The same is true of the phenomena bound up with radioactivity, which were discovered in direct consequence of Crookes's work. We know that the naturally radioactive elements are all in the group of those with the highest atomic weight. This fact, seen together with the characteristics of radioactivity, tells us that in such elements gravity has so far got the upper hand of levity that the physical substance is unable to persist as a spatially extended, coherent unit. It therefore falls asunder, with the liberated levity drawn into the process of dispersion. Seen thus, radioactivity becomes a symptom of the earth's old age.