From: The Newer Methods of Blood and Urine Chemistry, by  By Rutherford Birchard Hayes Gradwohl, Abraham Jacob Blaivas, 1920, Page 373.

“The Douglas Bag.  The Douglas Bag is made of rubber-lined cloth, and is capable of holding from 50 to 100 liters.  It is especially useful for investigations during exercise, since it is fitted with straps so that the bag can be fastened to the shoulders (Fig. 73).  It is then connected with the valves, the mouthpiece of which is placed between the lips.  Respirations are commenced with the three-way valve turned so as to allow the expirations to pass directly outside.  After respiratory equilibrium is established, the three-way valve is turned during an inspiratory period so that the succeeding expirations may pass into the bag.  The time required to fill the bag comfortably is determined with a stop-watch.  The air which has been collected in the bag during the period is thoroughly mixed and passed through a meter, the temperature and barometric pressure are noted, and a sample analyzed in the Haldane gas apparatus.  The bag should be emptied completely by rolling it up when nearly empty.”