42 PLANT RESPIRATION 



to o.i%.^ The following points are important for the reliabihty 

 of analysis: I. Exact calibration of the measuring tube and II. 

 Absolute cleanliness of the measuring tube and of the mercury.- 

 The column of mercury must move freely in the measuring tube 

 and not adhere to the walls of the tube. As soon as it is noticed 

 that the movement of mercury in the measuring tube is not 

 uniform, steps must be taken at once to clean the tube. For 

 this purpose an analysis tube filled with 15% nitric acid is placed 

 under the mercury on the end of the measuring tube and the 

 liquid drawn into the measuring tube. After repeated to-and- 

 fro movement, the acid is forced back into the analysis tube and 

 the measuring tube is rinsed with distilled water several times 

 by means of the procedure just described. Then the measuring 

 tube, the rubber tubing P and the pear-shaped bulb H are 

 emptied and dried by blasts of hot air.^ 



In carrying out the analyses, the adjustment of the column 

 of mercury at the line O in the measuring tube must always be 

 made by a movement in the direction of the arrow (Fig- 6 near 

 O). Readings on the measuring tube are also to be carried out 

 at a constant temperature. The fluctuations in temperature 

 which may possibly take place in the course of each analysis 

 are controlled with the aid of a thermometer dipped in the water 

 in the cylinder B and at once equalized by the addition of warm 

 or cold water. Any appreciable change in barometric pressure 

 can hardly take place with the short duration of the analysis. 

 In regard to the C02-determination in particular, its accuracy 

 can be increased by placing the gas for a short time in the 

 explosion pipette before its measurement after the absorption 

 of the CO2. The gas is deprived of a part of its water vapor 

 by the 30% potash and is again moistened'* in the explosion 

 pipette, for the latter always contains some water from the 

 previous determinations of oxygen.^ 



' The analysis figures of S. Kostytschew in Ber. d. bot. Ges. 39: 319. 1921 illustrate the 

 accuracy which may be reached with this apparatus. 



* For the purification of mercury see Hempei. Gasanalytische Methoden. 4th ed. P. 

 79- 1913 lor other standard texts]. 



' The use of alcohol and ether to dry the tube is expressly to be avoided. 



* A gas taken from the plant container is always moist. 



' If drops of water visible to the naked eye have gathered in the absorption [ = explosion) 

 pipette, they are driven out with mercury. 



