For the purpose of making a _controlled_ chain reaction, one may use the method of enrichment, or the method of moderation, or both. But to produce a _violent_ chain reaction, an atomic bomb, only the enrichment method will work. The reason is that all the energy of the bomb must be generated in a time that is as short as the time it takes the bomb to fly apart, which is a fraction of a microsecond. If natural uranium were used, the reaction would be slow and sluggish and would be extinguished before a substantial fraction of the nuclei could have reacted. It is interesting to consider that chain-reacting substances could have been obtained easily six billion years ago, before the U²³⁸ had time to decay and become a rare isotope. (The U²³⁵ was then about as abundant as U²³⁸.) A chemical separation would still have been necessary and so we do not need to imagine that chain-reacting mixtures accumulated spontaneously on the young earth. On the other hand, six billion years from now U²³⁵ will have become so rare that it will be impossible to get a reactor going by moderation. At the same time the isotope separation will have become most expensive since the isotope to be separated will be present in an abundance of less than 100 parts in a million. For those who like to worry about the distant future we should hasten to add that other methods of obtaining atomic energy will remain possible. And in any case there is good reason to believe that some stellar explosions produce fresh supplies of U²³⁵ which space merchants could undoubtedly make available. As to our present terrestrial supplies: uranium, like other heavy elements, is quite rare. But the earth is divided into layers of which the topmost 10 miles, forming something of a slag or scum, contain quite a few rare compounds. In particular almost all of the uranium in our planet is conveniently collected right under our feet, for us to use as we see fit. CHAPTER VIII Action of Radiation on Matter When an energetic particle moves through matter (living or nonliving), what happens is a question of chemistry. Chemistry is the subject that deals with the arrangement and rearrangement of electrons in atoms and molecules. A chemical rearrangement generally requires an energy in the neighborhood of a few electron-volts. (As we have seen, an electron-volt is the energy released when an electron moves through a potential of one volt, i.e., a little less than one per cent of the driving force in a standard electric outlet.) An energetic particle, such as might be emitted in a radioactive decay, typically has an energy of a few million electron-volts. Thus a single such particle has the potentiality of about a million chemical rearrangements.