RESPIRATION APPARATUS. 45 



current flowing at the rate of 2 to 3 liters a minute are met by using 

 pumice stone drenched with sulphuric acid to absorb the water vapor 

 and the specially prepared soda lime (see p. 29) to absorb the carbon 

 dioxide. These absorbents are held in glass U tubes. 



APPARATUS FOR ABSORPTION OF WATER. 



The pumice stone is broken into pieces approximately 5 mm. in 

 diameter, the finer dust carefully sifted out, and each arm of the U 

 tubes filled to within 10 mm. of the top. About 10 cc. of commercial 

 concentrated sulphuric acid is slowly poured over the pumice stone in 

 both arms of the tube, care being taken not to add so much acid as to 

 completely close the bend at the bottom of the U and thus retard the 

 free passage of gas. One-hole rubber stoppers provided with small 

 glass elbows are fitted in each arm of the U and a small label with 

 the number of the tube is placed on one arm. All connections are 

 made in such manner that the air leaves the U tube from the arm on 

 which the label is placed. This precaution is necessary, because as 

 the moist air enters the U tube the sulphuric acid with which it first 

 comes into contact takes up the water vapor from it, and consequently 

 becomes somewhat diluted. If during a subsequent use of this tube 

 the air were allowed to enter the other arm and pass out over the 

 dilute acid on the pumice stone, experience has shown that the re- 

 moval of water would not be complete, since the very dry air would 

 take up water from the dilute acid. 



Owing to the presence of chlorides in the pumice stone, the addition 

 of sulphuric acid is liable to produce a slight evolution of hydrochloric- 

 acid gas, and consequently, when the U tubes are freshly filled, it is 

 desirable to draw dry air through them for a few minutes, thus remov- 

 ing the hydrochloric-acid gas. This precaution is also taken with the 

 U tubes used for the oxygen purification, described on page 33. In 

 this case it is of even greater importance, as otherwise the hydrochloric- 

 acid gas would enter the main air current. 



The U tubes are 130 mm. long, 60 mm. wide, measured on the out- 

 side, and 15 mm. in diameter. They weigh, when fitted with rubber 

 stoppers, glass elbows, pumice stone, and acid, not far from 70 grams. 

 When not in actual use the glass elbows are closed by short pieces of 

 rubber tubing fitted with glass plugs. 



Efficiency of absorption, It has been found by repeated experiment 

 that a U tube filled as above described can be safely relied upon to 

 absorb one gram of water vapor. In the ordinary usage to which the 

 tubes are subjected in this laboratory, it is nearly always possible to 

 predict the gain in weight during an analysis. Thus if it is expected 



