THE BIOMETER: HOW TO USE IT 123 
the stimulated nerve is placed. Later the total amount 
of the precipitate grows much larger on the drop in this 
chamber. This clearly shows that the chamber in which 
the larger amount of the precipitate is found must have 
the higher concentration of carbon dioxide. Since we 
had exactly the same kind of air at the beginning, the 
conclusion is that the nerve when stimulated must give 
off more carbon dioxide than the resting one. This 
conclusion can easily be confirmed by exchanging the 
nerve in the chambers as usual. 
The following figures will illustrate the different stages 
of the granulation of barium carbonate and will show 
AAA Ae Me 
Fic. 4.—Different stages of the granulation of barium carbonate on 
the surface of the hemispherical drop of barium hydroxide; sa and 6a 
show the top views of the drops at the time when “‘iris effect” is pro- 
duced. 
definitely how easy it is to compare the amount of carbon 
dioxide production at several points. And such com- 
parison can be confirmed more exactly by the quantita- 
tive determination; the details of the method are given in 
the next paragraph. 
The method for quantitative measurement of carbon 
dioxide—While the apparatus is drying, prepare the 
tissue and weigh it. If everything is perfectly dry, fill 
both chambers with mercury up to the marks, as directed 
on p. 118. Remove the stopper from the right chamber 
only, which is to be used as a respiratory chamber, 
and the tissue is to be left in this; the other chamber 
