cause depends the permanent debility of those, who have been addicted to
intoxication, the general weakness of old age, and the natural debility or
inirritability of those, who have pale skins and large pupils of their
eyes.

There is a curious phenomenon belongs to this place, which has always
appeared difficult of solution; and that is, that opium or aloes may be
exhibited in small doses at first, and gradually increased to very large
ones without producing stupor or diarrhoea. In this case, though the opium
and aloes are given in such small doses as not to produce intoxication or
catharsis, yet they are exhibited in quantities sufficient in some degree
to exhaust the sensorial power, and hence a stronger and a stronger dose is
required; otherwise the medicine would soon cease to act at all.

On the contrary, if the opium or aloes be exhibited in a large dose at
first, so as to produce intoxication or diarrhoea; after a few repetitions
the quantity of either of them may be diminished, and they will still
produce this effect. For the more powerful stimulus dissevers the
progressive catenations of animal motions, described in Sect. XVII. and
introduces a new link between them; whence every repetition strengthens
this new association or catenation, and the stimulus may be gradually
decreased, or be nearly withdrawn, and yet the effect shall continue;
because the sensorial power of association or catenation being united with
the stimulus, increases in energy with every repetition of the catenated
circle; and it is by these means that all the irritative associations of
motions are originally produced.

2. When a stimulus is repeated at such distant intervals of time, that the
natural quantity of sensorial power becomes completely restored in the
acting fibres, it will act with the same energy as when first applied.