, the density of mass in this equation) is put proportional to a differential expression of the second order. This circumstance prescribes, as it were, the method of building up these equations, taking for granted that they are to assume a form similar to that of Poisson's equation.
In conformity with the deepened meaning we have assigned to the mutual relation between inertia and gravity, as well as to the connection between the inertia and latent energy of a body, we find that ten components of the quantity which determines the "energetic" state at any point of the field, and which was already introduced by the special theory of relativity as "stress-energy-tensor," duly make their appearance in place of the density of mass
, in Poisson's equation.
Concerning the differential expressions of the second order in the
's which are to correspond to the
of Poisson's equation, Riemann has shown the following: the measure-relations of a manifold based on the line-element