592 SCIENCE PROGRESS 



any of these types, associated with each molecule, would 

 produce a distortion in all the molecules and would give rise 

 to a new set of properties which would distinguish the crystalline 

 from the fluid state. If we examined a single crystal we should 

 no doubt find that the distortion, and therefore the properties, 

 are different along different directions. In a mass of small 

 crystals these directional differences would not be detectable, 

 but there would still be an outstanding mean distortion which 

 would produce a definite change of property on crystallisation. 

 Now we know that the effect of appfying a magnetic field to a 

 substance is to distort the configuration of each molecule, 

 and we can ascertain what must be the magnitude of such a 

 field which will produce the change of susceptibility which 

 was observed experimentally. Let us therefore assume that 

 the molecular field is a magnetic one and determine its 

 magnitude. 



If AM is the change of moment produced in an electron 

 orbit of moment M, by applying a magnetic field H, r being 

 the period of rotation of the electron (= io~ 16 sec), ejm the 

 ratio of charge to mass (17X io 7 in E.M.U.), we may write 

 (Part 3, p. 84): 



AM Ht€ 9 TT 



M 4-rrm 



The largest field which we can produce in the laboratory 

 is of the order io 4 gauss, and hence the largest value of AM is 

 io _5 .M. Suppose, however, on crystallisation that the mole- 

 cular forces were of the order io 7 gauss, then the value of -^ 



would be of the order io -2 , and would correspond to a change 

 of 1 per cent, in the specific susceptibility. This is the extent 

 of the change actually observed in aromatic compounds, and 

 therefore we could interpret the molecular force in such crys- 

 talline media as being of the order io 7 gauss. Other evidence 

 confirming a molecular field of this intensity is furnished by 

 double refraction, magneto-striction, and magneto-rotation 

 data, and this suggests an extension of the idea of an intense 

 molecular field to media which show an inappreciable change 

 of susceptibility on crystallisation. A liquid submitted to a 

 magnetic field becomes feebly doubly refracting and undergoes 

 aismall volume change. If we could apply a magnetic field of 

 io 7 gauss (such a field is, on the above view, brought into play 

 on crystallisation), the double refraction would be comparable 

 with the natural double refraction of quartz (Part 3, p. 87), 

 while the magneto-striction effect becomes the change of volume 

 on crystallisation (of the order o-i c.c. per c.c.) (Part 4). The 



