THE BIOMETER: HOW TO USE IT 121 
off carbon dioxide, will not be coated with the precipitate, 
but will have on its surface some crystals of barium car- 
bonate, which becomes more heavily precipitated as the 
respiration goes on. By repeating the same experi- 
ments after interchanging the chambers, using the left 
for the tissue and the right for a blank, it will be possible 
to eliminate any possible error which might come from 
some technical fallacy characteristic of one particular 
chamber. For a casual observer the initial granule 
will not be distinct from a granular spot on the glass. 
The granules, however, will soon increase over the surface 
of the drop and will gradually collect downward at the 
edge of junction of the drop with the glass tubing. 
The thick band of white precipitate around the bottom of 
the drop will gradually extend toward the top of the 
drop, so that after the band reaches more than half of the 
hemispherical drop of barium hydroxide one can see 
with the naked eye, not only from the side, but also 
from above, the whole drop, now resembling a contract- 
ing iris. When the very top of the drop is filled with the 
precipitate, the whole drop of barium hydroxide will 
look very opaque, covered with a thin layer of the car- 
bonate. If one take a small piece of sciatic nerve of a 
frog, say about 20 mg., he can see these different stages 
of precipitation very distinctly, but when the amount 
of tissue taken is very large it is very difficult to observe 
these phenomena on account of the too rapid formation 
of the precipitate all over the surface of the drop. It 
is therefore best to take a very small piece of the nerve 
for the purpose of following these different stages of the 
precipitation of carbon dioxide as carbonate, for the 
practice of distinguishing these different stages is very 
