6 RESPIRATION. [CH. I 



and the peas are therefore clearly visible, instead of being 

 partly hidden by mercury. 



A few drops of water are now passed in under the test- 

 tube rim with a bent pipette, and a fragment of caustic 

 potash added from below, in this way a strong solution 

 of KHO is supplied, by which the CO2 is absorbed. 



(5) Another method. 



The Torricellian vacuum was used by Wortmann in 

 his work on intramolecular respiration \ A tube closed 

 at one end, and of greater length than the height of the 

 barometric column, is filled with, and inverted over 

 mercury. Three or four peas are floated into the vacuum 

 at the top of the tube. After 24 hours a depression of 

 several cm. will be observed in the height of the column, 

 which will rise to nearly its original level on the addition 

 of KHO. 



(6) Pfeffer's method. 



To get -accurate results another method must be 

 followed; the following is taken from Pfeffer's paper in 

 his Tubingen Untersuchungen, Vol. i. p. 637. 



The principle is that already described for estimating 

 ordinary respiration, but instead of air, a current of 

 hydrogen is drawn through the vessel in which the plants 

 are contained. It is necessary to prevent the entrance 

 of extraneous CO2 and to make sure that the hydrogen 

 has no admixture of oxygen. 



1 Sachs' Arbeiten, 11. p. 500. 



