236 NUTRITIVE METABOLISM 



and place alongside a small empty flask. Smear the edges of a 

 bell-jar provided with a tubulure at the top with a cerate consist- 

 ing of equal parts of tallow, beeswax and linseed oil and cover 

 the plant and flask, taking care to seal the jar tightly to the plate. 

 Provide a tightly fitting rubber stopper with two holes for the 

 tubulure. A glass tube extending to the bottom of the flask and 

 projecting a few centimeters outside the stopper is inserted in one 

 opening and is connected with a burette, suitably supported, by 

 a short section of rubber tubing clamped by a pinchcock, or a 

 burette with a stopcock may be used. Insert a small section of 

 glass tubing with capillary bore bent twice at right angles in the 

 other opening of the stopper. This tube should be closed with 

 a section of rubber tubing and a pinchcock (Fig. 1 18, A). 



After the desired length of time has elapsed and it is necessary 

 to test the proportion of oxygen and carbon dioxide in the jar, 

 fill the burette attached to the tube with mercury. Next provide 

 a pair of Hempel's gas burettes (Fig. 1 18, B}. Fill slightly more 

 than half full with water. Raise the open burette c until the 

 graduated burette d is filled with water. Connect the rubber 

 tube with which the upper end of this burette is closed with the 

 capillary tube leading into the bell-jar and open the stopcocks ; 

 now lower the other burette until the level of the water in the 

 graduated burette is half way down its length, and about 50 cc. of 

 air have been withdrawn for analysis. Bring the fluid to the same 

 level in both burettes and measure exact amount of air in closed 

 burette which will be at normal pressure. Now allow the same 

 amount of fluid to flow into the bell-jar from the burette above it 

 to equalize the tension. 



Fill the absorbing pipette (Fig. 1 18, C) with a solution consisting 

 of I part potassium hydrate and 2 parts water until the liquid 

 rises a little into e. Force air in at g until the potassium so- 

 lution is forced up and fills the tube to the stopcock. Connect 

 with the burette containing the air to be tested and open the 

 stopcock and manipulate the pair of burettes to allow the air 

 to be drawn into the absorption pipette where it remains 3 to 



