METABOLIC PROCESSES IX THE PLANT. 27o 



Experiments in racuo, which I, for example, made with pea 

 seedlings after Wortmann's method, 1 are carried out as follows: 

 A thick glass tube, fused up at one end, and about 100 cm. long 

 and 1'5 cm. in diameter, is filled with clean and perfectly dry 

 mercury. As to the method of cleansing mercury, see 13. To 

 prevent bubbles of air from adhering to the walls of the tube in 

 filling it, it is best to run the mercury, by means of a funnel 

 rather finely drawn out at the end, through a thin glass tube 

 reaching to the bottom of the tube to be filled. When the tube 

 has been filled, it is closed, and inverted in a flat glass vessel par- 

 tially filled with mercury. We now have before us a barometer 

 with a fairly large Torricellian vacuum. A few seedlings, de- 

 veloped in moist sawdust, are freed from the seed-coats, dried 

 with blotting-paper, and passed up through the mercury of the 

 barometer tube, together with a little ball of blotting-paper soaked 

 in boiled-out water, which serves to keep them moist. In experi- 

 ments with Pisum sativum or Vicia Faba, we employ six to ten 

 seedlings ; if we experiment with lighter seedlings, a correspond- 

 ingly greater number must be used. When the mercury has 

 come to rest in the barometer tube, after introduction of the 

 seedlings, we at once proceed to observe the time, the tempera- 

 ture, and the barometer reading, and also the level of the column 

 of mercury (the upper level and the lower level, i.e. the point at 

 which the barometer tube touches the mercury in the flat vessel). 

 If we work with non-graduated barometer tubes, we mark the 

 upper and lower limits of the column of mercury by pasting strips 

 of paper on the tube, repeating this at each successive reading, 

 measure the heights of the columns of mercury thus indicated, 

 and determine later the volumes corresponding to them by run- 

 ning mercury from a burette up to the corresponding marks. All 

 the volumes are reduced to 0C. and 1,000 mm. of mercury. 



If at the commencement of the experiment : 



yb = the volume, 



h = the height of the Hg. in the barometer tube, 

 t =the temperature, and 

 I) the barometer reading, 

 and further, if 



ft the tension of the aqueous vapour over the mercury in the 

 tube at the corresponding temperature, and 



a the coefficient of expansion of air, the reduced volume V is 

 given by the following equation : 



