RADIOACTIVE SUBSTANCES AND THEIR RADIATIONS RADIOACTIVE SUBSTANCES AND THEIR RADIATIONS BY R. RUTHERFORD, D. Sc., PH. D., LL. D., F. R. S. - NOBEI. LAUREATE t. ANGWORTHY PROFESSOR OF PHYSICS, UNIVERSITY OF MANCHESTER Cambridge at the University Press PREFACE TN 1904 I published through the Cambridge University Press a collected account of radio-active phenomena entitled Radio activity. This was followed a year later by a revised and enlarged edition. In the seven years that have elapsed since the latter publication there has been a steady and rapid - growth of our knowledge of the properties of the radiations from active sub stances, and of the remarkable series of transformations that occur in them. In the present work I have endeavoured to give an accurate and concise account of the whole subject as it stands to-day within the compass of a single volume. A few pages from the earlier book have been utilised, but, otherwise, the present volume is an entirely new work, It is of interest to signalise some of the main directions of advance since the publication of the second edition of my Radio activity. There has been a very great increase in the amount and accuracy of our knowledge of the radiations from active substances, the nature of their absorption by matter, and of their connection with the transformations. The discovery of methods of counting single a particles has proved of great service not only in extending our knowledge of the a rays, but also in obtaining accurate data for the calculation of a number of important radio-active quantities and atomic magnitudes. The discovery of the recoil of radio active atoms resulting from the expulsion of a particles has proved VI PREFACEvery valuable as a means of separation of radio-active substances, and has also added another very interesting type of corpuscular radiation for study. Our knowledge of the series of transforma tions in active substances has been much extended. In 1905 twenty of these were known the number is now thirty-two, and there is some evidence that a few still remain undetected. The study of radio-activity is concerned not only with the radiations and transformations of active substances, but also with the distribution of active matter and the products of its trans formation in minerals and throughout the earths crust and atmosphere. The researches of Strutt, Joly, and others, have added greatly to our knowledge of the amount and distribution of active matter in typical rocks, and have indicated that the accompanying heating effect has an important bearing on the duration of the earths internal heat and on geological changes in general. The investigations of Elster and Geitel, Eve, arid others have brought out clearly that the electrical state of our atmosphere is undoubtedly influenced greatly by the distribution of active matter throughout it. The use of radio-active substances for therapeutic purposes has been widely extended, but a considera tion of this question is outside the scope of the present volume. In one respect the treatment of the subject has necessitated very little change for the transformation theory advanced in explanation of radio-active phenomena has undergone no essential modification in the interval. This theory has been found not only to give a reasonable explanation of all the facts that are at present known, but has suggested many lines of investigation resulting in awide extension of our knowledge. The passages in my previous work, which I have found it desirable to retain are, mainly, those dealing with the ionisatiori of gases and the methods of radio-active measurement, for it seemed to me that the book was in no sense complete without PREFACE Vll a brief reference to these important subjects. I have added three short Appendixes, the subject-matter of which could not be conveniently included in the main body of the volume. In the first an account is given of the methods of measurement of quantities of radium...