The soluble ferments or enzymes have always aroused the deepest interest, partly from the mystery which enshrouded their mode of action, partly from the importance of the processes with which they are associated. The peculiar power which each possesses of decomposing apparently unlimited quantities of a specific medium, without itself being used up in the process, has occasioned the confusion of enzyme action with processes truly vital in their nature. Although this action had only been demonstrated as subserving an alimentary function, its aid was invoked to explain many of 'the more obscure phenomena of biology. The series of decompositions which carbohydrates may undergo, known as the alcoholic, lactic, and butyric fermentations, were long ascribed to it. Even when Pasteur had proved that these processes were always correlated with a vital fact —the growth and multiplication of living cells—Traube, Hoppe- Seyler, and Liebig still contended that these might act only indirectly by the formation of soluble ferments. The analogy between fermentation and the infectious processes is so striking that the latter have long been grouped together under the term zymotic diseases, and these we are every day coming to recognise more and more as parasitic diseases conditioned by micro-organisms. Here again, however, many tend to regard the microbe as not acting directly; but through the production of soluble ferments.