These advances in knowledge all point to the possibility of effecting transmutations at will, but so far attempts to achieve this, as I have already indicated, cannot be regarded as altogether satisfactory. Several methods of making gold, or rather elements chemically identical with gold, once the method of controlling radioactive change is discovered (as assuredly it will be) are suggested by Sir Ernest Rutherford’s theory of the nuclear atom. Thus, the expulsion of two α-particles from bismuth or one from thallium would yield the required result. Or lead could be converted into mercury by the expulsion of one α-particle, and this into thallium by the expulsion of one β-particle, yielding gold by the further expulsion of an α-particle. But, as Professor Soddy remarks in his _Science and Life_ just referred to, “if man ever achieves this further control over Nature, it is quite certain that the last thing he would want to do would be to turn lead or mercury into gold--_for the sake of gold_. The energy that would be liberated, if the control of these sub-atomic processes were as possible as is the control of ordinary chemical changes, such as combustion, would far exceed in importance and value the gold. Rather it would pay to transmute gold into silver or some base metal.” In § 101 of the book I suggest that the question of the effect on the world of finance of the discovery of an inexpensive method of transmuting base metal into gold on a large scale is one that should appeal to a novelist specially gifted with imagination. Since the words were first written a work has appeared in which something approximating to what was suggested has been attempted and very admirably achieved. My reference is to Mr. H. G. Wells’s novel, _The World Set Free_, published in 1914. In conclusion I should like to thank the very many reviewers who found so many good things to say concerning the first edition of this book. For kind assistance in reading the proofs of this edition my best thanks are due also and are hereby tendered to my wife, and my good friend Gerald Druce, Esq., M.Sc. H. S. R. 191, CAMDEN ROAD, LONDON, N.W. 1. _October_, 1921. PREFACE