270 MAJOE A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 



In a diamagnetic crystalline medium, as the temperature is raised, the local positive 

 susceptibility will obey a similar law, the temperature of fusion now corresponding to 

 the critical temperature in the ferro-magnetic case. Although locally the relation 

 between susceptibility and temperature is the same in the two cases, the effect passes 

 unnoticed in the diamagnetic case because the molecule has a total zero magnetic 

 moment. Nevertheless, the effect of temperature acts in its two antagonistic ways in 

 diamagnetic as well as in ferro-magnetic media. When the temperature is above the 

 melting point, the rotational energy of the molecules annuls the local forcive (liquid 

 state), when it is very low the molecules become interlocked and cannot readjust 

 themselves in a space lattice (gel state). There is an intermediate region of 

 temperature where opportunity is offered for the tendency of self-orientation under 

 the mutual local forcives to display itself, and over this range crystallization may 

 take place. This intermediate temperature range defines the closed region of stability 

 of the crystalline form on the pressure temperature diagram of equilibrium of the 

 crystalline and amorphous states (see p. 263). 



A discussion as to how far we may regard it as proved that the local molecular field 

 in crystalline media is of magnetic nature was given in para. 8, of Part III. The 

 conclusion reached was that the molecular field is certainly in part magnetic. It is 

 possible to bring forward further evidence of the truth of this deduction. In some 

 noteworthy researches* published by TYNDALL, as long ago as 1870, it was shown that 

 magnetic properties of crystalline media bear a close relation to molecular aggregation. 

 About 100 different crystals were examined and from the deportment of these, when 

 subjected to a magnetic field, TYNDALL, found that " if the arrangement of the 

 component particles of any body be such as to present different degrees of proximity 

 in different directions, then the line of closest proximity, other circumstances being 

 equal, will be that chosen by the respective forces for the exhibition of their greatest 

 energy. If the mass be magnetic this line will stand axial, if diamagnetic, 

 equator ial."t 



The exactness of the dependence of magnetic deportment on the position of cleavage 

 planes is remarkably shown in these experiments. Whatever the crystal examined, it 

 was found that the magnetic deportment disclosed accurate information of the planes 

 of cleavage. TYNDALL describes the results of his important experiments in such 

 elegant language that it may be permissible to quote some of them at length. Thus 

 he continues : " From this point of view, the deportment of the two classes of crystals 

 represented by Iceland spar and carbonate of iron, presents no difficulty. This 

 crystalline form is the same, and as to the arrangement of the particles, what is true 

 of one will be true of the other. Supposing then, the line of closest proximity to 

 coincide with the optic axis ; this line, according to the principle expressed, will stand 

 axial or equatorial, according as the mass is magnetic or diamagnetic, which is 



* ' On Diamagnetism and Magneciystallic Action,' 1870. 

 t Loc. cil., p. 23. 



