124 A CHEMICAL SIGN OF LIFE 



is for the purpose of quantitative determination, and its 

 stopper need not be removed. The tissue is carefully 

 laid on the glass plate and on the platinum electrodes 

 fused into the chamber, or it can be laid on the cover- 

 slide and placed on the mercury. Close the stopper R 

 and seal both chambers with mercury. Wash the 

 apparatus with air free of carbon dioxide, as directed 

 before. At the end of the sixth or seventh washing 

 stopcocks G and F are closed and the time is recorded, 

 since it is plain that from this time on we are retaining 

 any gas given off by the tissue in the chamber. The 

 apparatus is filled once more with air free of carbon 

 dioxide by opening stopcock 7; the pressure is quickly 

 adjusted by raising the safety bottle D, while the stop- 

 cock I is still open, and then / is shut. After opening 

 stopcock C", barium hydroxide is introduced into the 

 tube d of the left chamber A only, but the solution is 

 never introduced into the respiratory chamber B. Turn 

 the stopcock L in such a way as to sever the connection 

 between these two chambers. It is imperative, not only 

 that the hemispherical drop formed at d in the left 

 chamber should be perfectly clear at the time of intro- 

 duction of this solution, but also that no visible granule 

 of any kind should be produced on standing. No 

 quantitative experiment can be performed unless the air 

 is absolutely free from carbon dioxide. We have thus 

 a control for each quantitative experiment. If at the 

 end of the desired period of respiration, say ten minutes, 

 the drop is perfectly clear, not having any deposit visible 

 with a lens, a portion of the gas from the respiratory 

 chamber B is introduced into the left chamber. This is 

 accomplished by drawing a designated amount of mer- 



