44 



PLANT RESPIRATION 



on the end of the measuring tube under mercury, the gas drawn 

 into the measuring tube by unscrewing the piston S, and sealed 

 with mercury by raising the analysis tube. Now the analysis 

 tube is removed from the bath and the length of the gas column 

 in the cahbrated part of the tube is measured by the use of a 

 microscope. The condition is somewhat troublesome that the 

 tube, as will be seen from subsequent statements, always 

 remains moist from previous analyses. Drying the tube after 

 each analysis is not only time-consuming but directly disturbing, 

 since the next readings must be carried out in the wet tube and 

 the measurements obtained are thus not comparable with an 

 original length of the gas column as read in the dry tube. 



Fig. 7. — The apparatus of Bonnier and Mangin. 



After reading the original relative volume of gas, the gas is 

 treated with caustic potash solution. To do this an analysis 

 tube filled with a concentrated solution of caustic potash is set 

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

 is introduced into the measuring tube by the suitable turning of 

 the piston S. The gas collects in the spherical distension of the 

 tube (above the socket joint). The potash is at once driven 

 back into the analysis tube. The gas column thereby occupies 

 its original position in the measuring tube and CO2 is absorbed 

 by the potash which adheres to the walls of the tube. The 

 length of the gas column is now read. The difference in the 

 two readings shows the relative quantity of carbon dioxide. 

 In exactly the same manner the amount of oxygen is determined 

 by the introduction of an alkaline solution of pyrogallol which 

 absorbs the oxygen. The observed data are expressed in per- 

 centages. After each analysis the measuring tube is rinsed with 



