Their importance emerges alike in the purely theoretical and in the technical aspects of the subject. In the natural order, also, investigation has shown how predominating a part the catalytic phenomenon plays. From the strictly utilitarian point of view, Industry was not slow to appreciate the advantages to be derived from the employment of agencies which should facilitate and speed-up manufacturing processes, nor was there lacking a body of scientific workers who steadily supplied the investigative and theoretical bases upon which such applications could be reared. As the scientific viewpoint developed the physiologist and biologist rapidly associated the simpler chemical phenomena with the more complex processes operating in living matter. To-day, therefore, catalysis intrudes prominently into all branches of natural science, both pure and applied. The need for an exposition of the fundamental principles involved in this particular field of general chemistry and of the applicability of such principles in modern life has been widely felt and has offered to the authors the necessary inducement to prepare the present volume. An excellent chapter in Mellor s Chemical Statics and Dynamics has formed, hitherto, the sole treatment in English of the theoretical aspects of the problem. From time to time various papers have catalogued the more important practical applications of catalysis. The monograph of Sabatier, La Catalyse en Chimie Organique, deals more particularly with the applications to preparative organic chemistry with slight reference to the physico-chemical theories involved.