The apparatus comprises two stoppered glass boiling tubes connected by three way taps to either limbs of graduated manometer. The other tube acts as a control thermo-barometer. One tube holds the organisms and has a syringe for altering the level of liquid in the manometer during setting up. CO2 given off by the respiring organisms is absorbed by potassium hydroxide in the experimental chamber resulting in a change in the level of liquid in the manometer tube.