of the colorific and coloring principles, so far as they are at present known, showing their chemical formulae and the authority therefor, and various relative information.  “It is highly probable that when the chemistry of the lichens has been more fully studied, and the whole subject of their color-educts and products better understood, we shall begin to reduce the present confused mass of complex substances, and find the same principles more extensively diffused through different lichen species.”  Dr. L. entered somewhat minutely on the chemical reactions of the better known colorific and coloring principles, and their derivatives, so far at least as these throw any light on the production and transmutation of the red or purple colors extracted from what may be termed par excellence, the dye-lichens.  After a few remarks on the chemical constitution of orchil and litmus, as given by Kane, Gelis, Pereira, and others, he discussed the subject of decolorisation of weak infusions of orchil and litmus by exclusion of atmospheric air, and by various deoxidising agents, and the different theories as to the causation of this phenomenon.  “I have repeatedly had occasion to notice that, when weak infusions of these substances are excluded for some time from atmospheric air, in a bottle, with a tightly fitting cork, they gradually lose color, but rapidly regain it on re-exposure.  It is curious that both orchil and litmus are what are called transient or false colors, i.e., they slowly lose their bloom and tint by long exposure to the atmosphere; the coloring matter, therefore, appears to be decolorised both by exposure to, and exclusion from the air, phenomena apparently of very opposite characters.  The cause of the latter phenomenon has never, so far as I am aware, been quite 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