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. 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before gave the precipitate is withdrawn into the left chamber 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? : 
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, 1.0X10—7 g., 
the amount carefully determined previously (taking known 
amounts of the exceedingly diluted gas) to be just sufficient to 
produce a noticeable precipitate. 
t The weight of this plate is known, hence the weight of the nerve 
can be determined very quickly (see p. 38). 
2 See footnote, p. 126. 
