IF a chemist were to purify lead from silver, and found on re-examining the lead that silver were present, and if, again and again, silver, initially absent, reappeared the doctrine of the unchangeability of the elements would be at an end. The conclusion in 1902 by Sir Ernest Rutherford and myself with regard to the element thorium was of this direct and simple character. As often as the constituents responsible for the radio-activity are separated by physical or chemical means, they reform. One of the constituents, the thorium emanation, is a gas which was shown to possess the complete absence of chemical character characteristic of the argon family of gases. It is formed from thorium through the intermediary of another constituent, thorium-X, which is left in the filtrate, when a solution of thorium is precipitated by ammonia, but not by other chemical reagents. In turn the emanation changes into non-volatile products causing the active deposit. The clear conception of the nature of chemical change, the distinction between atoms and molecules, which we owe to the founders of chemistry, made it possible to recognize .radio-active change almost instantly as a case of spontaneous transmutation. Novel as the explanation was, the phenomena explained are so novel as to transcend what to a generation ago would have appeared as the limits of the physically possible. But even today it is only in radio-active phenomena that the limits rea <:bed long ago in the chemical analysis of matter have been overstepped, and the rubicon, which many ha ve vaulted- over so lightly in imagination, has actually been crossed by science. The first phase of the study of radio-active change was mainly concerned with the disentanglement of the long and involved sequence of transformations which, starting from uranium and thorium, were ultimately found to include all the known radio-elements. Beyond the fact that the radio-elements were in present course of evolution, it added little to the conceptions of chemistry. But in the second and more recent phase—concerned with the chemical character of the successive products, the law connecting this with the type of ray expelled in the change, the discovery of elements with unique radio-active but identical chemical and spectroscopic character, the identification of these as isotopes, or elements occupying the same place in the periodic table, the interpretation of the latter and the recognition that the so-called chemical elements are in reality heterotopes, or substances occupying different places in the periodic table, and are not necessarily even homogeneous—conclusions, not merely novel, but upsetting, have been reached. The criterion at first relied upon in the analysis of matter into its elements, the possession of a unique *Summary in Na(ure of a lecture to the Chemical Society, London. chemical character, was added to by Dalton's atomic theory, which gave to each element a unique atomic weight. The periodic law apparently connected these two criteria, fitted the individual elements into families, and showed that, whatever the elements were, they were all of a class, the limits of chemical analysis, and, if complex, then all of the same kind of complexity. The periodic law introduced a third criterion of the element, that it occupied a place to itself in this scheme, and the discovery of spectrum analysis, a fourth, that it possessed a unique spectrum. The discovery of radio-activity introduced a fifth, the possession of a unique radio-active character, in the case of the radio-elements. Of the first three new elements discovered by the aid of the fifth criterion, polonium, actinium, and radium, the claim of the last to the title of element was brilliantly substantiated by the successive determination of its unique spectrum, unique chemical character, unique place in the periodic table, and unique atomic weight. The production of this element from uranium through the intermediary of ionium, and the production of helium from radium, and, in due course, from the other radio-elements, furnished conclusive proofs of the correctness of the first interpretation of the transmutational character of radio-active change. Then came a totally new departure. The possession of unique radio-active character does not always, as in the case of radium, connote unique chemical and spectroscopic character. As, one after another, the various members of the disintegration series were distinguished, by their breaking up in characteristic ways at definite rates, no further chemically new elements were found. All resembled known elements so closely that they could not be separated by chemical analysis, and those actually at work on these substances came to the conclusion that the chemical resemblances amount to identity. Radio-thorium is, for example, identical chemically with thorium. It was isolated from thorium and individually recognized by Sir William Ramsay and O. Hahn only because it is formed from thorium through an intermediate product, meso- thorium, chemically different from thorium, but chemically identical with radium. No more elegant addition, not merely to knowledge, but also to the means of winning knowledge, can be imagined. Two separate substances, radio-thorium and thorium, in the original analysis of the thorium disintegration series, taken for one, become individually knowable, because the first is formed from the second through a third substance chemically totally distinct from either. Radio-active change thus furnished a new means of analysis, for which, outside the radio-elements, there is as yet no equivalent. Further work on the chemical character of the various members of the disintegration series, notably by Fleck, who showed that practically all were chemically identical either with some common element or other radio-element, in 1913 paved the way for the generalization independently arrived at by Russell, Fajans, and myself, which is brought up to date and illustrated by the accompanying figure. Each a.-ray change was found to cause a shift of two places in the periodic table in one direction, and each {J-ray change a shift of one place in the other, the first change being accompanied by a reduction of four units of atomic mass, a helium atom being expelled, and the second not involving a sensible loss of mass. Thus the successive places in the periodic table were first associated with unit variation of atomic charge, for the {J-particle is the negative electron, and the a-particle a helium atom carrying two positive atomic charges. The elemoots with identical chemical character were found to occupy the same place in the periodic table, and were, therefore; termed isotopes. Conversely, the elements recognized by chemical and spectroscopic analysis may be termed heterotopes. In the figure, which is to be read at 45°, the numbers at the head of each place—92 for uranium, and so on—are the atomic numbers, or number of the place in the complete list of places in the periodic table, as determined by Moseley, on the assumption that the atomic number of aluminum, the thirteenth element in the list, starting from hydrogen, is 13. The period of average life of each member is shown above or below its symbol, a “?” indicating that the period is indirectly estimated from the range of the a.-ray expelled. The last member to be added, eka-tantalum or proto- actinium, the direct parent of actinium in an a.-ray change, was discovered this year independently by Cranston and myself, and by Hahn and Meitner. For this element, for actinium, and for polonium, but for none of the others, are the criteria of unique spectrum and chemical character, as found for radium, to be (Continued on page 77) Prof. S^My's diagram showing positions in the periodic table, and derivations of the radio-active elements