Kopp in 1842 state that the molar volume of a liquid at its boiling point is equal to the sum of the atomic volume of its constituent atoms. This is known as Kopp’s rule.
According to Kopp's rule it has been found that the molar volumes of two members of a homologous series of organic liquids differ by about 22 mL, for each CH2 group Kopp calculated the volume equivalent of each element by a simple arithmetic means.