RESPIRATORY EXCHANGE. 



119 



FIG. 33. Tissot-Carpenter Spirometer, Douglas mouthpiece and Pearce's valves. The dotted 

 lines in the diagram indicate the outline of the aluminium cylinder while in the water seal between 

 the outer and inner steel cylinders, the position of the latter of which is also indicated by dotted 

 lines. For further references see context. The inserts show the Douglas mouthpiece and Pearce 

 valves, the latter of which is made as follows: Prepared casings used in the manufacture of 

 bologna sausage are obtained preserved in salt, and they will keep indefinitely on ice. When 

 needed a short piece is taken, washed free from salt by allowing water from the tap to run through it, 

 and softened in a weak glycerine solution. The gut becomes very soft and pliable, and does not dry 

 quickly. A piece of the casing about 10 cm. long is threaded through a glass tube of about 15 mm. 

 bore and 4 to 6 cm. long. One end of the casing is brought around the outside of the tubing and 

 secured by means of a thread. The lower end of the membrane is pinched off and the casing is then 

 cut a little more than half way across its middle, so that the opening will lie just within the free end 

 of the tube when the casing is drawn back through it. The loose end of the casing is slightly 

 twisted an essential procedure and is then secured by a thread on the outer side of the tube. 



