le, owing to their inirritability, and consequent neglect of
nictitation.

The increase of transparency by filling the pores with fluid is seen by
soaking white paper in oil; which from an opake body becomes very
transparent, and accounts for a curious atmospheric phenomenon; when there
exists a dry mist in a morning so as to render distant objects less
distinct, it is a sign of a dry day; when distant objects are seen very
distinct, it is a sign of rain. See Botan. Garden, Part I. add. note xxv.
The particles of air are probably larger than those of water, as water will
pass through leather and paper, which will confine air; hence when the
atmosphere is much deprived of moisture, the pores of the dry air are so
large, that the rays of light are refracted by their edges instead of
passing through them. But when as much moisture is added as can be
perfectly dissolved, the air becomes transparent; and opake again, when a
part of this moisture collects into small spherules previous to its
precipitation. This also accounts for the want of transparency of the air,
which is seen in tremulous motions over corn-fields on hot summer-days, or
over brick-kilns, after the flame is extinguished, while the furnace still
remains hot.

2. _Deglutitio irritativa._ The deglutition of our saliva is performed
frequently without our attention, and is then an irritative action in
consequence of the stimulus of it in the mouth. Or perhaps sometimes for
the purpose of diffusing a part of it over the dry membranes of the fauces
and pharinx; in the same manner as tears are diffused over the cornea of
the eye by the act of nictitation to clean or moisten it.

3. _Respiratio et Tussis irritativae._ In the acts of respiration and of
coughing there is an increased motion of the air-cells of the lungs owing
to some stimulating cause, as described above in Class I. 1. 2. 8. and I.
1. 3. 4. and which are frequently performed without our attention or
consciousness, and are then irritative actions; and thus differ from those
described in Class II. 1. 1. 2. and 5. To these increased actions of the
air-cells are superadded those of the intercostal muscles and diaphragm by
irritative association. When any unnatural stimulus acts so violently on
the organs of respiration as to induce pain, the sensorial power of
sensation becomes added to that of irritation, and inflammation of the
membranes of them is a general consequence.

4. _Exclusio bilis._ The excl