For the convenience of those who have read the first edition,

a list of the sections and chapters which contain the most important additions and alterations is added below the table of contents.

The writing of a complete account of a subject like radio-activity, in which so much new work is constantly appearing, has been a matter of no little difficulty. Among other things it has involved a continuous revision of the work while the volume was passing through the press.

I wish to express my thanks to my colleague Professor Harkness for the care and trouble he has taken in revising the proofs and for many useful suggestions; also to Mr R. K. M^cClung for his assistance in correcting some of the proofs and in preparing the index.