eads_ that are denser, that
are uniform in texture, and that are shielded in an unusual manner from
disturbing forces, except at their two extremities."

The last consideration is that form of force (thought power) which
appears in manifestations called mental. It must be noticed at the
outset, that every external manifestation of thought force is a muscular
one, as a word spoken or written, a gesture, or an expression of the
face always takes place; hence this force must be intimately correlated
to nerve force. It is very certain, then, that thought force is capable
in external manifestations of converting itself into actual motion. But
here the question arises, can it be manifested inwardly without such a
transformation of energy? Or is the evolution of thought entirely
independent of the matter of the brain?

This question can be answered by actual experiment, strange as it may
appear. Experiments have demonstrated that any change of temperature
within the skull was soonest manifested externally in that depression
which exists just above the occipital protuberance. Here Lombard[45]
fastened to the head at this point two little bars, one made of bismuth,
the other of an alloy of antimony and zinc, which were connected with a
delicate galvanometer;[46] to neutralize the result of a gradual rise of
temperature over the whole body, a second pair of bars, reversed in
direction, was attached to the leg or arm, so that if a like increase of
heat came to both, the electricity developed by one would be neutralized
by the other, and no effect would be produced by the needle unless only
one was affected. By long practice it was ascertained that a mental
torpor could be induced, lasting for hours, in which the needle remained
stationary. But let a person knock on the door outside of the room, or
speak a single word, even though the experimenter remained absolutely
passive, the reception of the intelligence caused the needle to swing
twenty degrees. "In explanation of this production of heat," says
Barker,[47] "the analogy of the muscle at once suggests itself. No
conversion of energy is complete, and as the heat of muscular action
represents force which has escaped conversion into motion, so the heat
evolved during the reception of an idea is energy which has escaped
conversion into thought, from precisely the same cause." Dr. Lombard's
experiments have shown that the amount of heat developed by the
recitation to one's self of