120 A CHEMICAL SIGN OF LIFE 
minute trace of carbon dioxide, the drop of barium 
hydroxide should be clear, not only at the time of 
introduction of the drop at the beginning, but also after 
standing for several hours, having not a single granule 
of the precipitate visible with a lens. 
Since the main point of accuracy in our apparatus 
depends on having the air free from carbon dioxide— 
indeed, it is the most difficult part of the manipulation 
of the biometer to have good air—particular care must 
be taken to have every point of junction perfectly air- 
tight. The points most susceptible to leaking will be the 
stopcocks and the mouth of the carboy where the air is 
preserved. A strong suction is essential for a complete 
washing of the apparatus with the air free of carbon 
dioxide. 
« Methods for the qualitative detection of carbon dioxide 
production in the tissue —After the apparatus is cleaned 
and dried and the air is ascertained to be pure for use, 
a prepared tissue is placed on a cover-slide or the glass 
plate shown in Fig. 2 (p. 38)- and introduced into the 
chamber B, no tissue being put in the left chamber A. 
The detailed method is exactly the same as the one 
just described. After both chambers are closed with the 
stoppers S and R and sealed with mercury, the apparatus 
is washed with the air free of carbon dioxide in the usual 
manner. Barium hydroxide solutions are introduced 
into d and /, forming hemispherical drops, and the con- 
nection between the two chambers is severed by turning 
the stopcock between them (ZL); then watch the drop 
with a lens. If the air is free from carbon dioxide, the 
drop in the left chamber ought to be perfectly clear, 
while the drop in the right chamber, if the tissue gives 
