As the concentration of phosphate is still further increased, it is first observed that the maximum velocity, which is still attained almost immediately after the addition of the phosphate, is maintained for a certain period before the fall commences, and then, as the increase in concentration of phosphate proceeds, that the maximum is only gradually attained after the addition, the period required for this increasing with the concentration of the phosphate. Moreover, with still higher concentrations, the maximum rate attained is less than that reached with lower concentrations, and further, the rate falls off more slowly. The concentration of phosphate which produces the highest rate, which may be termed the optimum concentration, varies very considerably with different specimens of yeast-juice [Harden and Young, 1908, 1]. All these points are illustrated by the accompanying curves (Fig. 7) which show the rate of evolution per five minutes plotted against the time for four solutions in which the initial concentrations of phosphate were (A) 0·033, (B) 0·067, (C) 0·1, and (D) 0·133 molar, the volumes of 0·3 molar phosphate being 5, 10, 15, and 20 c.c. in each case added to 25 c.c. of yeast-juice, and made up to 45 c.c, each solution containing 4·5 grams of glucose. The time of addition is taken as zero, the rate before addition being constant, as shown in the curves. It will be observed that 5 and 10 c.c. (A and B) give the same maximum, whilst 15 c.c. (C) produce a much lower maximum, and 20 c.c. (D) a still lower one, the rate at which the velocity diminishes after the attainment of the maximum being correspondingly slow in these last two cases. By calculating the amount of phosphate which has disappeared as such from the amount of carbon dioxide evolved, [p072] it is found that the maximum does not occur at the same concentration of free phosphate in each case. [Illustration: FIG. 7. (Graph CO{2} rate vs time--Ed.)] These results suggest that the phosphate is capable of forming two or more different unstable associations with the fermenting complex. One of these, formed with low concentrations of the phosphate, has the composition most favourable for the decomposition of sugar, whilst the others, formed with higher concentrations of phosphate, contain more of the latter, probably associated in such a way with the fermenting complex as to render the latter partially or wholly incapable of effecting the decomposition of the sugar molecule. As the fermentation proceeds slowly in the presence of excess of phosphate, the concentration of the latter is reduced by conversion into hexosephosphate, and a re-distribution of phosphate occurs, resulting in the gradual change of the less active into the more active association of phosphate with fermenting complex, and a consequent rise in the rate of fermentation.