54 A RESPIRATION CALORIMETER. 



By means of the screw pinchcocks 5j s 2 the amount of air bubbling 

 through the water can be regulated at will. Some difficulty was expe- 

 rienced in getting an air pressure sufficient to force air through such a 

 long column of water, but compressed air from a cylinder was eventu- 

 ally found satisfactory. The air is first saturated with water vapor 

 by bubbling through water in a gas-washing bottle, thus diminishing 

 the cooling effect in the water-jackets due to the evaporation of water. 

 In case there is clogging of the tubes and consequent increased pressure, 

 a mercury trap provides a safety escape. 



Inasmuch as the percentage of oxygen in the carbon-dioxide free air 

 from the respiration chamber is seldom less than 17 to 18 per cent, the 

 graduations on burette B 2 , which extend only from 90 to 100 cc., do 

 not permit of reading directly the volume of unabsorbed gas when drawn 

 from the pipette back into B 2 . This volume may be as great as 83 cc. 

 or as small as 78 cc. To overcome this difficulty, we have adopted the 

 plan of driving a definite volume of nitrogen from the burette Bj, 

 through the 3-way stopcock C, into burette B 2 , in order to depress 

 the level of the water in B 2 to such a point that it can be read on the 

 graduations from 90 to 100 cc. In general, about 10 to 14 cc. of nitro- 

 gen are thus expelled from the burette B r At the beginning of an 

 analysis the burette B : is nearly filled with pure nitrogen, obtained 

 either from a previous anatysis of air or from the gas above the reagent 

 in the Hempel pipette. 



Having filled the burette B, with nitrogen, the neck of the rubber bag 

 used to collect the sample of air to be analyzed is slipped over the end of 

 the capillary tube T. On opening the stopcock C the air is drawn into 

 the burette B, until the water level in the burette is the same as that in 

 the reservoir R 2 . The stopcock C is then closed, the screw pinchcock 

 on the neck of the rubber bag closed, and the bag removed. Theoreti- 

 cally, it is better to leave the bag on until just before reading the volume, 

 but the difference in composition of the room air and the small sample in 

 the open portion of the tube is so slight that practically no difference in 

 results is to be expected. After allowing the water in burette B 2 to drain 

 down the customary time, readings are taken of the volume in the burette, 

 of the thermometer, and of the barometer. The gas is then driven over 

 through the stopcock C into the Hempel pipette described for oxygen 

 analysis (p. 37), all the gas in the capillary tubes being forced out by the 

 pressure of the water in the elevated reservoir R 2 . After closing the stop- 

 cock C and the pinchcock P on the pipette, the air is shaken vigorously 

 with the reagent for five minutes. The residual unabsorbed gas is then 

 returned to the burette B 2 , and by lowering the reservoir R 3 the reagent 

 is drawn up through the rubber connection R and along the capillary 



