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considered magnetic. For this purpose it is only necessary to 

 plunge the magnetic body into a yielding medium more magnetic 

 than itself. When thus exposed to the action of a magnet it recedes 

 from the poles, because the volume of the medium which it dis- 

 places is more powerfully attracted. 



This fact naturally suggested the idea, that all repulsion by the 

 magnet might be owing to the attraction exercised on the medium 

 being stronger than that on the body repelled ; just as balloons are 

 driven upwards by the superior weight of the displaced volume of 

 air. And as phenomena of diamagnetism are observed in a so-called 

 vacuum, it was thought that some "magnetic medium" might be 

 present there. 



I do not purpose on this occasion to enter upon the general 

 question of the evidence which may be adduced for or against this 

 important conclusion ; for it could only be proved satisfactorily by 

 considerations including phenomena of the most varied kind, such 

 as electricity, light, chemical action, &c., to which it must neces- 

 sarily apply. But it might be disproved by any one well-understood 

 fact contradictory to it. 



Now it appears to me, that the facts adduced by Dr. Tyndall are 

 not inconsistent with the notion of a magnetic medium, but follow 

 naturally from it ; and that his argument involves a tacit assumption 

 foreign to the theory under consideration. 



The first fact adduced is, that compression increases the attraction 

 of magnetic bodies, and the repulsion of diamagnetic bodies by the 

 magnet, in the direction of the line of compression. Now it is evident, 

 that a variation of pressure on a number of particles surrounded 

 by a magnetic medium may alter the attraction of the mass by a 

 magnet in two ways ; first, by altering the density of the matter*; 

 secondly, by altering the density of the medium. 



In a cubical mass of carbonate of iron the material particles are 

 more magnetic than the medium which they displace, and the force 

 with which it is attracted is proportional to this excess. 



If it becomes more magnetic by compression, we must conclude 

 that the loss of magnetic medium from its interstices is more than 

 supplied by the magnetic matter which takes its place. 



Carbonate of lime is less magnetic than the quantity of medium 



* The word " matter" is here used for brevity to denote ponderable matter. 

 VOL. VII. 2 H 



