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 (L); 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 



