The first experimental attempts to provoke a transformation by means of the radiation from a radioactive substance were made by A. T. Cameron and Sir William Ramsay in 1907 and 19o8. They first found that radium emanation in pure water produced not only helium, but also neon and a trace of argon. They also announced that in a solution of copper salts a con version of a minute part of the copper into lithium took place as a result of the action of the a-particles. Rutherford and Royds, however, found that emanation in water produced only helium, which is to be expected, for it is now abundantly estab lished that the a-particles are doubly charged atoms of helium. The neon and argon observed by Cameron and Ramsay are attributable to a small leak of air, very minute traces of these gases being detectable by the spectroscope. Other workers were unable to confirm the production of lithium, and it was soon generally accepted that the presence of this metal was due to the admission of traces of impurity, and not to transformation.
The difficulty of these experiments is that the amounts of ele ments supposedly produced by transmutation are excessively minute and the spectroscopic methods used fot detection exceed ingly sensitive, so that the slightest defect in the manipulation is sufficient to produce a spurious appearance of transformed matter. In 1913 N. Collie and H. Patterson described experiments which they considered to show that the passage of an electric discharge through hydrogen in X-ray bulbs produced helium and neon, the source of which was supposed to be a transmutation of some one or more of the elements present in the walls of the tube, or of the hydrogen or mercury vapour. However, J. J. Thomson and other workers showed that the gases were already in the walls of the tube, and that the action of the discharge was merely to liberate, not to manufacture, them.
After this the question of transmutation was left alone for some years, until interest was revived by the experiments of Rutherford on the disruption of atomic nuclei by bombardment witha-particles. These experiments are described under NUCLEUS. Rutherford and his collaborators showed by the method of scintil lations, which enables single atoms to be observed, that, in the case of certain elements, swift a-particles can knock a proton from the nucleus, the process resulting in the formation of a nucleus of a fresh kind, that is, in a transmutation of the element in question. The expulsion of a proton, however, only takes place very rarely, since, for it to occur, the a-particle must strike the nucleus with a full collision, and not at a glancing angle, and the nucleus is very minute compared with the atom itself. Rutherford's calculations, confirmed by the ray-track photographs of Blackett, show that of the incident a-particles only something of the order of 2 per 1oo,000 produce a disintegration of the nucleus in the case of nitrogen, which means that the whole of the a-rays from 1 gramme of radium in equilibrium with its products (and 1 gramme of radium, costing £20,000 or so, is far more than a physical laboratory can hope to possess) would only transmute about two ten-millionths of a gramme of nitrogen in a year—an unweighable quantity. There is, therefore, no question
of producing by this method sufficient transmuted matter for ordinary chemical or physical tests to be applied, and, in fact, although the expulsion of the proton is established beyond doubt in the case of the elements boron, nitrogen, fluorine, neon, sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, argon and potassium, it is not known what elements are formed when atoms of these elements are so transmuted. The uncer tainty is as to what becomes of the a-particle. If it expels the proton and itself passes on a given atom will be transformed to one whose atomic number is lower by a, and whose atomic weight is lower by I : if, on the other hand, the a-particle sticks in the nucleus the resulting atom will have its atomic number greater by 1 than its parent, and atomic weight greater by 3. Blackett's photographs of ray tracks indicate that in the case of nitrogen the latter is the case, so that each nitrogen atom transmuted be comes an oxygen atom, which, having atomic weight 17 instead of 16, is an isotope of ordinary oxygen. On the other hand, the ray-track photographs of Harkins and Ryan, and of Akiyama, taken in air (which comes to the same thing as nitrogen, since no evidence for the disruption of the oxygen nucleus has ever been obtained) show cases of disintegration with tracks of expelled proton, struck nucleus, and a-particle after collision. This would indicate that the a-particle does not always stick in the nucleus. This point awaits further elucidation.
Recent Attempts at Artificial Transmutation.—The many lines of work which are described in the articles NUCLEUS and ISOTOPES have confirmed the hypothesis that the nucleus is built up of electrons and protons, the net positive charge of the nucleus giving the atomic number and determining the number of electrons in the extranuclear structure of the neutral atom, and so the chemical properties of the atom, while the number of protons gives the atomic mass. These ideas, and the experiments on nuclear disintegration by a-particles to which reference has just been made, have encouraged many workers to endeavour once more to effect a transmutation of the elements in weighable quantities, or, failing that, in quantities demonstrable by chemical and physical tests. If it were possible either to add an electron to, or to subtract a proton from, the nucleus of an element of atomic number Z, then an element of atomic number Z-i would be formed. Attempts have been made to effect both these changes. As the naturally unstable, or radioactive elements, are the heaviest ones in the periodic table most workers endeavouring to provoke a transmutation have made their choice among the heaviest of the non-radioactive elements, namely the following, the atomic numbers being given in brackets: gold (79), mercury (8o), thallium (81), lead (82), bismuth (83). On account of the apparent ease of purification, mercury has been a popular choice.