126 A CHEMICAL SIGN OF LIFE 
Thus in order to determine the concentration of 
carbon dioxide in question, one must first determine how 
many cubic centimeters of the gas must be introduced 
before we can obtain the precipitate in ten minutes; 
this volume must contain then 1.0X1077 g. Since we 
know the volume of the original respiratory chamber 
from which this known amount of gas is withdrawn, 
we can easily determine how much total carbon dioxide 
is present at the time of analysis. That is to say, the 
original capacity of chamber B divided by this mini- 
mum quantity of the gas which gave the precipitate, 
multiplied by 1.0X1077 g., corresponds to the total 
amount of the carbon dioxide given by the known weight 
of the tissue for the known period of time. 
The following example will make the details of the 
method and calculation clear: 
We took to mg. of the sciatic nerve of a frog and 
after ten minutes of respiration we drew 1 c.c. from the 
respiratory chamber into the left chamber, and found no 
precipitate visible within ten to fifteen minutes. Instead 
of now taking more gas from the respiratory chamber, we 
should take another fresh nerve, and, after it has respired 
ten minutes or longer, draw, say, 1.5 c.c. to the analytic 
chamber. As will be noticed, we have three variables 
which we can choose from, namely, the weight of the 
nerve, the time of respiration, or the amount of gas 
withdrawn from the respiratory chamber at the time of 
analysis. To estimate the carbon dioxide production 
from the isolated tissues it is far better to keep the time 
constant and vary the other two, for in many cases the 
rate of respiration varies as the time elapses. As far 
as the weight of the tissue is concerned, we cannot but 
