CHEMICAL SIGNS OF IRRITABILITY 19 
of the biometer and no nerve in the left, the biometer 
being properly sealed with mercury and filled with air 
which is free from carbon dioxide, and if barium hydrox- 
ide is allowed to rise to the top of each tube in such a 
way as to form hemispherical drops of approximately 
equal size in both chambers, we observe that the drop 
in the right chamber, where the nerve is, will soon be 
coated with a white precipitate of barium carbonate, 
but that no precipitate whatever can be seen, even with 
a lens, in the left chamber. Carbon dioxide is thus 
shown to be produced by this resting nerve of the spider 
crab. By interchanging the nerve from the right to the 
left chamber, no nerve being now put in the right, we 
find that the precipitate is now in the left-hand side of 
the biometer, and we have no difficulty in convincing 
ourselves that the carbon dioxide has come from the 
nerve, for we have thus eliminated any technical error 
which might have produced the different results in the 
different chambers. The rate at which the precipitate 
appears and its quantity depend on the size of the nerve 
and the length of time we leave it in the chamber. That 
an unstimulated nerve gives off carbon dioxide is a fact 
which can thus be demonstrated easily to anyone if the 
proper apparatus is at hand. The rate of production 
of carbon dioxide by the normal resting nerve of the 
spider crab is found to be proportional to its weight, 
other things being equal, and is fairly constant. The 
quantitative determination shows that for 10 mg. of 
nerve per ten minutes it gives off 6.7107 g. of carbon 
dioxide at 15° to 16° C. 
The quantitative determination of this amount is made in 
the following manner: The claws of the crab are carefully removed 
