Brown. — Some Studies on Yeast. 
217 
initially present, and from the course of the curve we may conclude that, 
with a liquid seeded with yeast at the rate of one cell per unit volume, the 
stimulation to reproduction ceases when the initial oxygen supply is 
equivalent to about 0*5 c.c. per 100 c.c. of the liquid. 
Since one cell of yeast per unit volume is equivalent to about o*i c.c. of 
yeast per 100 c.c. and the whole of the oxygen is rapidly taken up from the 
liquid, it is evident that under the above conditions the volume of oxygen 
taken up by each cell and constituting the ■ oxygen charge * is about five 
times the volume of the cell itself. 
PART II. 
The Metabolism of the Yeast-cell, with Special Reference 
to the Thermal Phenomena of Fermentation. 
The earlier ideas that the yeast-cell, by its mere presence, is able 
to exert an extraneous transforming influence on certain substances dis- 
solved in the surrounding medium have now proved to be erroneous. All 
known facts point consistently to these changes taking place exclusively 
within the cell, and they therefore connote a constant centripetal flux of 
material through the cell-wall, and a counter flux of the metabolites of equal 
or nearly equal amount in the opposite direction. 
We can obtain some idea of the extraordinary activity of this metabolic 
‘ mill ’ by determining the ratio of the mass of the cell to that of the sugar 
fermented in a given time, or we can, if we prefer it, indicate the activity 
under given conditions by stating the time necessary for the cell to trans- 
form its own weight of sugar into alcohol and carbon dioxide. 
Another way of illustrating the activity of the cell is to study the 
thermal phenomena of fermentation, and to refer the liberation of energy 
to the cell in such a manner as to show to what extent the temperature of 
the cell would be raised in a given time if the liberated heat were all 
concentrated on the cell, and had not to be shared by its environment. 
The advantage of this second method is that it gives us a rough means 
of comparing the metabolic activity of a yeast-cell with that of a warm- 
blooded animal ; but it takes for granted that we know the amount of heat 
energy evolved during the breaking down of a unit weight of sugar by the 
cell, a value which is still to some extent in dispute. 
The earliest attempts to ascertain the actual amount of heat evolved 
during the splitting up of sugar into alcohol and carbon dioxide were made 
by Berthelot and by C. V. Rechenberg, whose methods, however, were 
entirely indirect, since they were based on a determination of the differences 
between the calculated heat of formation of dextrose and the calculated 
heat of formation of the equivalent amounts of alcohol and carbon dioxide. 
