The centre-piece of this book is an English translation of the Leiden thesis of 1873 of J.D. van der Waals, of which the original version must be the most cited and least read of any thesis in physical science. In an introductory essay the editor has put this work in the context of its time and explained why it is still of value to physicists and chemists working in statistical mechanics, on phase transitions and on the properties of liquids. The thesis is followed by a translation of Van der Waals first great paper on the theory of liquid mixtures, which is also frequently quoted, but which has not hitherto been available.