262 MAJOR A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 



unknown functions of the molecular structure and space lattice of each substance. It 

 will be noticed that those substances showing a small value of (5V have low compressi- 

 bilities. 



The case of iron at the A 3 transformation is particularly interesting. From the 

 curves given by CHARPY and GRENET,* dealing with the expansion of iron and iron- 

 carbon alloys between 200 C. and 1000 C., we can show that the extent of linear 

 contraction which occurs suddenly at the A 3 point (900 C.) is of the order 0'003 centi- 

 metre per centimetre. The change of volume will be of this order of magnitude, 

 which is small in comparison with the change of volume accompanying the crystalliza- 

 tion of many organic compounds, but is very large compared with the magneto- 

 striction effect which can be induced in either a ferro-magnetic or diamagnetic 

 substance with a field of 50,000 gauss. 



Taking into account the small compressibilty of iron,t which is only 0'4x 10~ 18 , or 

 about 1/200 that of the liquids above referred to, this change of volume may be inter- 

 preted as due to a change of internal energy represented by ^ . N . P where N is the 

 constant of the ferro-magnetic field, of the order 0'38x 10*4 I the saturation intensity 

 of magnetization, of the order 1760. For we have 



<5V = i . c . N . F 



= xO'4x 10- ia xO'38x 10 4 x 17G 2 x 10" 

 = 0-002 (4) c.c./c.c. 



which is of the order of magnitude found experimentally. The molecular field exists 

 in an umnagnetized piece of iron and is accompanied by the large spontaneous 

 magnetization of that element throughout an individual grain, but as these grains 

 have all types of orientation, the large molecular field and the accompanying 

 spontaneous magnetization are hidden in a piece of iron large enough to contain many 

 grains. The molecular field will nevertheless produce the magneto-striction effect 

 referred to above. Let us suppose that such a piece of iron is subjected to an external 

 magnetic field. The molecules of all the grains will tend to come into alignment with 

 the applied field and there will be a new distribution of stress. In an unmagnetized 

 piece of iron, taken as a whole, the stress may be regarded as equal in all directions, 

 but when an external field is applied, this is no longer the case and the iron shows a 

 new magneto-striction effect consisting of an expansion in one direction and a 

 contraction in the other. The extent of the redistribution of stress should be deter- 

 mined by a term of the form ^ . a' c . f where a' e is the constant of the molecular field 



* Loc. cit. 



t RICHARDS, ' Journ. Chem. Soc.,' vol. 99, p. 1201, 1911. 



I WEISS and BECK, 'Journ. de Phys.,' se"r. iv., vol. 7, p. 249, 1908. 



See infra, p. 265. It is assumed here that the molecular field disappears just above the A 3 point, at 

 least in so far as it is effective in causing spontaneous magnetization. This is in accordance with the 

 small paramagnetic susceptibility of iron above AS. 



