CHEMICAL SIGNS OF IRRITABILITY 43 
sodium salt solutions of varying concentrations and has 
already discovered confirmatory evidence for the increase 
of metabolism during chemical stimulation. It may 
be added here in passing that the different solubility of 
carbon dioxide in these salt solutions cannot alone explain 
our results, for there is not enough difference in solu- 
bility of this gas in such dilute equimolecular solutions 
of potassium and sodium chloride whose effects on carbon 
dioxide production are so divergent, the former salt 
diminishing, the latter increasing, it. 
The fact that during chemical stimulation the nerve 
gives off more carbon dioxide is made evident, also, by 
the use of low concentrations of anesthetics. If the 
concentration is so low as to give a primary stimulation 
to the nerve, the production of this gas is greatly acceler- 
ated at the beginning of immersion of the nerve in the 
narcotics. This is an additional evidence that there 
is a relation between excitation and metabolic activity. 
Stimulation in hydrogen.—The last experiment which 
we shall describe in this connection is on the quantitative 
estimation of the carbon dioxide production in a nerve 
when the latter is in an atmosphere of hydrogen and when 
it is being stimulated by an electrical current. We 
expected to find here one of two things. First, there 
is evidence, to which reference has already been made, 
that nerves left in hydrogen gas show diminished irri- 
tability and that they give off smaller amounts of carbon 
dioxide than do the same nerves in air. This fact led 
us to anticipate that these nerves, being thus less irritable 
than normal nerves, would produce less than the usual 
increment of carbon dioxide on excitation. This would 
be the case if the increment were a proper measure of the 
