What clew have we in regard to the nature of this synthetic power? We know that the comparatively great velocity of chemical reactions in a living organism is due to the presence of enzymes (ferments) or to catalytic agencies in general. Some of these catalytic agencies are specific in the sense that a given catalyzer can accelerate the reaction of only one step in a complicated chemical reaction. While these enzymes are formed by the action of the body they can be separated from the body without losing their catalytic efficiency. It was a long time before scientists succeeded in isolating the enzyme of the yeast cell which causes the alcoholic fermentation of sugar; and this gave rise to the premature statement that it was not possible to isolate this enzyme since it was bound up with the life of the yeast cell. Such a statement was even made by a man like Pasteur, who was usually a model of restraint in his utterances, and yet the work of Buchner proved him to be wrong. The general mechanism of the action of the hydrolyzing enzymes is known. The old idea of de la Rive, that a molecule of enzyme combines transitorily with a molecule of substrate; the further idea, which may possibly go back to Engler, that the molecule of substrate is disrupted in the “strain” of the new combination and that the broken fragments fall off or are easily knocked off by collision from the ferment molecule which is now ready to repeat the process, seems to be correct. On the assumption that the velocity of enzyme reaction is proportional to the mass of the enzyme and that de la Rive’s idea was correct, Van Slyke and Cullen were able to calculate the coefficients of the velocity of enzyme reactions for the fermentation of urea and other substances, and the agreement between calculated and observed values was remarkable.[16] [16] Van Slyke, D. D., and Cullen, G. E., _Jour. Biol. Chem._, 1914, xix., 141.