My thanks are due to the Council of the Royal Society, and to the Publishers of the "Journal of Physiology" for permission to make use of blocks (Figs. 2, 4 and 7) which have appeared in their publications. A. H. PREFACE TO THE SECOND EDITION. In the New Edition no change has been made in the scope of the work. The rapid progress of the subject has, however, rendered necessary many additions to the text and a considerable increase in the bibliography. A. H. /May/,1914. CONTENTS. CHAPTER PAGE I. HISTORICAL INTRODUCTION 1 II. ZYMASE AND ITS PROPERTIES 18 III. THE FUNCTION OF PHOSPHATES IN ALCOHOLIC FERMENTATION 41 IV. THE CO-ENZYME OF YEAST-JUICE 59 V. ACTION OF SOME INHIBITING AND ACCELERATING AGENTS ON THE ENZYMES OF YEAST-JUICE 70 VI. CARBOXYLASE 81 VII. THE BY-PRODUCTS OF ALCOHOLIC FERMENTATION 85 VIII. THE CHEMICAL CHANGES INVOLVED IN FERMENTATION 96 IX. THE MECHANISM OF FERMENTATION 119 BIBLIOGRAPHY 136 INDEX 155 [p001] CHAPTER I. HISTORICAL INTRODUCTION. The problem of alcoholic fermentation, of the origin and nature of that mysterious and apparently spontaneous change which converted the insipid juice of the grape into stimulating wine, seems to have exerted a fascination over the minds of natural philosophers from the very earliest times. No date can be assigned to the first observation of the phenomena of the process. History finds man in the possession of alcoholic liquors, and in the earliest chemical writings we find fermentation, as a familiar natural process, invoked to explain and illustrate the changes with which the science of those early days was concerned. Throughout the period of alchemy fermentation plays an important part; it is, in fact, scarcely too much to say that the language of the alchemists and many of their ideas were founded on the phenomena of fermentation. The subtle change in properties permeating the whole mass of material, the frothing of the fermenting liquid, rendering evident the vigour of the action, seemed to them the very emblems of the mysterious process by which the long sought for philosopher's stone was to convert the baser metals into gold. As chemical science emerged from the mists of alchemy, definite ideas about the nature of alcoholic fermentation and of putrefaction began to be formed. Fermentation was distinguished from other chemical changes in which gases were evolved, such as the action of acids on alkali carbonates (Sylvius de le Boë, 1659); the gas evolved was examined and termed gas vinorum, and was distinguished from the alcohol with which it had at first been confused (van Helmont,