Harvey and Danielli (1939) proposed a lipid bilayer membrane covered on each side with a layer of protein to account for measurements of surface tension. In 1941 Boyle & Conway showed that the membrane of frog muscle was permeable to both and , but apparently not to , so the idea of electrical charges in the pores was unnecessary since a single critical pore size would explain the permeability to , , and as well as the impermeability to , , and .