I WAS much honoured by the invitation to give the first course of the Joly Memorial Lectures. As is well known, Joly was for many years not only intensely interested in the problems of radioactivity, but also made numerous original and important contributions to our knowledge of this subject. When the large and continuous emission of heat from the radioactive bodies was made clear in 1903, Joly's alert and original mind was at once attracted to the problem of the effect of this steady generation of heat by the radioactive bodies present in the earth's crust on the geological history of our planet. To obtain reliable data, he devised simple but ingenious methods for measuring the amount of the primary radioactive bodies, uranium and thorium, in typical rocks constituting the earth's crust. He was the first to point out the far-reaching significance of this small but steady supply of heat on the internal temperature gradient of the earth, resulting in violent movements in the earth's crust. Indeed, he was of opinion that the rise and fall of continents and the elevation of mountain chains were intimately connected with the heating effect of radioactive bodies over long intervals of time. A fascinating account of these bold and original ideas has been given by Joly in his books and papers.