448 



IX. " On the Existence of a Magnetic Medium." By T. A. 

 HIRST, Esq. Communicated by Dr. TYNDALL, F.R.S. 

 Received April 14, 1855. 



In a note on the above subject, communicated to the Royal So- 

 ciety on March 16, 1855, Professor Williamson objects to a certain 

 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- 

 magnetic 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 ma- 

 terial particles to be diminished, without in any way altering the 

 distances between those particles, and consequently without in any 

 way displacing 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 with 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- 



