RESPIRATION 



425 



at least 2 liters capacity. Between the bag and mouthpiece there is inter- 

 posed a cylindrical metal vessel containing moist granulated soda lime 

 or other suitable absorbent to absorb COg. The end of this vessel may be 

 made to screw on and off, with an air-tight rubber washer; or may be 

 made in two pieces, the outer of which slides over the inner, as shown 

 in the figure, the junction being made air tight with plasticine. The soda 



Figure 104. 

 Apparatus for determining blood volume in man. 



lime is kept in position by two circular pieces of wire gauze, one of which 

 is pushed into the end of the inner vessel, and the other into the end of 

 the outer vessel. Good soda lime can be made by stirring fresh slaked 

 lime in powder with a strong solution of caustic soda till the mixture 

 granulates, and then sifting off the fine powder and coarse lumps by 

 means of two sieves. Granulated caustic soda will also answer. There 

 should be no appreciable resistance to breathing, and one tin of soda 

 lime should last for several experiments. When the soda lime is spent it 

 ceases to heat, and the breathing begins to become increased, owing to 

 un absorbed CO 2. 



The narrow graduated cylinder D is filled under water with CO, of 

 which a stock, prepared from formic and pure sulphuric acids, can be 

 kept in a large bottle. Just before the experiment, some of the CO is, by 

 turning the water tap E, driven out through the test tube and 3-way tap 

 F to the outside. In this way all the air is expelled up to the 3-way tap. 

 The water tap is then closed, and afterwards the 3-way tap. Oxygen from 

 a steel cylinder is now turned on through the tube C to displace CO 



