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. 7 X io~ 7 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 



