Radio-activity a permanent property--Induced activity--Discovery of Uranium X--Conclusions drawn--Search for new radio-active bodies--Methods of investigation--Nature of the radiations--Life-periods--Equilibrium series 17 CHAPTER IV NATURE OF THE ALPHA PARTICLE Disintegrating of the elements--Identification of the rays--The alpha rays--Alpha rays consist of solid particles--Electrical charge--Helium formed from alpha particles--Discovery of Helium--Characteristics of Helium--Table of constants 25 CHAPTER V THE STRUCTURE OF THE ATOM Properties of Radium--Energy evolved by radium--Necessity for a disintegration theory--Disintegration theory--Constitution of the atom--Rutherford's atom--Scattering of alpha particles--Stopping power of substances 32 CHAPTER VI RADIO-ACTIVITY AND CHEMICAL THEORY Influence upon chemical theory--The periodic system--Basis of the periodic system--Influence of positive nucleus--Determination of the atomic number--Use of X-ray spectra--Changes caused by ray-emission--Atomic weight losses--Lead the end product--Changes of position in the periodic system--Changes from loss of beta particles--Isotopes--Radio-activity in nature--Radio-active products in the earth's crust--Presence in air and soil waters--Cosmical radio-activity 41 INDEX 53 A BRIEF ACCOUNT OF RADIO-ACTIVITY CHAPTER I DISCOVERY OF RADIO-ACTIVITY The object of this brief treatise is to give a simple account of the development of our knowledge of radio-activity and its bearing on chemical and physical science. Mathematical processes will be omitted, as it is sufficient to give the assured results from calculations which are likely to be beyond the training of the reader. Experimental evidence will be given in detail wherever it is fundamental and necessary to a confident grasp of some of the marvelous deductions in this new branch of science. Theories cannot be avoided, but the facts remain while theories grow old and are discarded for others more in accord with the facts. The Beginning As so often happens in the history of science, the opening up of this new field with its fascinating disclosures was due to an investigation undertaken for another purpose but painstakingly carried out with a mind open to the truth wherever it might lead.