RELATION OF ALGAE TO DISSOLVED GASES. 201 
is the fact that the first time this color reaction was noticed 
by me was in a small flask of pond water left in a south 
window during the very bright and very hot days of the 
summer of 1911. The color appeared faintly at first and 
increased from day to day until the cause was searched out. 
At first the flask seemed to contain nothing but the water, 
no plant material being visible; but closer investigation 
showed that the change in color was due to a few microscopic 
alge, chiefly Desmids, Pediastrum and the like, which, as 
before mentioned, have little tolerance for calcareous waters, 
and do not, as a rule, have lime deposited on their surfaces. 
These algee developed mostly as a result of multiplication in 
the flask. 
INCREASED AND REDUCED CO2 AND 02 PRESSURE. 
Tn order to test the conduct of higher pressures of COz upon 
the dissolved carbonates and contained alge, the following 
experiments were prepared :— 
A. February 10. A half-pint milk bottle was half-filled 
with distilled water; three small crystals of KHCOs were 
added and three drops of phenolphthalein, as in former ex- 
periments; this gave no color. Into this were put about 5 
grams of Pithophora. The bottle was then stoppered with a 
rubber stopper, attached to a suction pump and exhausted to 
one-half vacuum, then refilled to atmospheric pressure with 
COz from the attached generator. The jar was then closed 
air-tight and placed in the light of a south window. In a 
few days the color appeared, indicating photosynthesis and 
the reduction of bicarbonate to normal carbonate. The color 
faded during the night and cloudy days, reappeared on 
bright days, more especially when in the bright sunlight, 
just as the previous cases with open vessels and tap water. 
B. A control jar like “A,” except that tap water alone 
was used, without KHCOs. This went through the usual 
color changes. 
February 24. Jar “A” was opened and a lighted match 
applied. The flame flared up more brightly, showing that 
