CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 275 



The theory propounded by TYNDALL was called by him the " theory of reciprocal 

 induction," and the direction within the crystalline medium where the molecules had 

 the closest proximity and along which the greatest energy was displayed, he called the 

 " line of elective polarity." This theory is identical with our hypothesis of mutual 

 molecular distortion enunciated at the beginning of Part I. and subsequently confirmed 

 in a variety of ways by other physical phenomena. The direction of closest approach 

 of the molecules, i.e., the line of elective polarity is the line along which the crystal 

 shows the maximum elastic properties. TYNDALL'S explanations of the phenomena he 

 had discovered were prophetic. The diarnagnetic forces were known to be so minute 

 that the theory of reciprocal induction appeared incredible, and, as a correspondence 

 between Lord KELVIN and TYNDALL shows,* the former expressed emphatically his 

 view that this theory was quite incapable of accounting for the effects observed. 

 On our modern conception of the magnetic structure of matter, this doubt is dispelled 

 and the smallness of the diamagnetic property is no barrier to the theory of reciprocal 

 induction. The effect of applying pressure to a diamagnetic medium, produced, in tin; 

 direction of the pressure, an increase in the diamagnetic property. This was 

 attributed by TYNDALL to the mutual actions of the diuinagm-tic polarities which 

 are so minute that their effects, as then understood, would be of such a small order 

 of magnitude that they could not be detected by experiment. On our view of 

 a diamagnetic molecule, which maintains that such a molecule is paramagnetic 

 locally, the effects observed by TYNDALL can be accounted for quantitatively for 

 the local molecular forcives are comparable with those in para- and ferro-magnetic 

 media. 



But even in the case of ferro-magnetic media it is not obvious that the magnetic 

 forces are sufficient to explain the mechanical phenomena unless we realise the 

 localised nature of the forcive. If we take, for example, a crevasse of the usual 



assigning a magnetic nature to the forces, of valency is clear, for in this way, without admitting an 

 electron transfer between the various atoms forming the molecule, we can secure the necessary attraction, 

 and this by a fixed or directed force which at the same time is compatible with a characteristic orbital 

 frequency such as appears to be necessary to account for ordinary absorption, magnetic rotation, and 

 diamagnetic phenomena. The possibility of a satisfactory interpretation of many problems suggested by 

 stereochemistry, in terms of the magnetic force due to revolving electrons, has been ably expounded by 

 A. L. PARSON ('Smithsonian Miscellaneous Collections,' vol. 65, No. 11, a paper to which, on account of 

 war service, I have only recently had access). Though PARSON'S theory involves new difficulties in 

 connexion with the distribution of positive electricity in the atom, the advantages from a chemical stand- 

 point which he secures by the introduction of magnetic forces of chemical combination cannot be denied. 

 Granting this, it is natural to suppose that the cohesive forces, which hold the molecules in position in a 

 space lattice, will be residual magnetic forces, and that they will closely resemble, in distribution at least, 

 the atomic forces determining the configuration of the molecule. It will be of great interest to see how 

 far such' magnetic cohesive forces are capable of interpreting the spacing of molecules in a crystalline 

 lattice in accordance with the distribution disclosed by X-ray analysis. Magnecrystallic action, as we 

 have seen, is explicable in this way. 



* Various letters, ' On Diamagnetism and Magnecrystallic Action," 1870. 



