All the chemical transformations which protoplasm is able to accomplish are of the nature of fermentations. The term =fermentation= was first applied to the effervescence which occurs in grape-juice when its sugar is being converted into alcohol, carbonic acid gas, and certain substances which appear in relatively small quantities. It was discovered later that the yeast which effects this change is a unicellular plant. The term “fermentation” was extended to the production of vinegar from alcohol, and eventually to all such reactions as are carried out by living organisms, or by the secretions or products of living organisms, without the destruction of the agent which is effective in the process. A ferment is an organic body which brings about changes in other bodies without itself undergoing change. At the end of the process, however prolonged, there is as much ferment as there was at the beginning, and its chemical nature is the same. Rennin has been made to curdle nearly a million times its weight of milk, pepsin to digest half a million times its weight of fibrin. As the ferment is not consumed, there is no relation, except one of speed, between the ferment and the quantity of fermentable substance which it is able to transform. We said that a ferment is an organic body. It is necessary to introduce the qualification “organic,” because certain reactions termed “catalyses” which occur in mineral chemistry resemble fermentations in respect of the non-destruction of the agent which serves as intermediary. If a solution of cane-sugar containing a very small quantity of sulphuric acid is boiled, the cane-sugar is “inverted.” It is changed into a mixture of fruit-sugar and levulose. The ferment invertin of the gastric juice and of intestinal juice produces a similar effect; and just as invertin remains unchanged, so also the sulphuric acid is found in the mixture unchanged in nature and in amount after an unlimited inversion of cane-sugar. Great stress was formerly laid upon the similarity between fermentation and catalysis. It has now been shown that catalytic actions are not necessarily of the same nature as fermentation, although the results and, as far as is visible, the means are similar. For example, finely divided platinum (or, better, palladium) causes an indefinite quantity of oxygen and hydrogen to unite. The reaction comes within the category of catalyses. But it is widely different from a fermentation. The metal causes hydrogen to condense, and actually absorbs it into its surface layer. In the liquid form hydrogen cannot resist combination with oxygen. This may be termed a “physical phenomenon,” adopting the common distinction between chemistry and physics. There is no reason for thinking that fermentations can be explained in so simple a way. They may, however, be grouped under the designation “catalyses.” As the initial conditions and final results are similar, it is inevitable that fermentations and