IN writing this book, my object has been to give as clear and simple an account of the phenomena of radio-activity as the subject admits of, without sacrificing accuracy. The extraordinary properties of radium have excited general interest outside the scientific world, and there are probably many who would be glad to learn something of the subject, if they could find it explained without the use of technicalities. However essential mathematical methods may be in developing the subject, they are seldom really necessary in presenting the results. Moreover, some idea of the train of reasoning can generally be given in ordinary language. I have not found it possible to avoid assuming some elementary scientific knowledge on the part of the reader, but this has been reduced to the smallest limits, and probably a great part of the book will be intelligible without it. I have not attempted to describe all the pheno mena which have been recorded, but have confined myself to those which seemed most significant and interesting.