Amagat's law

Amagat's law or the Law of Partial Volumes of 1880 describes the behaviour and properties of mixtures of ideal (as well as some cases of non-ideal) gases. Of use in chemistry and thermodynamics, Amagat's law states that the extensive volume V = N·v of a gas mixture is equal to the sum of volumes V_i of the K component gases, if the temperature T and the pressure p remain the same:

This is the experimental expression of volume as an extensive quantity. It is named after Emile Amagat.