184 INTRODUCTION TO GENERAL PHYSIOLOGY 



B 



lower end and also to a reservoir of mercury. The measuring 



tube A is first filled with air by lowering the reservoir while the 



tap is open. The tap is then closed and the mercury made level in 



the reservoir and the tube. The volume is read off. Two or 3 



c.c. of strong (40 per cent.) caustic soda are placed in the funnel, 



the reservoir lowered somewhat, and the reagent allowed to run in 



slowly by opening the tap. Close the tap and raise and lower the 



reservoir a few times to spread the 



soda over the sides of the tube. 



Any carbon dioxide present is 



absorbed, but its concentration in 



air is so small that probably no 



diminution in volume will be 



noticed when the level of the 



mercury is again adjusted. We 



know, however, that it is present 



because lime-water left exposed 



becomes covered with a film of 



calcium carbonate, and the amount 



of it can be determined with a more 



sensitive method of analysis. 



Place next in the funnel 2 or 

 3 c.c. of a strong solution of pyro- 

 gallol, and allow it to run into 

 the measuring tube and mix with 

 the caustic soda already present 

 there. Alkaline pyrogallol is a 

 powerful absorbent of oxygen, 

 and there will be found to be a 

 marked decrease in the volume 

 of the gas. Repeat the process of 

 raising and lowering the reservoir 

 until no further reduction in 

 volume occurs. The difference 

 between this reading and that 

 obtained after the addition of soda 



gives the volume of oxygen in the mixture, and its percentage 

 in the air can be calculated. The remainder is nitrogen 

 and inert gases. After use, the tube is washed out repeatedly 

 with water, by running in through the funnel and expelling 

 by the tube C. Finally 5 per cent, sulphuric acid should be run 

 through. 



Breathe backwards and forwards from a rubber football bladder 

 until the asphyxial effect is too great to continue. A tube has 

 been tied into the bladder for the purpose, and should have a piece 



FlG. 13. Apparatus for Analysis 

 of Gas Mixtures. 



