Mr. T. A. Hirst on the Existence of a Magnetic Medium. 445 



according as Pj is greater or less than ?„, Mj is less or greater than 

 M^, so that the two hypotheses 



( 1 ) Pi > P., and Ml < Mj, and 



(2) P,<P2andM,>]\l2 

 are both compatible with the sole condition, 



Pi + Mi>P2+M,. 

 In order to test the apphcability of Professor Tyndall's method of 

 argument in each of these cases, let us conceive, with him, that the 

 magnetic capacity of the particles is so far diminished that their 

 attractions Pj and P., are reduced to p, and p^. This may evidently 

 be done by making j!?j = aP, and p.^=a?.,, where n represents a posi- 

 tive fraction whose magnitude can be diminished indefinitely. If, 

 under this supposition, the resultant attractions F, and F^ become 

 /i and/2, tben it can easily be proved that, however small the value 

 of a may be, we shall always have 



in the cases I., II. and III. (1). That is to say, with the distribu- 

 tions of the internal medium assumed in these cases, the resultant 

 attraction of the cubical mass will always be greatest, or repulsion 

 least, when the force acts in the Hne of compression, no matter how 

 diamagnetic the cube may have become by diminishing the magnetic 

 capacity of its particles. It may just as easily be proved, however, that 

 in case III. (2) a value of a may be chosen sufficiently small to make 



that is to say, with the distribution and properties of the internal 

 mediuni here supposed, it is quite possible so far to diminish the 

 magnetic capacity of the particles as to obtain a cube which will be 

 attracted least, or repelled most strongly when the force acts in the 

 line of compression. This conclusion involves nothing contradictory 

 to experimental facts, whereas the former one does. 



I will not here enter into the question of the relative probability 

 of these three cases, supposing the medium to exist. My sole ob- 

 ject has been to show that, although the method of argument adopted 

 by Professor Tyndall is strictly applicable in a great number of 

 cases, even when the medium is supposed to fill the interstices of 

 the body, yet it is possible to attribute properties to this medium of 

 such a nature as to avoid the conclusion, contradictory to fact, which 

 he has deduced. This may be done in two ways. First, the den- 

 sity of the internal medium may be such as to render it impossible 

 to produce a diamagnetic from a magnetic cube in the manner 

 assumed, i. e. by diminishing the magnetic capacity of the material 

 particles. Secondly, granting that a diamagnetic cube may be so 

 produced, the distribution and properties of the internal medium 

 may still be such as to cause the cube to be attracted least, or repelled 

 most .strongly when the force acts on the line of compression, and 

 thus, if the substance be diamagnetic, to cause it to agree, in its 

 deportment, with experimental results. On the other hand, if these 



