MOBILITY OF CAESIUM ATOMS 3 87 



If at any temperature the rate of arrival of Cs is changed from |j,o to \ii, 

 the velocity of the movement of the boundary is given by 



v= (D/2Ay'-([ii -[io) 



This was derived on the assumption that D is independent of o and that v 

 is small. 



The experiments show in fact that Ai = A2 and that v varies in pro- 

 portion to Hi — i^Iq. From measurements at 967° K. a value of D =6 X 10"* 

 was obtained. The concentration of the a phase was 7,3 X 10^^. From 

 Eq. (i) for this temperature we calculate D = i.i X 10'^. The higher 

 concentration is the probable reason for the higher observed value of D 

 by this method. 



The strong repulsive forces between the adatoms explains the variation 

 of D with o. It is planned to develop a quantitative theory of this efifect. 

 The coefficient of the last term of Eq. (i) corresponds to an activation 

 energy for diffusion amounting to 0.61 volt. The atoms are normally held 

 in definite positions with respect to the underlying tungsten lattice, but by 

 acquiring thermal energy of agitation of 0.61 volt or more, may hop into 

 adjacent vacant spaces. With definite knowledge of the lattice and the forces 

 acting on the atom, it is hoped that the average life t in each elementary 

 space may be determined. 



