The constitution of Venus is discussed in the light of the density-pressure hypothesis stated by W. H. Ramsey. It is shown that on this theory, an increase in the precision of observations of the diameter of Venus may lead to a useful test on the question of the chemical homogeneity of the Earth's mantle. The bearing of the writer's compressibility-pressure hypothesis on the density distribution in Venus is discussed. Values of the density and pressure in Venus are also given on the older hypothesis that the Earth's mantle and core are composed of chemically distinct materials.