196' Mr., J. Parker on the Theory of Magnetism and 



tism rests on insufficient experimental evidence, or rather, 

 on none at all, I may add that no one has ever attempted 

 to support the notion by theory. This will prepare the way 

 for my alternative idea, based on theory, that every substance 

 placed near one pole of a magnet is attracted by the pole 

 when the magnet is excited and can only be prevented from 

 approaching the pole by some antagonistic cause, such as 

 the pressures exerted by the contiguous objects. In the 

 common theory of diamagnetism, it is supposed that bismuth 

 is more powerfully repelled by a magnet pole than any 

 other substance; according to my theory, bismuth is more 

 feebly attracted than any other substance. 

 • Before giving the theoretical grounds of the new theory, 

 I cannot help observing that those who think the old theory 

 of diamagnetism sufficient, should try to prove that the 

 ascent of a balloon in the air is due to the diagravitational 

 repulsion of the earth. Those who accept the new theory, 

 will be content to regard diagravitation and diamagnetism as 

 companion absurdities. 



I was first led to reason theoretically on diamagnetism 

 by reading the following remarkable article in Prof. Tait's 

 6 Sketch of Thermodynamics \ : — " The commonly received 

 opinion, that a diamagnetic body in a field of magnetic force 

 takes the opposite polarity to that produced in a paramagnetic 

 body similarly circumstanced, is thus attacked by Thomson 

 by an application of the principle of energy. Since all para- 

 magnetic bodies require time for the full development of their 

 magnetism, and do not instantly lose it when the magnetizing 

 force is removed, we may, of course, suppose the same to be 

 true for diamagnetic bodies ; and it is easy to see that in 

 such a case a homogeneous non-crystalline diamagnetic sphere 

 rotating in a field of magnetic force would, if it always tended 

 to take the opposite distribution of magnetism to that acquired 

 by iron under the same circumstances, be acted upon by a 

 couple constantly tending to turn it in the same direction 

 round its centre, and would therefore be a source of the per- 

 petual motion/' 



This argument, which, for some unaccountable reason, seems 

 to have been entirely neglected, requires to be slightly mo- 

 dified; for no notice is taken of the frictional resistance of the 

 air to the rotation of the sphere, and it is not shown that the 

 motion supposed to be thus derived in unlimited quantities 

 may not be due to the absorption of heat at one temperature 

 and its partial rejection at a lower. These defects may be 

 obviated by supposing the sphere placed in a " perfect 

 vacuum," and making provision to prevent the system 



