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 J; the pressure is quickly 
adjusted by raising the safety bottle D, while the stop- 
cock J is still open, and then J 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 Z 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- 
