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 i.oXio" 7 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 from which this known amount of gas is withdrawn, 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 i.oXio" 7 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 10 mg. of the sciatic nerve of a frog and 

 after ten minutes of respiration we drew i 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 withdrawn from the respiratory chamber at the time of 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 



