12 A CHEMICAL SIGN OF LIFE 
is that by chemical transformations it sets free energy 
and moves itself. 
It is much better, too, to take the carbon dioxide pro- 
duced, rather than the oxygen consumed, as the measure 
of the metabolism associated with irritability, for the 
reason that sometimes organisms get their oxygen from 
sources other than the air, whereas their carbon dioxide 
production is always something positive and universal. 
Indirect evidence of the presence of metabolic activity 
in the nerve fiber —The search for some kind of metab- 
olism, such as the production of carbon dioxide, in 
nerves had been made by many physiologists on many 
occasions, but it was impossible for them to discover 
this substance because their methods were not sufficiently 
delicate. No carbon dioxide could be found, and for 
this and other reasons the conclusion was incorrectly 
drawn that there was none produced, or that, if it were 
produced, it had no connection with the vital functions 
of thenerve. Most physiologists were accordingly of the 
opinion that the conduction of the nerve impulse was 
a physical process and involved no transformation of 
energy and no consumption of material. There was 
one exception to this rule. Professor A. D. Waller, 
the eminent English physiologist, maintained that, 
because of their electrical behavior, nerves certainly 
produced carbon dioxide. In 1896 he showed that 
carbon dioxide when applied to a nerve produced a very 
characteristic change in the electrical response which a 
nerve exhibits when it is irritated. It will be remem- 
bered that when a nerve or, in fact, any kind of proto- 
plasm is irritated in any way, if one applies two electrodes 
to the living tissue in such a way that one electrode is on 
