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 n 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 



