““The writer found that certain freshwater crustaceans, namely Californian species of Daphnia, copepods, and Gammarus when indifferent to light can be made intensely positively heliotropic by adding some acid to the fresh water, especially the weak acid CO2. When carbonated water (or beer) to the extent of about 5 c.c. or 10 c.c. is slowly and carefully added to 50 c.c. of fresh water containing these Daphnia, the animals will become intensely positive and will collect in a dense cluster on the window side of the dish. Stronger acids act in the same way but the animals are likely to die quickly. . . Alcohols act in the same way. In the case of Gammarus the positive heliotropism lasts only a few seconds, while in Daphnia it lasts from 10 to 50 minutes and can be renewed by the further careful addition of some CO2.””
