ELEMENTARY EXPERIMENTAL PHYSIOLOGY 183 



The Alveolar Air. The composition of the alveolar air is deter- 

 mined, according to the method introduced by Haldane and Priestley, 

 by an analysis of the last portion of the air expired in an ordinary 

 expiration. The experiment may be performed in the following way. 

 An anaesthetic mask is connected by a T-piece to a piece of tubing 

 80 cm. long and 1'8 cm. internal diameter; to the free end of the T- 



FIG. 181. Apparatus for collection of a sample of Alveolar Air. 



piece is connected (Fig. 181) a gas-sampler with a capacity of 50 cubic 

 centimetres. The subject of the experiment fits the mask to his face 

 and makes an ordinary expiration ; as soon as the expiration ceases, 

 the tap of the gas-sampler, the air of which has previously been 

 removed by a vacuum-pump or gas-pump, is opened and a sample 

 of the last portion of the expired air is collected before the mask is 

 removed from the face. The analysis of the air is performed in the 

 manner already described. The percentage composition is about 5*5 

 carbon dioxide, 14*5 oxygen and 80 nitrogen. 



It is an advantage to determine the volume of each expiration by a 

 spirometer attached to the end of the tubing, and it is important that 

 the subject of the experiment should by a little practice with the 

 apparatus learn to breathe naturally, otherwise a fair sample will not be 

 obtained. 



The partial pressure, or, as it is often called, the tension of the 

 component gases is : 



Dry atmospheric air : 



21 

 Oxygen approximately ^-^ x 760= 159*6 mm. of mercury or 21 



per cent, of an atmosphere. 



79 

 Nitrogen approximately y^ x 760 = 600'4 mm. of mercury or 79 



per cent, of an atmosphere. 



