16 A CHEMICAL SIGN OF LIFE 
method is described in detail in the Appendix. The 
special apparatus, the biometer, as I have named it, 
which was constructed for the investigation, is shown in 
Fig. 1, and its use is detailed in the Appendix. It will 
detect one ten-millionth of a gram of carbon dioxide and 
estimate it with accuracy. 
As is shown in the figure, the biometer has two 
respiratory chambers each provided with a small tube, at 
the top of which the hemispherical drop of barium 
hydroxide can be formed. Exceedingly minute amounts 
of carbon dioxide produced in the chamber by the small 
piece of nerve will be precipitated as barium carbonate 
on the surface film of these hemispherical drops and may 
beseen withalens. As the apparatus has two chambers, 
not only can we detect very small amounts of carbon 
dioxide which the nerve may produce, but we can also 
compare the output of carbon dioxide of different 
tissues under the same conditions, by placing one 
tissue in one respiratory chamber and another in the 
other. 
To discover whether nerve fibers, as distinct from 
nerve cells, respire, particular care was taken to select 
at first those nerves which are known to be free from 
such cells and, as far as possible, free also from connective 
tissue. It was necessary to do this because the work of 
several investigators seemed to indicate that tissue oxi- 
dation was in some way dependent on the cell nucleus. 
Certain biologists even went so far as to believe that a 
nerve fiber ought not to respire at all, since it contained 
no nucleus. The fact that the blood supply to the 
brain, where most of the nerve cells are located, is so 
copious, whereas the blood supply to the nerve fibers is 
