In this article we show how to verify that in a fluid surface or film the value of the surface tension (i.e. the free energy per unit area) does not depend on the area of the film itself. The experimental evidence discussed can be obtained extremely simply yet with great accuracy. This experiment is important in that it leads to a deeper understanding of the actual microscopic nature of the fluid state of matter and to its comparison with the solid state, as well as to a practical example of a differential (zero) measurement procedure.