88 MR. A. E OXLEY ON THE INFLUENCE OF MOLECULAR 



But COTTON and MOUTON have shown that the magnetic double refraction induced in 

 water by the largest magnetic field they were able to apply (3 . 10 4 gauss) is 

 insignificant compared with that of liquid nitrobenzene subjected to the same field. 

 In general the natural double refraction of a crystalline medium is of a high order 

 compared with the artificial double refraction which we can produce in a liquid using 

 a field of 3 . 10 4 gauss. The intrinsic molecular field, if interpreted magnetically, must 

 therefore be large compared with 3 . 10 4 gauss in all crystalline diamagnetic media. 

 This result will be used later in the extension of our ideas concerning the 

 molecular field to diamagnetic crystalline media in general. 



In Part II. the change of specific susceptibility due to the crystallization was 

 represented by the difference, (a c a t ) AH, of the mean molecular fields of the 



ATT 



crystalline and liquid states. -==- , where H is the applied field, defines the distortion 



produced in an electron orbit by the forces exerted by surrounding molecules when 

 the substance is in the liquid state and subjected to a field H. AH is the polarization 

 (proportional to the applied field H) due to this distortion. a t is the constant of the 

 molecular field for the liquid state and is approximately equal to -j. Now the 

 molecules of a liquid are moving about rapidly in all directions, and therefore 

 the polarization defined by AH is an average effect. In the crystalline state, on the 

 other hand, the molecules are orientated into definite positions with respect to one 

 another, and therefore the polarization due to the mutual influences of the molecules 

 in this case (as disclosed by applying an external field H) is large compared with AH. 

 Let a c . AH be this new polarization. Then a c , which is large compared with a h is a 

 factor defining the polarization which results from the complete orientation of the 

 molecules in the crystalline medium. a c will also depend upon the proximity of the 

 molecules. 



Now reverting to equation (8) of Part II., the variation of x on crystallization may 

 be written 



9 X / x AH AH 



: ~ \ a e~ a l) -TJ- a c ~TT~ > 



if a c is large compared with a t . The large value of a c implies a correspondingly small 

 value of AH/H, i.e., a small value for the molecular distortion in the liquid state in 

 order to account for a given change of x on crystallization. 



As in 3 we may write the mean molecular field a c . AH, which accounts for the 

 change 3 X on crystallization, in the form a' c x (diamagnetic moment per unit volume) 

 = a! c . N . AM . p, and the value of a' c , the new constant of the molecular field, 

 is of the order 10 5 for those substances which show a small percentage change of x on 

 crystallization. 



Now the local molecular field H,. is of the order 10 7 gauss. We may write 

 H c = a' c . I where I is the aggregate of the local intensity of magnetization per unit 

 volume. a' n the constant of the local molecular field, will be equal to the constant of 



