136 



PRACTICAL PHYSIOLOGY 



the reading of the burette independent of changes in temperature 

 and barometric pressure during analysis a control tube N con- 

 nected with the potash solution by means of a T-tube is employed. 

 The tap at P makes it possible to render 

 the pressure in N equal to that of the atmo- 

 sphere. At the beginning of the experi- 

 ment the potash is adjusted to the mark R 

 by altering S, P being open. P is then 

 closed, and not opened again till the an- 

 alyses are complete. The barometer and 

 the temperature of the water-jacket are 

 read. Each time a reading of the burette 

 is made the potash is brought to the mark 

 R by altering S, and to the mark M by 

 means of the levelling tube B. As the con- 

 trol tube and the gas-burette are kept 

 moist, variations in the tension of aqueous 

 vapour in the burette are also corrected by 

 the control tube. 



A sample of expired air is obtained with 

 the burette (Fig. 133), or may be collected 

 in a gas-sampler (Fig. 134). 



In order to test the apparatus the student 

 should make several analyses of the air of 

 the room. 



Special care should be taken to avoid 

 drawing the absorbent fluids into the taps ; 

 if such an accident should occur the taps 

 and tubes will require a thorough clean- 

 ing. 



FIG. 133. Hempel's 

 burette for collecting 

 a sample of expired 



Oxygen 



Carbon dioxide 

 Nitrogen 

 Argon 



Atmospheric Air, measured dry at stan- 

 dard temperature and pressure, and 760 

 mm., has the following composition : 



20-94 volumes per cent. 

 . 0-03 

 . 78-09 

 0-94 



There are also traces of helium, krypton, neon, xenon, and 

 hydrogen. The nitrogen and argon appear to be inert as far as 

 the higher animals are concerned, and in ordinary analyses are 

 given together as nitrogen. 



The Expired Air varies in composition according to the rate and 

 depth of respiration ; this is shown by the following analyses 

 made by Speck: 



