From: Yandell Henderson, AND INDIRECT CALORIMETRY VI. THE RESPIRATORY EXCHANGE APPLICATIONS OF GAS ANALYSIS: ARTICLE: J. Biol. Chem. 1918, 33:47-53.

“A form of valve which has proved convenient has recently been devised at the University of Minnesota by professors A.D. Hirschfelder and E.D. Brown.  By their kind permission a diagram of two of these valves arranged for inspiration and expiration on a T-tube is shown in Fig. 2.  Each valve is made of a large tin salve box with a disk of sheet rubber inside, held in place by a ring spring wire soldered to the box at one point.  The crack around the box is made tight with adhesive plaster.  In the figure the cover of one valve is removed to show the rubber and wire.”