Adopting Thomson's (1871) thought experiment, a simple equation for the size of an equilibrium gas bubble embedded in a liquid is derived. It accounts for both a lowering of equilibrium pressure due to concave interface and changes of physical properties with changing composition of the gas-liquid solution. The gaseous phase is either a single component or a mixture of ideal gases, the phase equilibrium is assumed to obey Henry's law. This equation reduces to the one commonly encountered in literature when the physical properties are taken as constant and the gas density is neglected in comparison with the liquid density.