e
state are termed irritability, sensibility, voluntarily, and
associability; in their active state they are termed as above
irritation, sensation, volition, association.

Irritation is an exertion or change of some extreme part of the
sensorium residing in the muscles or organs of sense, in consequence
of the appulses of external bodies.

Sensation is an exertion or change of the central parts of the
sensorium, or of the whole of it, beginning at some of those extreme
parts of it, which reside in the muscles or organs of sense.

Volition is an exertion or change of the central parts of the
sensorium, or of the whole of it, terminating in some of those extreme
parts of it, which reside in the muscles or organs of sense.

Association is an exertion or change of some extreme part of the
sensorium residing in the muscles or organs of sense, in consequence
of some antecedent or attendant fibrous contractions; see Zoonomia,
Vol. I.

The word sensorium is used to express not only the medullary part of
the brain, spinal marrow, nerves, organs of sense and muscles, but
also at the same time that living principle, or spirit of animation,
which resides throughout the body, without being cognizable to our
senses except by its effects.

ADDITIONAL NOTES. III.

Next when imprison'd fires in central caves
Burst the firm earth, and drank the headlong waves.
CANTO I. l. 302.

The great and repeated explosions of volcanoes are shown by Mr.
Mitchell in the Philosoph. Transact. to arise from their communication
with the sea, or with rivers, or inundations; and that after a chink
or crack is made, the water rushing into an immense burning cavern,
and falling on boiling lava, is instantly expanded into steam, and
produces irresistible explosions.

As the first volcanic fires had no previous vent, and were probably
more central, and larger in quantity, before they burst the crust of
the earth then intire, and as the sea covered the whole, it must
rapidly sink down into every opening chink; whence these primeval
earthquakes were of much greater extent, and of much greater force,
than those which occur in the present era.

It should be added, that there may be other elastic vapours produced
by great heat from whatever will evaporate, as mercury, and even
diamonds; which may be more elastic, and consequently exert greater
force than the steam of water even though heated red hot.