50 A CHEMICAL SIGN OF LIFE 
is no heat produced in excited nerves. How shall we 
explain the fact that this relatively tremendous chemical 
transformation can occur without heat formation ? 
There are several explanations which might be given 
of this fact, but before considering them we may see 
first what the evidence is that there is no heat produced. 
Although there have been in the literature many 
contradictory statements as to heat formation in the 
active nerve, the original negative results of Helmholz, 
Stewart, and Rolleston have been confirmed recently 
by A. V. Hill’s work, which shows that there is no meas- 
urable liberation of heat when the nerve is stimulated. 
Since his apparatus is exceedingly sensitive, being sus- 
ceptible to the change of 1/1,000,000 of a degree Centi- 
grade, the lack of observed heat production is not ap- 
parently to be explained by any lack of a proper method 
of measuring temperatures. His work is remarkably 
significant in that according to his calculation not more 
than one single oxygen molecule in every cube of a 
nerve containing 3.7 cubic » can be used up by a single 
propagated nerve impulse, since more than this amount 
would produce a measurable amount of heat. Thus he 
is convinced that a nerve impulse is not of an irreversible 
chemical nature, but must be of a purely physical nature. 
Negative evidence of this kind cannot be taken at 
its face value without considering the limitations of the 
method. Stewart calls attention to the fact that we 
should not forget that if the axis cylinder is the only 
portion which is conducting a nerve impulse, as we 
believe, the measurement in medullated nerves with 
which most experiments were made does not express 
the true state in the axis cylinder. We should consider, 
