The essays contained in this volume were written on different occasions mostly in response to requests for a popular presentation of the results of the author's investigations. The title of the volume characterizes their general tendency as an attempt to analyze life from a purely physico-chemical view point. Since they deal to a large extent with the personal work of the author, repetition was unavoidable, but in view of the technical difficulties presented by some of the topics this may serve to facilitate the understanding of the subject.The author wishes to thank the editors and publishers who gave their consent to the reprinting of these essays: Professor. J. McKeen Cattell, of Columbia University, Professor Albert Charles Seward, of the University of Cambridge, England, Ginn & Co., of Boston, G. P. Putnam's Sons, of New York and London, and the J. B. Lippincott Company, Philadelphia.