From: A laboratory manual of experimental physiology, by Lois McPhedran Fraser, 1919, page 103.
From: The respiratory exchange of animals and man by August Krogh, 1916, page 40.
“Valves such as Muller’s and other fluid valves, generally filled with water or mercury, were formerly used extensively. They have the advantage that leakage backwards is impossible, but their resistance is generally considerable. Zentz uses the “Darmventile” invented by Speck. These are certainly effective and the resistance very slight, but the valves are large and cumbrous. Reliable metal valves with a minimum resistance have been constructed by Chauveau (Tissot, 1904) and by the firm Siebe, Gorman (Douglas, 1911). Bohr constructed rubber valves which, slightly modified, have given entire satisfaction in Danish laboratories (fig. 14).”