It is shown that the difference in thermal properties of dilute solutions against those of the pure solvent is due to the superposition of two effects: 1. A physical effect of the ion on the solvent which is manifested as an internal pressure varying from point to point in the solution. This pressure distribution is calculated for aqueous solutions of monovalent salts by an approximation theory in which the effect of the so-called ion atmosphere is estimated by means of the Debye-Huckel potential. In this, as in ensuing calculations, the thermal properties of the solvent are assumed to be known. The effect of this internal pressure on the thermal coefficient of expansion and on the compressibility of dilute binary solutions is calculated and the results are found to be in satisfactory agreement with experimental values. 2. Definite evidence is obtained that the second effect determining the thermal properties of solutions is due to a chemical action of the ion on the solvent. This effect increases linearly with the molal concentration. As a result of the present paper and previous work, to which reference is given, we can say that the caloric as well as the thermal behaviour of dilute solutions is duly accounted for.