  We derive the Teixeira, Wolk and Som method, for obtaining electrostatic
solutions from given vacuum solutions, in its inverse form. Then we use it to
obtain the geometrical mass $M_S$ in the Schwarzschild spacetime, and we find
$M_S^2=M^2-Q^2$, where $M$ and $Q$ are, respectively, the mass and charge
parameters of the Reissner-Nordstr\"om spacetime. We compare $M_S$ to the
corresponding active gravitational mass and mass function.
