20 A CHEMICAL SIGN OF LIFE 



from the body, and by gentle cracking the long fiber of the nerve 

 trunk is easily isolated. After the last drop of water is removed 

 by a filter paper, the nerve, with the aid of glass chopsticks, is 

 carefully placed on the glass plate (Fig. 2) and quickly weighed. 

 The glass plate with the nerve is now hung on the platinum hooks in 

 the right respiratory chamber, and the chamber is then sealed with 

 mercury. The left, or analytic chamber, is now partially filled with 

 mercury in the manner described elsewhere, and then the appara- 

 tus is washed as usual by air free from carbon dioxide. The 

 time at which barium hydroxide is introduced into the top of the 

 tube in the left chamber is recorded and the stopcock between 

 the two chambers is closed. When at the end of ten minutes the 

 drop on the tube in the left chamber is perfectly clear, having not 

 a single granule of the precipitate visible to a lens, thus insuring 

 that the air used for washing is absolutely free from carbon 

 dioxide, a known amount of the gas from the right respiratory 

 chamber is introduced into the left chamber in which the clear drop 

 of barium hydroxide has been exposed, and it is determined 

 whether or not the amount of the gas taken contains enough 

 carbon dioxide to give a precipitate in several minutes. Usually 

 ten minutes will be sufficient for the reaction. 1 If it does not give 

 a precipitate in this time, a larger volume should be taken until 

 the precipitate appears within ten minutes. If it does, the ap- 

 paratus is washed, dried, and with a fresh nerve the procedure is 

 repeated, but a less volume of the gas than the amount which 

 before gave the precipitate is withdrawn into the left chamber 

 from the right. 2 



In this way, by the use of several fresh nerves, a minimum 

 volume of the gas for a known weight of the nerve which gives a 

 precipitate is determined. This minimum volume should con- 

 tain a definite quantity of carbon dioxide, namely, i.oXio~7 g., 

 the amount carefully determined previously (taking known 

 amounts of the exceedingly diluted gas) to be just sufficient to 

 produce a noticeable precipitate. 



1 The weight of this plate is known, hence the weight of the nerve 

 can be determined very quickly (see p. 38). 



3 See footnote, p. 126. 



