During the discussions that have arisen as to the internal heat of the earth, the writer has never seen any reason given for supposing that there was a time when the earth was a highly heated fluid mass, and he believes that view to have originated by analogy from the case of the sun; and no other cause of the heat seems to be generally assumed than the collisions of the parts that came together to form the earth's mass. He has expected to find some one maintaining that gradual gravitational compression of the mass was the main source of the earth's internal heat, but till recently he has never tried to find out if it could possibly be a sufficient cause. It has always seemed to him that the formation of the earth's mass must have been accomplished under circumstances so different from the case of the sun that an analogy could scarcely be drawn between the two cases; indeed, it is obvious that the amount of heat produced by the formation of planets from nebulae will depend principally on their masses, and will be in a, higher ratio than that of the masses.