RELATION OF ALGAE TO DISSOLVED GASES. 199 
The above-named plants had been kept in stock in open 
jars or flasks supplied with tap water, which contains con- 
siderable lime in solution and is slightly alkaline to phenolph- 
thalein. This usually loses its color when plants are first 
introduced, but becomes deep red again after a day or two in 
bright sunlight or passing CO2 through the water. A period 
of darkness had the same effect of destroying or withdrawing 
the color. But the color returned again within a few hours 
of bright sunlight, and the process was repeated with the 
alternation of darkness and light. 
This I attributed to the action of photosynthesis during the 
day, withdrawing COz from the water, thus changing the solu- 
tion from a weakly acid one to a neutral and alkaline one, 
causing the red color; and respiration by night or the balance 
of respiration over photosynthesis on cloudy days restoring 
the lost COz and again rendering it neutral, or acid and 
colorless. 
This I believed at the time, to be an original discovery and 
interpretation of my own, but on investigation found that, 
among a number of others, Hassak (’88) had given the same 
interpretation, backed by proofs at considerable length. To 
this, however, Loew (’93) found objections, and claimed that 
the coloring was due, not to a normal carbonate of lime, as 
explained by Hassak, but to the excretion of a colloidal com- 
pound of calcium, which is alkaline to phenolphthalein. 
Hassak, in proof of his stand-point, states that sodium bi- 
carbonate gave the same results as calcium bicarbonate, and 
he believed that other carbonates would do the same if he only 
had the opportunity to try them. 
Hassak used 0.1% and 0.2% solutions in his experiments. 
Accordingly I prepared 0.1% and 0.2% solutions of sodium 
bicarbonate, using melted snow, as it was necessary for the 
experiment that the water be free only from carbonates. 
January 9. Into these were put Elodea, Pithophora, 
Cladophora, Oedogonium, Hydrodictyon and Spirogyra. 
Another set of blanks was used as a control, containing only 
water and plants, and another with nothing but water. To 
each tube were added three drops of phenolphthalein (1% 
