38 PLANT RESPIRATION 



shifted so that the rear meniscus takes the exact position of the 

 forward one and the new position of the forward meniscus is 

 noted. By continuing in this manner the whole measuring tube 

 is measured with the same column of mercury. Since the 

 volume of the mercury does not change (care must be taken that 

 the temperature of the water in the cyhnder B remains constant) 

 the apparent inequality of the lengths of the column in various 

 parts of the tube is due to the unequal widths of the inner space 

 of the measuring tube. Thus a table of corrections can be 

 drawn up, which are to be taken into consideration for each 

 analysis. The calculations of the corrections are carried out 

 according to the exact directions of Bunsen.^ To all data 

 obtained there is to be added the number ascertained for the 

 space between and which is not divided into millimeters, 

 since this space is to be included in the similar analysis of gas. 

 The gas analysis is carried out in the following manner. 

 The gas to be analyzed must be sealed in an analysis tube with 

 mercury (see below). The analysis tube is transferred to the 

 bath C, placed under mercury on the end of the measuring 

 tube, a suitable amount of gas drawn into the measuring tube 

 by lowering the pear-shaped bulb H and opening the stopcock 

 a, and sealed with mercury by raising the analysis tube. The 

 analysis tube with the surplus of gas is then removed from the 

 bath and replaced by another full of pure, dry mercury. This 

 analysis tube is allowed to float in the bath with the lower end 

 dipped in mercury while the glass support rests against the 

 hook //. The level of mercury in the bath C is now adjusted 

 exactly to the line Im. The mercury meniscus in the right- 

 hand, narrow side of the measuring tube is set at the Une O 

 with the aid of the screw e and a microscope; then the position 

 of the mercury meniscus in the left side of the measuring tube 

 is noted with the aid of the reading microscope and the relative, 

 corrected volume of gas is calculated according to the table of 

 corrections." It is recommended that the accurate setting on 

 the line O be done twice in succession with subsequent readings 



1 Bunsen, R. Gasometrischen Methoden. 2nd. ed. Pp. 34-35. 1877. 



2 If one wishes to know the absolute volume of the measuring tube, the column of mercury- 

 used for the calibration of the measuring tube must be carefully weighed. Cf. Bunsen. 

 loc. cil. This can be done only in cases of exceptional importance. 



