This dialysable, thermostable substance, without which alcoholic fermentation cannot proceed, has been provisionally termed the co-ferment or co-enzyme of alcoholic fermentation. This expression was first introduced by Bertrand [1897], to denote substances of this kind, and he applied it in two instances--to the calcium salt which he considered was necessary for the action of pectase on pecten substances, and to the manganese which he supposed to be essential for the activity of laccase. Without inquiring whether these substances are precisely comparable in function with that contained in yeast-juice, the term may be very well applied to signify the substance of unknown constitution without the co-operation of which the thermolabile enzyme of yeast-juice is unable to set up the process of alcoholic fermentation. The active agent of yeast-juice consisting of both enzyme and co-enzyme may be conveniently spoken of as the fermenting complex, and this term will occasionally be employed in the sequel. The co-enzyme is present alike in the filtrates from fresh yeast-juice and from boiled yeast-juice, and is also contained in the liquids obtained by boiling yeast with water and by washing zymin or dried yeast with water. Practically the only chemical property of the co-enzyme, other than that of rendering possible the process of alcoholic fermentation, which has so far been observed, is that it is capable of being decomposed, probably by hydrolysis, by a variety of reagents, prominent among which is yeast-juice. This was observed by Harden and Young in the course of their attempts to prepare a completely inactive residue by filtration. In many cases a residue was obtained which still possessed a very limited power of fermentation, only a small amount of carbon dioxide being formed and the action ceasing entirely after the expiration of a short period; on the subsequent addition of boiled juice, however, a very considerable evolution of carbon dioxide was produced. This was interpreted to mean that the residue in question contained an ample supply of enzyme but only a small proportion of co-enzyme, and that the latter was rapidly destroyed, so that the fermentation soon ceased. The boiled juice then added provided a further proportion of co-enzyme by the aid of which the surplus enzyme was [p064] enabled to carry on the fermentation. This view was confirmed by adding to a solution of a completely inactive filtration residue and glucose successive small quantities of boiled juice and observing the volumes of carbon dioxide evolved after each such addition. Thus in one case successive additions of volumes of 3 c.c. of boiled juice produced evolutions of 8·2, 6, and 6 c.c. of carbon dioxide. In another case two successive additions of 15 c.c. of boiled juice produced evolutions of 54 and 41·2 c.c. On the other hand, the enzyme itself also gradually disappears from yeast-juice when the latter is incubated either alone or with sugar (p. 20).