42 A CHEMICAL SIGN OF LIFE 
evidences which establish the fact that the nerve chemi- 
cally stimulated gives off more carbon dioxide, and that 
when rendered less excitable by reagents it produces 
less carbon dioxide than the normal resting nerve. 
When the two sciatic nerves of a frog are isolated and 
one is left in physiological salt solution—o.75 per cent 
sodium chloride—and the other in the body of the frog 
for the same length of time, and when they are trans- 
ferred to the two chambers of the apparatus, it is found, 
if the quantities of the carbonate precipitates are com- 
pared, that the nerve which has been in the saline solu- 
tion produces more carbon dioxide than that which has 
remained in the body. It is known that such a saline 
solution raises irritability and ultimately stimulates the 
frog’s sciatic nerve. 
The different rates at which carbon dioxide is pro- 
duced from different nerves treated by various con- 
centrations of potassium chloride are equally instructive. 
When a nerve is placed in a molecular solution of 
potassium chloride, stimulation takes place for a con- 
siderable time. Then finally the nerve becomes inex- 
citable. But if the nerve is put in o.2 mol. solution 
of the same salt, nervous excitability is abolished in a 
short time without any primary stimulation. The 
carbon dioxide production follows exactly analogously to 
this. The nerve treated with the stronger solution 
gives off more carbon dioxide than the one treated 
with the weaker solution. This was true even after 
the nerve became inexcitable, showing that the nerve 
must still be giving off more carbon dioxide while being 
stimulated by the stronger solution. Mr. Riggs is 
making an extensive study of the effect of various 
