1 4 8 D O C U M E N T 1 2 6 D E C E M B E R 1 9 2 5 , or according to (2), if one takes into account that on this plane , , . Thereby is expressed said contradiction. This equation would demand , , , , d. h. , and specifically, this conclu- sion could be drawn for any meridional plane. In the end, one would have no field and consequently no charges at all. With kind regards, yours truly, H. A. Lorentz 126. From George Y. Rainich Johns Hopkins University, Baltimore, Md., U.S.A., 8 December 1925 Dear Colleague, I have received your kind letter of 13 Nov.[1] You write: “What I have been en- visioning during the past few years is the endeavor to find a conception according to which the gravitational field and the electromagnetic field appear as components of one and the same mathematical construct, and in which the field equations like- wise appear logically unified.” The first desideratum can, I think, easily be fulfilled: the mathematical construct which contains gravitational and the electromagnetic field as components is the complete (noncontracted) Riemann tensor (of your original general theory of rela- tivity). It can be decomposed into two parts: (1) as indicated in my letter in Nature, which appeared on 4 April (I [enclose] a copy of the letter).[2] The second part, , is a quadratic function of the electromag- netic tensor[3] and has the same components as the contracted Riemann tensor (in that when con- tracting (1), the first part is eliminated for the sake of simplicity, I assume ).[4] The fact that depends quadratically on implies that for a given the sign of (or also the sign of the charge) remains undetermined: perhaps this is connected to the presence of positive and negative electricity.[5] ³Xxdσ 0= Ex 0= Hx 0= Ey 2 Ez 2 Hy 2 Hz 2)dσ + + + ³( 0= Ey 0= Ez 0= HY 0= Hz 0= E 0= H 0= Rij kl , Gij kl , Eij kl , += Eij kl , fij Eij kl , 1 2 -- - fij fkl rij rkl = Gij kl , R 0= Rij, kl fij Rij, kl fij