The Debye and Keesom contributions to the boiling points of methane, ethane, propane and ethene derivatives are studied, and several remarkable numerical relationships between them are found. These relationships can be explained theoretically. Moreover, they permit the calculation of the Debye and Keesom contributions to the boiling points of the propane derivatives, which hitherto could not be estimated empirically. A method is developed, according to which it is possible to calculate theoretically the values k in the formulae for the London contribution to the boiling points of the methane, ethane and propane derivatives. The agreement with the empirical values is satisfactory. Some minor questions, left open for further discussion in earlier papers, are treated.