IN a paper, "On the Constitution of the Terrestrial Planets", by W. H. Ramsey (Mon. Not. Roy. Astro. Soc., 108, 5 ; 1948), it is suggested that the earth' core and mantle are not chemically distinct. Seismological data show that the earth has a central core which occupies about one-sixth of its total volume and contains about one-third of its mass, and it has been generally assumed that the material of this core is an alloy of iron and nickel. The core is mainly responsible for the earth' high mean density–the highest in the solar system. The high masses of the four great outer planets have enabled them to retain hydrogen and other volatile substances, and this explains their comparatively low densities ; but the fact that the terrestrial planets have mean densities smaller than that of the earth presents a difficulty.