IN the five-dimensional theory of the connexion between electromagnetism and gravitation first proposed by Th. Kaluza (Sitzungsberichte d. Berl. Akad., 1921, S. 766; see also O. Klein, Zs. für Phys., 37, 875, 1926), the equations of motion of an electrified particle may be shown to be the equations of geodetics belonging to the following line element: where x1, x2, x3, x4 are the co-ordinates of ordinary space time with the line element gikdxidxk, while x° is a fifth co-ordinate, and the φi are the four co-variant components of the electromagnetic potential vector. If the constant β is given the value κ being the Einstein gravitational constant, the 14 field equations of the Einstein theory may, moreover, be simply expressed by means of the curvature tensor belonging to this line element.