In view of recent research in the domain of Radioactivity and the consequent advance in knowledge that has resulted since this book was first published, I have carefully considered the advisability of rewriting the whole of the last chapter, but came to the conclusion that the time for this was not yet ripe, and that, apart from a few minor emendations, the chapter had better remain very much as it originally stood. My reason for this course was that, whilst considerably more is known to-day, than was the case in 1911, concerning the very complex transmutations undergone spontaneously by the radioactive elements--knowledge helping further to elucidate the problem of the constitution of the so-called “elements” of the chemist--the problem really cognate to my subject, namely that of effecting a transmutation of one element into another at will, remains in almost the same state of indeterminateness as in 1911. In 1913, Sir William Ramsay[1] thought he had obtained evidence for the transmutation of hydrogen into helium by the action of the electric discharge, and Professors Collie and Patterson[2] thought they had obtained evidence of the transmutation of hydrogen into neon by similar means. But these observations (as well as Sir William Ramsay’s earlier transmutational experiments) failed to be satisfactorily confirmed;[3] and since the death of the latter, little, if anything, appears to have been done to settle the questions raised by his experiments. Reference must, however, be made to a very interesting investigation by Sir Ernest Rutherford on the “Collision of α-Particles with Light Atoms,”[4] from which it appears certain that when bombarded with the swiftly-moving α-particles given off by radium-C, the atoms of nitrogen may be disintegrated, one of the products being hydrogen. The other product is possibly helium,[5] though this has not been proved. In view of Rutherford’s results a further repetition of Ramsay’s experiments would certainly appear to be advisable. [1] See his “The Presence of Helium in the Gas from the Interior of an X-Ray Bulb,” _Journal of the Chemical Society_, vol. ciii. (1913), pp. 264 _et seq._ [2] See their “The Presence of Neon in Hydrogen after the Passage of the Electric Discharge through the latter at Low Pressures,” _ibid._, pp. 419 _et seq._; and “The Production of Neon and Helium by the Electric Discharge,” _Proceedings of the Royal Society_, _A_, vol. xci. (1915), pp. 30 _et seq._ [3] See especially the report of negative experiments by Mr. A. C. G. Egerton, published in _Proceedings of the Royal Society_, _A_, vol. xci. (1915), pp. 180 _et seq._ [4] See the _Philosophical Magazine_ for June, 1919, 6th Series, vol. xxxvii. pp. 537-587. [5] Or perhaps an isotope of helium (see below).