ALVEOLAR AIR 111 



walls of the spiral tube becomes much more transparent. 

 Before the final volume is read in A, the air entrapped in d 

 must be removed as above described 1 . Since it takes some 

 time for all O 2 to be absorbed, the final volume should not be 

 read in A until time has been allowed for the temperature of 

 the gas to come to that of the water in the jacket. 2 



After completion of the analysis the taps J .and C are both 

 turned so that the tubing is completely shut off. 



Calculation of the percentage of Og is shown in the example 

 on p. 121. 



The Tension of Carbon Dioxide in Alveolar Air. By 

 the gas laws each gas in the alveolar air, and also the water 

 vapour, will exert a partial pressure or tension which is equal 

 to that which it would exert were it alone present in the space 

 occupied by the mixture of gases. This mixture consists, 

 approximately, of 5.5 per cent. CO2, 16 per cent, oxygen, 

 79.5 per cent. Nitrogen and it is saturated with water vapour 

 at body temperature, 37 C. Assuming that the barometric 

 pressure is 760 mm. Hg., then, to find the tension of CO 2 in 

 alveolar air we must first of all subtract from 760 the aqueous 

 tension at 37 C. which equals 48 mm. Hg. and multiply by 

 5.5 

 100' 



x Since absorption of C>2 is relatively slow, it is advisable for routine work to 

 have the reservoir B raised and lowered automatically. The most convenient 

 way for doing this is to attach it by string to one end of a hinged metal rod 

 which rests on a can placed on shafting that is made to revolve at a suitable 

 rate by means of a small motor. 



2 To correct for possible changes in the temperature of the water Haldane's 

 dummy tube is used. This consists of a 10 c.c. tube in the water jacket, con- 

 nected by capillary glass tubing with the lower end of D. The capillary tubing 

 is furnished with a stop cock so that it can be opened to the outside. The position 

 of this system is shown on the dotted lines in Fig. 32. At the start of an analysis 

 the stop cock is connected outside and the level of the meniscus of NaOH solution 

 marked on the tube /. The stopcock is then turned. If this level remians un- 

 altered after the analysis there can have been no change of temperature. If it 

 has shifted, it is brought back to the original level, while g and d are in com- 

 munication, by raising or lowering F. By this procedure the gas in A is com- 

 pressed or decompressed in proportion to the extent to which it may have ex- 

 panded by heat or contracted by cold. Before taking the final reading the 

 meniscus in d is of course readjusted by manipulating the screw clip a. 



