122 A CHEMICAL SIGN OF LIFE 
useful for a quick comparative estimate of the different 
rates of carbon dioxide production from two different 
samples of tissue. 
Methods for a quick comparative estimate of carbon 
dioxide production from two different samples of tissue.— 
By repeating quantitative experiments it was found that 
the speed with which the first precipitate appears, the 
sizes of the precipitates, and the shapes of the aggrega- 
tion of the deposits at different stages represent different 
quantities of carbon dioxide, if compared simultaneously 
under the same conditions. Thus, with this remarkably 
simple means we can determine quickly the comparative 
output of carbon dioxide from two different tissues at 
the same time. The method of procedure is best illus- 
trated by the following example: 
Two pieces of the sciatic nerve are isolated from 
the same frog and weighed into approximately the same 
mass. One piece is laid on one glass plate and the other 
on the other plate in such a way that one part of the nerve 
lies across the electrodes of the glass plates as shown in 
Fig. 2 (p. 38). In this way, when the plates are hung on 
the electrodes and m either nerve desired can be stimu- 
lated with the induction current. These plates are now 
hung on the electrodes in each chamber, and the usual 
procedure is followed for eliminating carbon dioxide from 
the apparatus. After the connection between the two 
chambers is closed by means of stopcock L, having the 
drops of barium hydroxide in each chamber as usual, 
the nerve in chamber A is stimulated by the current. 
Then if one watches over the surface of the drops care- 
fully from the start, the deposit of carbonate will be 
seen to appear first on the drop in chamber A, in which 
