of both voii Wogau and Xeyer. Using the data of I\-eischedel, Cohen and Bruins, and those furnished 1 ~ the - polarographic method, the authors found the relation, D d * p = constant, to be invalid for the diffusion of metals in mercury. T t appeared to the authors that if the values of the diffusion coefficients were to give reliable indications of compound formation, a more fundamental approach to the problein was necessary From kinetic theory i t is known that the diffusion coefficient is related to the iiieaii free path of the diffusing particles whether they be atoms or iIiolecuIes aiid that mean free path depends on the tliiiiensions and velocity of thc particles In Fig. 1 the diffusion coefficients obtained bv II'eischedcl, Colicn and 13ruins and the authors are plotted LiKciiiistthc radii of tlic iiict'd atoii~s.'l I t woultl be
expected that copper would diffuse a t about the same rate as zinc since these metals have approximately the same atomic radius and atomic weight. However, copper diffuses much more slowly than does zinc, a fact which indicates that copper does not exist in dilute mercurial solution in monatomic form but in all probability in molecules of a coppermercury compound. Since bismuth has about the same atomic weight as lead put a much smaller atomic radius (1.46 as. 1.74 A , ) , it would be expected that bismuth would diffuse more rapidly than lead. However, this is not the case and the existence of molecules of a bismuth-mercury coinpound in dilute mercurial solution is indicated. ADDEDIX PRooF.--After the completion of tlic present paper, the authors' attention was directed to a paper by Turner and Winklerx2in which these =I 1 authors, using the equation of Strehlow aiid vori -StackelbergZ3for the anodic diffusion current of a dropping amalgam electrode, reported the values 1.52 X 10-j and 1.28 X set.-' for the diffusion coefficients of cadmium and lead in mercury. Acknowledgment.-The authors express their thanks to E. I. du Pont de Neinours Sr Co., Inc., ant1 the Wisconsin Alumni Research Foundation for financial assistance to one of thein (W. C. C.). The authors are also grateful to James C. Huber of the University of Wisconsin for his work on bismuth amalgams. (2.2) I
