The extent of the action of arsenite appears from the following results. In one case a rate of 1·7 c.c. was increased to 7 c.c. by 0·06 molar arsenite. In another experiment it was found that the optimum concentration was 0·04 molar arsenite, the addition of which increased the rate three-fold. As in the case of arsenate the optimum concentration and the corresponding maximum rate of fermentation are considerably greater for fructose than for glucose. The relative rates produced by the addition of equivalent amounts of arsenate and arsenite (1 c.c. of 0·3 molar solution in each case to 20 c.c. of yeast-juice) were 27·5 and 3·1, the original rate of the juice being 1·7. In general the optimum concentration of arsenite is considerably greater than that of arsenate. The inhibiting effects of higher concentrations of arsenite and arsenate also present close analogies, but this most interesting aspect of the question has not yet been sufficiently examined to repay detailed discussion. Buchner [Buchner, E. and H., and Hahn, 1903, pp. 199-205] has suggested that the inhibition is due primarily to some change in the colloidal condition of the enzyme and has shown that certain colloidal substances appear to protect it, as does also sugar. The possibility is also present that inactive combinations of some sort are formed between the fermenting complex and the inhibiting agent, in the manner suggested to account for the inhibiting effect of excess of phosphate (p. 72). It seems most probable that the effect is a complex one, in which many factors participate. NATURE OF THE ACCELERATION PRODUCED BY ARSENATE AND ARSENITE. In explanation of the remarkable accelerating action of arsenates and arsenites two obvious possibilities present themselves. In the [p079] first place the arsenic compound may actually replace phosphate in the reaction characteristic of alcoholic fermentation, the resulting arsenic analogue of the hexosephosphate being so unstable that it undergoes immediate hydrolysis, and is therefore only present in extremely small concentration at any period of the fermentation and cannot be isolated. In the second place it is possible that the arsenic compound may accelerate the action of the hexosephosphatase of the juice, and thus by increasing the rate of circulation of the phosphate produce the permanent rise of rate. With this effect may possibly be associated a direct acceleration of the action of the fermenting complex.