The first instalment of this part of my paper deals mainly with the theory of the behaviour of mixtures of CO2 and N, for which some remarkable experimental results were given by Andrews about 1874. His full paper, so far as he had drawn it up for press, was published posthumously in the Phil. Trans, for 1886, and is reprinted in his Scientific Papers, No. L. One special reaaon for the introduction of this question at the present stage of my work was the desire to attempt a correction of Amagat's numbers, for the (very small) admixture of air with his CO2. The virial equation for a mixture is formed on the same general principle as that I employed for a single gas. There are, of course, more undetermined constants :—and, unfortunately, the data for their determination are barely adequate. The general results, however, agree in character with those described by Andrews :—the particular phenomenon which is most closely studied being the increase of volume, at constant pressure, when the gases (originally separated by the liquefaction of one) were allowed to diffuse into one another.