only 

 with this difference, that in a bar of magnetic substance the in- 

 tensity of the distribution upon the molecules increases from the 

 ends to the middle, while in a bar of diamagnetic substance it 

 decreases from the ends to the middle. The currents of Ampere in 

 magnetic substances would be consequently more feebly directed 

 in a given particle than in those nearer the centre of the excita- 

 tion, but stronger than in those more distant ; but in diamag- 

 netic substances the reverse would be the case. 



These suppositions are pennitted, if we attribute to the two 

 groups of substances a diverse resistance to the magnetic excita- 

 tion (a different coercive power). The particles of a magnetic 

 body have a veiy feeble coercive power : then the distribution 

 of magnetism must take place in such a manner, that the mag- 

 netism evolved in every particle by the primitive excitation acts 

 on its part on the others, and particularly on the neighbouring 

 particles. Because the molecules are situated very near one to 

 the other, it may be supposed that this part of magnetism is 

 stronger than that of the primitive excitation. But in dia- 

 magnetic bodies the coercive power is so considerable, that this 

 portion of magnetism which appears by the excitation of the 

 molecules one on the other is more feeble than that which is pro- 

 duced by the primitive excitation. 



A bar of magnetic or diamagnetic substance may be excited in 



* I have not adopted the view referred to. See Phil. Trans. 1850, 

 p. 171.— M. F. 



t Memoirs of the Netherlands Institution, vol. xii. 



