be the concentration of either of these, or the product of the two concentrations (see p. 122). The hexosephosphatase appears invariably to outlast the enzyme and co-enzyme. The condition at any moment could be determined experimentally if it were possible to add enzyme, co-enzyme and hexosephosphatase at will and so ascertain which of these produced an acceleration of the rate. Unfortunately this can at present be only very imperfectly accomplished, owing to the impossibility of separating these substances from each other and from accompanying matter which interferes with the interpretation of the result. A third condition can also be established by adding to the fermenting mixture of the juice and sugar a solution of phosphate. The supply of phosphate is now almost independent of the action of the hexosephosphatase, and the measured rate represents the rate at which reaction (1), p. 51, can occur between sugar and phosphate in the presence of the fermenting complex consisting of enzyme and co-enzyme. This change is controlled, so long as sugar and phosphate are present in the proper amounts, by the concentration of the fermenting complex or possibly of either the enzyme or the co-enzyme. If only a single addition of a small quantity of phosphate be made, the rate falls as soon as the whole of this has been converted into hexosephosphate and the reaction then passes into the stage just considered, in which the rate is controlled by the production of free phosphate. Although these varying reactions have not yet been exhaustively studied from the kinetic point of view, owing to the experimental difficulties to which allusion has already been made, investigations have nevertheless been carried out on the effect of the variation of concentration of yeast-juice and zymin as a whole, as well as of the carbohydrate. Herzog [1902, 1904] has made experiments of this kind with zymin, and Euler [1905] with yeast-juice, whilst many of the results [p121] obtained by Buchner and by Harden and Young are also available. The actual observations made by these authors show that the initial velocity of fermentation is almost independent of the concentration of sugar within certain limits, but decreases slowly as the concentration increases. When the velocity constant is calculated on the assumption that the reaction is monomolecular [see Bayliss, 1914, Chap. VI], approximate constancy is found for the first period of the fermentation. This method of dealing with the results is, however, as pointed out by Slator, misleading, the apparent agreement with the law of monomolecular reactions being probably due to the gradual destruction of the fermenting complex.