Mr. T. A. Hirst on the Existence of a Magnetic Medium. 443 



conclusion deduced by Professor Tyndall in a letter addressed by the 

 latter to Mr. Faraday and recently published in the Philosophical 

 Magazine. Professor Tyndall's conclusion was that, according to 

 the hypothesis of the existence of a magnetic medium in space and 

 of the identity of magnetism and diamagnetism, a compressed dia- 

 magnetlc cube ought to be less repelled when the magnetic force 

 acts on the line of compression than when it acts at right angles to 

 that line ; a result which his own experiments have contradicted. 

 Against the legitimacy of this conclusion Professor Williamson 

 urges that "Dr. Tyndall seems to have assumed, that on the com- 

 pression of an aggregate of particles of a diamagnetic substance, the 

 medium is not displaced by the particles in their change of posi- 

 tion." We shall be better able to estimate the value of this objec- 

 tion by recalling the steps of Professor Tyndall's argument. 



A magnetic cube was taken which had already been compressed : 

 its deportment before a magnet was experimentally examined, and 

 deductions drawn concerning the changes that would occur in that 

 deportment by merely conceiving the magnetic capacity of the 

 material particles to be diminished, without in any way altering 

 the distances between those particles, and consequently without 

 in any way dis2Jlacing the magnetic medium in the interstices of 

 the body. 



Instead of the assumption attributed to Dr. Tyndall, he might, 

 with greater justice, be accused of having disregarded the possible 

 presence of the medium within the body ; but in his own defence he 

 may Avith perfect justice reply, that in Mr. Faraday's experiments, 

 which originally gave rise to the discussion, no such interpenetration 

 of two media existed. 



Admitting, however, that the interstices of a body are occupied 

 by the medium, it may be interesting to inquire whether, from an 

 argument similar to Professor Tyndall's, the same decided conclu- 

 sion could, with equal accuracy, be deduced. To answer the in- 

 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 culjc 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 disj)laced 

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

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

 sibly render such a cube diamagnetic. 

 ^02 



