Of course, if we could utilize the forces which we now know to exist inside the atom, we should have such capacities for destruction that I do not know of any agency other than divine intervention which would save humanity from complete and peremptory annihilation. But the remoteness of the day when we shall use these forces may best be judged by an analogy. Some thousands of years ago someone first realized that the sun, moon and stars were not mere bodies as large as a plate or a house, but very large, and moving very fast. It was an obvious idea that their motions might be exploited in some way. Wise men observed them and hoped, for example, to increase the probability of success in their own enterprises by beginning them when Jupiter was in the ascendant. These attempts were unsuccessful, though far more valuable to humanity than most of the methods successfully employed for the same purposes, such as fraud, violence and corruption. They led to astronomy, and so to all modern physics. We now know that the only probable way of harnessing the kinetic energy of the heavenly bodies is to employ tidal power to create electric currents. But five thousand years ago “hitching one’s wagon to a star” was a reasonable project and not a poetic metaphor. The reason we cannot do it is a simple matter of scale. And the reason why we cannot utilize subatomic phenomena is just the same. We cannot make apparatus small enough to disintegrate or fuse atomic nuclei, any more than we can make it large enough to reach to the moon. We can only bombard them with particles of which perhaps one in a million hit, which is like firing keys at a safe-door from a machine-gun a mile away in an attempt to open it. We do occasionally open it, but the process is very uneconomical. It may be asked why we cannot bring our machine-gun nearer, or improve our aim. To do this we should require to construct apparatus on the same infinitesimal scale as the structure of the chemical atom. Now we can arrange atoms into various patterns. For example, we can arrange carbon, hydrogen and oxygen atoms in patterns which constitute the molecules of sugar, glycerine, or alcohol at will. This is called chemical synthesis. We have been doing it by rule-of-thumb methods for thousands of years, and are just beginning to learn a little about it. But even chemical molecules are much too large for our purposes. We can no more ask a chemist to build our apparatus than expect a theatrical scene-painter or a landscape-gardener to do us a miniature. We know very little about the structure of the atom and almost nothing about how to modify it. And the prospect of constructing such an apparatus seems to me to be so remote that, when some successor of mine is lecturing to a party spending a holiday on the moon, it will still be an unsolved (though not, I think, an ultimately insoluble) problem.