ABSTRACT
This chapter deals with the properties of the radioactive bodies themselves and the other with the distribution of radioactive matter throughout the surface of the earth. It considers briefly the ordinarily accepted theory of the origin of the earth's internal heat and of the age of the earth. The production of the radioactive emanation by radium offers an extremely simple and reliable method of determining not only whether radium is present, but also of measuring accurately the quantity present. The study of radioactivity has profoundly altered our views of the earth's internal heat. The conclusions advanced are by no means completely proved, but sufficient has been done to cast grave doubt on the validity of the older theories of the origin and variation of the earth's internal heat. According to the condensation hypothesis of Kelvin and Helmholtz, the heat of the sun is derived from the gradual concentration of its material due to gravitation.
