Effects somewhat similar to those produced by arsenates were observed by Buchner [Buchner and Rapp, 1897; 1898, 1, 2, 3; 1899, 2; Buchner, E. and H., and Hahn, 1903, pp. 184-205] when potassium arsenite was added to yeast-juice. This substance, the action of which on yeast had been adduced by Schwann as a proof of the vegetable nature of this organism, was employed by Buchner on account of its poisonous effect on vegetable cells as an antiseptic and as a means of testing for the protoplasmic nature of the agent present in yeast-juice. Its effect on the fermentation was, however, found to be irregular, and at the same time it did not act as an efficient antiseptic in the concentrations which could be employed. Even 2 per cent. of arsenious oxide, added as the potassium salt, had in many cases a decided effect in diminishing the total fermentation obtained with cane sugar, and this effect increased with the concentration. A number of irregularities were also observed which cannot here be discussed. It was further found that in some cases 2 per cent. of arsenious oxide inhibited the fermentation of glucose but not of saccharose, or of a mixture of glucose and fructose, whilst its effect on fructose alone was of an intermediate character. The important observation was also made by Buchner that the addition of a suitable quantity of arsenite as a rule caused a greatly increased fermentation during the first sixteen hours even in experiments in which the total fermentation was diminished. By examining the effect of arsenite on fermentation in a similar manner to that of arsenate, Harden and Young [1911, 1] have found that a close analogy exists [Pg 078] between the effects and modes of action of these substances, but that arsenite produces a much smaller acceleration than arsenate. An optimum concentration of arsenite exists, just as in the case of arsenate, which produces a maximum rate of fermentation. Further increase in concentration leads to inhibition, and in no case is there any indication of the production of an exactly equivalent amount of fermentation as in the case of phosphate. In various experiments with dialysed, evaporated, and diluted yeast-juice in which 2 per cent. of arsenious oxide was found by Buchner to inhibit fermentation, it is probable that, owing to the small amount of fermenting complex left, this amount of arsenious oxide was considerably in excess of the optimum concentration, although Buchner ascribes the effect to the removal of some of the protective colloids of the juice, owing to the prolonged treatment to which it had been subjected.