194 Mr. J. Parker on the Theory of Magnetism and 



Experiments, which need not here be described, have been 

 made with many solids and with liquids and gases, from 

 which it is inferred that all substances are susceptible to 

 magnetic influence, and it is supposed necessary to divide 

 them into two classes : paramagnetic, those which, when 

 placed near one pole of a magnet, are supposed to be attracted; 

 diamagnetic, those which, under the same circumstances, are 

 supposed to be repelled. 



I first of all deny that (a) can be correct. For let an 

 ideal right circular cylinder PQRS, indefinitely small in all 

 its dimensions, be described in the air anywhere near the 

 pole P of the electromagnet such that at no point within the 

 cylinder is the magnetic force at right 

 angles to the axis of the cylinder. Also let Fig. 4. 



the ends PQ, ES of the cylinder be normal 

 sections and suppose that the radius of each 

 is indefinitely small in comparison with the 

 length of the cylinder. Then since, in the 

 state of equilibrium, the pressure of the mag- 

 netized air on any surface is evidently & 

 normal to that surface, just as if gravitation 

 were the only action at a distance, it follows that the pressure 

 of the air, per unit of area, at the end PQ of the cylinder is 

 not the same as the pressure at the end PS. Thus in the 

 space around the excited pole P, the pressure of the magne- 

 tized air varies from point to point, and therefore the assump- 

 tion (a) cannot be correct. 



The explanations (a) and (b) must therefore be modified 

 as follows : — It must be admitted that in the above experi- 

 ment the pressure of the magnetized air is not quite uniform 

 over the surface of M, but it may be supposed that the 

 resultant pressure is so small in comparison with the action 

 of the pole P that it is still necessary to divide bodies into 

 two classes, viz. those which are attracted by the pole P, 

 and those which are repelled by it. 



Now if we recollect that the so-called diamagnetic bodies 

 (like bismuth), which, when the magnet is excited, recede 

 from the pole P, are caused to do so by a force which is 

 evidently very minute, it will be easy to show that a very 

 slight inequality (too small to be detected) in the pressure of 

 the air on the surface of M is quite competent to produce the 

 phenomena observed. Diamagnetic bodies may therefore be 

 really attracted to the pole P, and only caused to recede from 

 it by an excess in the pressure of the air on the side of M 

 next to P over the pressure on the side remote from P ; just 

 as a balloon is attracted to the earth and only caused to 

 ascend by the pressure of the air. 



