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STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



valve which pierces just through a piece of lead L. By means of 

 this valve the rate at which the air-stream passes through the appa- 

 ratus could be very accurately regulated. The valve is attached to 

 a small vessel containing a little phosphorus pentoxide. 



From the control-valve the air-stream passes through narrow-bore 

 glass tubes to the distributing tube. This consists of glass stopcocks 

 by means of which the air-stream can be directed to the absorption 

 tubes. Each stopcock is connected to a narrow glass tube slightly 

 bent up at the end. Over these tubes are fitted the absorption tubes 

 by means of a small rubber stopper. The air-stream can thus be 

 directed from one absorption tube to another 

 without interrupting the rate of flow. 



The absorption tubes consist of 10-bulb glass 

 tubes, the lower end of which is slightly bent. 

 They are filled by means of a pipette, carefully 

 graduated. For these experiments 68.12 c. c. 

 of barium hydroxide was used for each determina- 

 tion. It was found that the bulb-tubes break up 

 the air-bubbles more thoroughly and thus assure 

 more complete absorption than the straight-walled 

 Pettenkoffer tubes. 



Fig. 22. — Needle control valve by means of which the rate of the 

 au--stream passing through the apparatus is regulated. The valve is 

 shown in section; a fine needle pierces the sheet of soft lead L, and 

 is raised and lowered by means of a packed screw. 



From the absorption tubes connection is made to a bottle which 

 served to determine the rate of the gas-stream. This bottle is 

 partly filled with water, over which is a layer of mineral oil. The 

 glass tube from the absorption tubes extend to the bottom of this 

 bottle. The rate of the gas-stream is determined by counting 

 the gas-bubbles passing through the water in the bottle during a 

 certain period. The time measurements were made with a stop- 

 watch, and by means of the control-valve the rate of flow could be 

 very accurately adjusted. A rate of 70 bubbles per minute repre- 

 sented a flow of 310 c. c. per hour. Care was taken to maintain 

 constant the pressure in the aspirator. This was indicated by a 

 manometer filled with aniline oil. Blank experiments showed that 

 with the constant temperature of the room and in the water ther- 

 mostat a very regular flow of air, and consequently of carbon dioxid, 

 is obtainable and obviates the use of cumbersome and inaccurate 

 gasometers. 



The aspirator consists of an inverted galvanized-iron cylinder 

 which is drawn out of a vessel of water. The cyhnder is attached 

 to a weight by means of a light wire cable. This passes over the rim 



