A very simple theoretical equation given by the Principle of Natural Proportionality for the compressibility of geomaterials: V/V₀=[(i+σ)/i]^−γ is applied to the experimental data obtained by P.W. Bridgman in thirty-nine substances from 0 to 100,000 kg/cm². Independently of polymorphic transitions, the compression response of the natural solid substances presented in this paper is described by this simple theoretical equation with constant natural internal pressures i and constant natural coefficients of compressibility γ<1.