For a pure liquid, despite its low compressibility, the variation of density near a solid surface can be detected and measured. The total volume $V$ of a system consisting of solid and pure liquid is different from (usually less than) that calculated assuming a constant liquid density. If the densities of bulk solid (${\rho }^{\mathrm{sol}}$) and liquid (${\rho }^{\text{l}}$) are known then an excess volume (usually negative) can be defined as: $$V^{\sigma }=V-V^{\mathrm{sol}}-V^{\text{⚬}}=V-\frac{m^{\mathrm{sol}}}{{\rho }^{\mathrm{sol}}}-\frac{m^{\text{l}}}{{\rho }^{\text{l}}}$$ where $m^{\mathrm{sol}}$ is the mass of solid, $V^{\mathrm{sol}}$ its volume calculated from the bulk density, $V^{\text{⚬}}$ is the initial volume of liquid and $m^{\text{l}}$ is the mass of liquid. The excess mass is given by: $$m^{\sigma }=m^{\text{l}}-(V-V^{\mathrm{sol}}){\rho }^{\text{l}}$$