ion and the phenomena of mental activity. The former are the
simpler, and offer the greatest hope of solution. If we are to find any
correlation between nervous energy and other physical energy, we must do
so by finding some way of measuring nervous energy and comparing it with
the latter. This has been very difficult, for we have no way of
measuring a nervous impulse directly. In the larger experiments upon the
income and outgo of the body, in the respiration apparatus mentioned
above, nervous phenomena apparently leave no trace. So far as
experiments have gone as yet, there is no evidence of an expenditure of
extra physical energy when the nervous system is in action. This is not
surprising, however, for this apparatus is entirely too coarse to
measure such delicate factors.

That there is a correlation between nervous energy and physical energy
is, however, pretty definitely proved by experiments along different
lines. The first step in this direction was to find that a nervous
stimulus can be measured at least indirectly. When the nerve is
stimulated there passes from one end to the other an impulse, and the
rapidity with which it travels can be accurately measured. When such an
impulse reaches the brain it may give rise to a conscious sensation, and
a somewhat definite estimation can be made of the amount of time
required for this. The periods are very short, of course, but they are
not instantaneous. The nervous impulse, can be studied in still other
ways. We find that the impulse can be started by ordinary forms of
energy. A mechanical shock, a chemical or an electrical shock will
develop nervous energy. Now these are ordinary forms of physical energy,
and if, when they are applied to a nerve, they give rise to a nervous
stimulus, the inference is certainly a legitimate one that the nerve is
simply a bit of machinery adapted to the conversion of certain kinds of
physical energy into nervous energy. If this is the case, then it is
necessary to regard nervous energy as correlated with other forms of
energy.

Other facts point in the same direction. Not only can the nervous
stimulus be developed by an electric shock, but the strength of the
stimulus is within certain limits proportional to the strength of the
shock which produces it. Again, not only is it found that an electrical
shock can develop a nervous stimulus, but conversely a nervous stimulus
develops electrical energy. In ordinary nerves, even when not act