449 



quiry, it must be remembered that the force of the argument in 

 question depends essentially upon the justness of the supposition, 

 that a diamagnetic cube may, theoretically, be produced from a 

 magnetic one by conceiving the magnetic capacity of the particles 

 of 'the latter to be sufficiently diminished. It is evident that the 

 total attraction of the cube by a magnet will be equal to the sum of 

 the attractions of the material particles, and of the medium con- 

 tained in its interstices. If this sum be greater than the attracting 

 force upon the quantity of medium which the cube and its contents 

 displace, the substance is called magnetic, for it will be drawn 

 towards the magnet ; if less, it is called diamagnetic, for it will be 

 repelled from the magnet. But in our present knowledge of the 

 properties of the medium there is nothing incompatible with the 

 supposition that the density of the internal medium may so far ex- 

 ceed that of the external, that the attraction of the former by the 

 magnet is itself greater than the attraction of the medium displaced 

 by the cube and its contents. If so, however, no conceivable dimi- 

 nution of the magnetic capacity of the material particles could pos- 

 sibly render such a cube diamagnetic. 



This is sufficient to show that, admitting the presence of the me- 

 dium within the cube, the method of argument adopted by Professor 

 Tyndall would not be strictly applicable, unless the density of the 

 internal medium were subjected to limits which the advocates of its 

 existence might possibly be unwilling to grant. 



But it may be asked, if, whilst admitting that the medium may 

 exist in the interstices of the body, it be granted that a diamagnetic 

 may be produced from a magnetic cube in the manner assumed by 

 Professor Tyndall, does it still follow, necesarily, that attraction is 

 always greatest repulsion least when the force acts in the line of 

 compression ? In other words, can a conclusion contradictory to 

 experimental facts be then legitimately deduced ? 



In attempting a reply to this question, it will, perhaps, be best to 

 employ the following symbols. Let W represent the attracting 

 force of the magnet upon the medium displaced by the cube and its 

 contents. The value of W will, of course, be unaltered, no matter 

 whether the force acts in, or at right angles to the line of compres- 

 sion. When the force acts in the line of compression, let P l repre- 

 sent the attracting force upon the particles, W l the attracting force 



