CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 85 



can be produced in the laboratory is of the order 10 5 gauss, and therefore the largest 

 moment which we can induce is given by 



| AM] < 10- 4 .M. 



This induced moment is sufficiently large to account for the diamagnetic suscepti- 

 bility of the substance. 



From equation (3) we have 



M, 



(5) 



er c H 



where the subscripts I and c refer to the liquid and crystalline states respectively. 



In passing from the liquid to the crystalline state the alteration of M, produced by 

 the local molecular field H c is AM'; where 



AM', 6 T,H C * 

 M, 



M C = M,AM', ........... (7) 



It should be noted that although H c alternates as we pass from molecule to molecule 

 of the crystalline structure, still the sign of AM', will everywhere remain the same, 

 for when H c changes sign M, also changes sign, so that every molecule of the 

 crystalline structure suffers a definite distortion due to the action of the local 

 molecular field. The ambiguity of sign in the above formulae simply implies that the 

 arrangement of molecules may be such that M, is increased or decreased by the 

 particular kind of packing which the molecules assume and corresponds to a reversal 

 of the sign of 3 X . (See Part I., p. 133, and Part II., p. 143.) 

 From equations (6) and (7), we find 



and therefore from (5) 



M, 

 Hence 



AM C -AM, = erJIA T er,HA , 

 M, 47rm\ 47rm/ ' 



which gives by (4) 



AM e -AM, _ r c /, -P er,HA 

 ~AMT " 



* Hj, the molecular field within the liquid, is insignificant compared with H c and is therefore neglected 

 in equations (6) and (7), 



