Brown. — Some Studies on Yeast. 
213 
well adapted for this purpose, since the presence of the yeast-cells does not 
sensibly affect its accuracy. A description of the method has been given 
by Clowes and Biggs (Journ. Soc. Chem. Ind., April 15, 1904). 
A stoppered bottle of from 300 to 400 c.c. capacity is filled with the 
water to be examined, due precautions being taken to avoid agitation 
during the process. About 1 c.c. of a strong solution of manganous 
chloride is then added, followed by 2 c.c. of a solution containing 33 per 
cent, of alkaline hydroxide and 10 per cent, of potassium iodide. The 
bottle, which must be quite full, is then closed with its stopper, taking care 
to avoid the inclusion of any air bubbles, and is inverted several times in 
order to mix the liquid thoroughly. The manganous oxide which is 
precipitated rapidly absorbs any free oxygen which is present, and in order 
to complete the reaction the bottle is put in the dark for fifteen minutes. Its 
contents are then acidified with from 2 to 3 c.c. of strong hydrochloric or 
sulphuric acid, which dissolves the precipitate, leaving the liquid slightly 
coloured with iodine. This liberated iodine is a measure of the original 
amount of free oxygen present in the water, and is titrated with a standard 
solution of thiosulphate, of which the oxygen value is known, using starch 
as the indicator. 
If the experiment is repeated with another sample of the water which 
has been fully aerated at the same temperature, then the ratio of the 
amount of thiosulphate solution employed in the two experiments gives 
the degree of oxygen-saturation of the first sample when full saturation is 
represented as unity. 
The curves in Fig. 7 show the rates of absorption of dissolved oxygen 
from fully aerated water at 1 5 0 C. by amounts of yeast corresponding to 
i*o, 2*o, and 3-0 cells per unit volume respectively, the actual amounts of 
pressed yeast per 100 c.c. being o*i, 0*2, and 0.3 grm. 
It will be noted that above 1 cell per unit volume the absorption of 
oxygen is approximately proportional to the number of cells per unit volume, 
and that with the small amount of only 0-3 grm. of pressed yeast per 100 c.c. 
or 3-0 cells per unit volume, the absorption is complete in two and a half 
hours. 
It might reasonably be expected that a reaction of this kind would 
show a tendency to slow down as the concentration of the dissolved oxygen 
gradually decreased. Absorption of dissolved oxygen by the yeast-cell 
progresses, however, as a linear function of the time , just as the invertive, 
fermentative, and reproductive actions do. There is no doubt a common 
cause controlling all these functions, which is to be found in the fact that 
they are all dependent on cell metabolism, and that the processes involved, 
although in some cases rapid as judged by ordinary standards of time, are 
very slow compared with the relatively rapid processes of diffusion by which 
the materials involved get access to the cell -protoplasm from the surrounding 
Q 
