The electrochemical reduction of oxygen was discovered by Heinrich Danneel and Walter Nernst in 1897. Polarography using dropping mercury was discovered accidentally by Jaroslav Heyrovsky in Prague in 1922. This method produced the first measured oxygen tension values in plasma and blood in the 1940s. Brink, Davies, and Bronk implanted platinum electrodes in tissue to study oxygen supply, or availability, from about 1940, but these bare electrodes became poisoned when immersed in blood. Leland Clark sealed a platinum cathode in glass and covered it first with cellophane; he then tested silastic and polyethylene membranes. In 1954 Clark conceived and constructed the first membrane-covered oxygen electrode having both the anode and cathode behind a nonconductive polyethylene membrane. The limited permeability of polyethylene to oxygen reduced depletion of oxygen from the sample, making possible quantitative measurements of oxygen tension in blood, solutions, or gases. This invention led to the introduction of modern blood gas apparatus.