Happy Birthday Johannes van der Waals
Johannes van der Waals, born November 23, 1837, is famous for his work on the equation of state for gases and liquids which describe the relation between the pressure, volume, and temperature of fluids (gases and liquids). He developed a model in which the liquid and the gas phase of a substance merge into each other in a continuous manner. It shows that the two phases are in fact of the same nature. In deriving his equation of state, van der Waals assumed not only the existence of molecules (which in physics was disputed at the time), but also that they are of finite size, and attract each other. Since he was one of the first to postulate an intermolecular force, however rudimentary, such a force is now sometimes called a van der Waals force.