A There have been many theoretical discussions of the Hall effect. Some of these have made much of the fact that in certain crystals conductivity, both thermal and electric, is known to be aeolotropic, different in different directions, and have explained or, rather, described, the effect in question as due to oeolotropy set up by action of the magnetic field. Others have made prominent the direct action of the magnetic field on elements of the electric current, the moving electrons or ions. I have long been of the opinion that, in some metals at least, the Hall effect is not to be explained by this ponderomotive action alone, even if more than one mode of electric conduction is admitted. Thus, the rotation of the equipotential lines of the electric current by action of the magnetic field is about one hundred times as great in bismuth as in gold. In tellurium it is perhaps greater than in bismuth but in the opposite direction. It seems unlikely that the very large effects, of opposite sign, in bismuth and tellurium, respectively, can be explained without recourse to conductive seolotropy produced or modified by the action of the magnetic field. Until recently, however, I have had a hope that the small Hall effects, of negative sign, observed in certain metals, aluminium, copper, gold, silver, palladium and platinum, could be satisfactorily accounted for by the hypothesis of direct action on the electric current itself, this current having the dual character which my writings have so frequently described. In this hope I wrote a paper' which was read at the November, 1922, meeting of the National Academy. In that paper I undertook to state the relations of the four transverse effects to each other in terms derived directly from my theory of electric and thermal conduction, and in particular to calculate from the observed values of the Hall and Ettingshausen coefficients in certain metals values of the Nernst and the Righi-Leduc coefficients for the same metals. For brevity, however, I left out the very complicated formula which I had obtained for the relation of the Nernst effect to the Hall effect, giving merely numerical results derived by application of this formula. In a paper presented in April, 1924, to the Solvay Conference in Brussels I continued the effort described and gave at great length the derivation and application of the formula in question, using data which I had obtained from my own recent experiments on gold. It thus appeared that, while I could deal more or less satisfactorily with the problem in hand so long as