757 



is the consequence of tlie conveyance of iron and dissolved substance. 

 The quantity of dissolved substance supplied resp. extracted in the 

 transition layers is of the order §, just as the quantity of electricity 

 e. As the volume of the transition layers is of (he order of magnitude 



/, the change ot state inside these layer will be of the order — . 



Now there was equilibrium in the transition layers before the 

 variation ; hence a variation of the free energy per unity of mass 



of the order of magnitude ( — ) will correspond to a change of state 



of the order — (the external work is zero). The variation of the 

 total free energy of the transition layers will therefore be of the 

 order — . Thence we see that this variation may be neglected with 

 respect to the other variations of the free energy, which are of the order |. 



5. We shall now still examine what will be the equilibrium 

 concentration in the magnetic field, i. e. that concentration which 

 will finally exist after the diffusion has been active between the 

 different volume elements. For this purpose we consider an infini- 

 tesimal variation of the total free energy W of the system. We 

 choose this so that all the parts of the system, with the exception 

 of the solution, remain unvaried ; moreover we leave the magnetic 

 induction '^ unvaried. We can, therefore, restrict ourselves to the 

 variation of W^, the free energy of the solution in the magnetic 

 state. For this free energy holds the expression according to (8) 

 and (9): 



^,=jL.xp-\-Js:>.cm 



dxj 



when we use the expression (23) for \p. 



As the susceptibility may be considered as small, we may put 

 for it : 



"F 



,=J[Q.^pi^{l-4jtx,)'\dr (27) 



We shall now let the variation consist in a change of the con- 

 centration, accompanied with a change of the specific volume ; in 

 this we leave the volume of every volume element unvaried, so 

 that the external work is equal to zero. We get the relation between 

 concentration variation and volume variation by eliminating (Sq from 

 (17) and (18), by which we get: 



