satisfactorily explained; but it has been variously supposed to be
due:--

1. To the mere negation of oxygen.

2. To the development, in the liquids, of various substances,
capable of exerting a decolorising influence on the coloring matter.

3. To deoxidation of the coloring matter by substances, which have a
great tendency to become oxidised or peroxised; _e.g._ hydrogen, in
the case of decolorisation by sulphuretted hydrogen, nascent
hydrogen, and the protoxides of iron and tin, &c.

4. To the fixation of an additional amount of hydrogen in a new
colorless body, formed by the union of the sulphuretted hydrogen or
other substances with the coloring matter of the liquid. This view
is chiefly supported by Kane, who says, "that precisely as the
coloring matters combine with water, to form different shades of
red-colored bodies--with ammonia to produce a series of bodies,
which are blue and purple--so they combined with sulphuretted
hydrogen to form colorless compounds in solution, which, if solid,
very probably would be white." He supposes, in a word, that for
every colored substance existing in orchil and litmus, there is a
corresponding white one, producible by the action of sulphuretted
hydrogen, &c.; and, in proof of this theory, he mentions having
obtained from Azolitmine and Betaorceine colorless bodies, to which
he gave the respective names of Leuco-litmine and Leuco-orceine.

The author then gave a short summary of Dr. Westring's experiments
on the dyeing powers of the Swedish lichens, which he found might be
conveniently divided into four classes, according to the degree of
heat employed in their maceration, viz.:--

1. Lichens, whose coloring matter was easily extractable by _cold_
water alone.

2. Those which required for the elimination of their coloring
matter, maceration in _tepid_ water (_i.e._ below 258 degs. Swedish
thermometer).

3. Those which required maceration in _warm_ water (_i.e._between 50
and 60 degs. Swedish thermometer).

4. Those requiring _boiling_ water alone, or with the aid of
solvents.

"It must be admitted that our knowledge of the true nature of the
colorofic and coloring principles of the lichens is, as yet, very
imperfect and confused, and one great cause of the dubity and
obscurity overhanging the subject, is the