The emission of rays from radioactive bodies is a process which takes place in the individual atoms alone: nothing else is a participant in it. One could see that the radiation is accompanied by the ejection of material particles from the body, even from the atoms themselves. In consequence of this loss of matter the nature of these bodies must of necessity be changing. What remains after the atom has sent out such a ray is quite a new body, with a lower atomic weight, and consequently with different physical and chemical properties—it is a new element, but, like all other elements, it belongs to our periodic system of elements and must find a place therein. When an atom of radium emits an alpha particle the remainder of the original atom forms an atom of the emanation. Its atomic weight is less than that of the emanation, and at ordinary temperatures it is a solid.