great as that which he occupies, would completely account for solar radiation. It was only necessary to suppose that this volume of matter contracted in consequence of the mutual attraction of its parts. As it diminished in hulk, the quantity of potential energy would be of course lessened. But as energy could not be lost, that disappearing potential energy must be manifested in some other form. This was accomplished by its transformation into heat, which kept the sun so far supplied as to maintain its radiation unabated for uncounted thousands of years. It was easily demonstrated that a shrinkage in the solar diameter too small to be appreciable by any measurements we could make, would, nevertheless, set free a quantity of heat sufficient to maintain the radiation for a period of two thousand years. The molecular theory of gases stands in a significant relation to this beautiful discovery of the great German philosopher. It is quite clear that the necessary energy is indeed afforded by the contraction, but it is not quite so easy to learn the precise character of the process by which the energy after disappearing from the potential form reappeared as heat. We want as it were to see the mechanism by which this is effected. This it is which the molecular theory of gases enables us to do; we can now follow the entire process of transformation which the energy undergoes.
Gravitation at the surface of the sun is of course very much greater than at the surface of the earth. It is easy to show that if two globes had the same mean density the gravitations at the surface of each would be simply proportional to its radius. As the radius of the sun is 109 times as great as the radius of the earth, it would follow that if the earth and the sun had the same mean density
