The experimental decision between these alternative explanations is rendered possible by the use of a mixture of enzyme and co-enzyme free from phosphate and hexosephosphate. As has already been described (p. 55) a mixture of boiled yeast-juice, which has been treated with lead acetate, glucose or fructose, and washed zymin can be prepared which scarcely undergoes any fermentation unless phosphate be added. If now arsenates or arsenites can replace phosphate, they should be capable of setting up fermentation in such a mixture. Experiment shows that they do not possess this power. For fermentation to proceed phosphate must be present and it cannot be replaced either by arsenate or arsenite [Harden and Young, 1911, 1]. The effect of these salts on the action of the hexosephosphatase can also be ascertained by a modification of the foregoing experiment. If a hexosephosphate be made the sole source of phosphate in such a mixture as that described above, in which it must be remembered abundance of sugar is present, the rate at which fermentation can proceed will be controlled by the rate at which the hexosephosphate is decomposed with formation of phosphate. Experiment shows that in the presence of added arsenate or arsenite the rate of fermentation is largely increased, so that the effect of these salts must be to increase the rate of liberation of phosphate, or in other words, to accelerate the hydrolytic action of the hexosephosphatase. This conclusion is even more strikingly confirmed by a comparison of the direct action of yeast-juice on hexosephosphate in presence and in absence of arsenate, as measured by the actual production of free phosphate. In a particular experiment this gave rise to 0·0707 gram of Mg{2}P{2}O{7} in the absence of arsenate and 0·6136 gram of Mg{2}P{2}O{7} in the presence of arsenate. The results obtained with arsenite are throughout very similar to those given by arsenate, but are not quite so striking. It may therefore be affirmed with some confidence that the chief action of arsenates [p080] and arsenites in accelerating the rate of fermentation of sugars by yeast-juice or zymin, consists in an acceleration of the rate at which phosphate is produced from the hexosephosphate by the action of the hexosephosphatase.