catalyses should obey the same “laws” as to mass action, speed, effect of accumulation of products of action, and the like; but it does not follow that invertin and sulphuric acid produce their effects in the same way. Fermentations are instances of catalysis, but all catalytic actions are not fermentations. So far from dwelling upon the resemblance between fermentation and the catalysis of mineral chemistry, chemists nowadays incline to regard fermentation as essentially a reaction of life. It is very difficult, when attempting to present ideas which are new to thought, to adapt, without ambiguity, existing words. It would be absurd to talk of a substance removed from yeast or bacteria or blood-corpuscles by a process which involves cooling with liquid air, grinding with powdered glass, solution in water, precipitation with absolute alcohol, and resolution in water, as alive. Yet, unlike any known mineral product, it is easily killed. Ferments are not destroyed by cold, but their activity is arrested. They are most active at about the body temperature. Their activity is annihilated by heating them, in solution, to the temperature at which albumin coagulates—a little over 50° C. Although they are not alive, their behaviour very closely resembles that of living matter. They can be obtained only from living things. They produce their effects even though they are present in almost infinitely small quantity. It is impracticable to make a chemical analysis of a ferment, owing, in the first place, to the very small amount available for analysis, and, in the second place, because of the impossibility, with existing methods, of obtaining a ferment pure. The amount of ferment present in even a great mass of yeast, or in many pounds of salivary gland or pancreas, is extremely small. However prepared, it is always accompanied with proteid substances. It is impossible to say whether ferments, like proteins, have heavy nitrogen-containing molecules. The fact that they are not diffusible suggests that they have. It would be straining language to term fermentation a phenomenon of life; worse, to define life as a sequence of fermentations. Yet it is safe to say that all the chemical changes carried out by living organisms are fermentations. Fermentation and the chemistry of life are almost synonymous terms.