Although as indicated above Euler's theory cannot apply to zymase itself, if applied to the hexosephosphatase it would afford a consistent explanation of the facts. According to this modified view it would be the hexosephosphatase of yeast which existed largely in the combined form, so that in extracts, in dried yeast and in presence of toluene only the small fraction which was free would remain active. The zymase on the other hand would have to be regarded as existing to a large extent in the free state so that it would pass into extracts comparatively unimpaired in amount and capable under proper conditions (i.e. when supplied with sufficient phosphate) of bringing about a very vigorous fermentation. The theory of combined and free enzymes is undoubtedly of considerable value, although it cannot be considered as fully established. FERMENTATION BY LIVING YEAST. Much important information as to the nature of the processes involved in fermentation has been acquired by the direct experimental study of the action of living yeast on different sugars. This phenomenon has formed the subject of several investigations from the kinetic point of view, and its general features may now be regarded as well established. The difficulty, which must as far as possible be avoided in quantitative experiments of this sort with living yeast, is the alteration [p128] in the amount or properties of the yeast, due to growth or to some change in the cells. This has been obviated in the work of Slator [1906] by determining in every case the initial rate of fermentation, so that the process only continues for a very short period, during which any change in the amount or constitution of the yeast is negligible. The method has the further advantage that interference of the products of the reaction is to a large extent avoided. The pressure apparatus already described (p. 29) was employed by Slator, the rate of production of carbon dioxide being measured by the increase of pressure in the experimental vessel. INFLUENCE OF CONCENTRATION OF DEXTROSE ON THE RATE OF FERMENTATION. With regard to this important factor it is found that the action of living yeast follows the same law as that of most enzymes (p. 121); within certain wide limits the rate of fermentation is almost independent of the concentration of the sugar. This conclusion has been drawn by many previous investigators from their experiments [Dumas, 1874; Tammann, 1889; Adrian Brown, 1892; O'Sullivan, 1898, 1899] and is implicitly contained in the results of Aberson [1903], although he himself regarded the reaction as monomolecular. [Illustration: FIG. 8. (Graph Fermentation Velocity vs Conc. Dextrose--Ed.)] Slator, working with a suspension of ten to twelve yeast-cells per 1/4000 cubic millimetre at 30°, obtained the results which are embodied in the curve (Fig. 8).