Joseph Barcroft, in 1920, spent six days in a sealed glass chamber to investigate respiration at altitude. The partial pressure of oxygen was initially 163 mmHg falling to 84 mmHg (equivalent to an altitude of 18,000 ft) as the experiment progressed. Barcroft was attempting to disprove a theory of John Scott Haldane that the lungs actively secrete oxygen into the blood (rather than just through the process of passive diffusion) under conditions of low oxygen partial pressure. Barcroft suffered from severe hypoxia. At the end of experiment, part of Barcroft's left radial artery was removed for investigation.