and
processes--pointing out their respective advantages and
disadvantages--and showing their practical value and applications.
He stated that he had made use of these, and various other tests, in
upwards of 300 experiments, and the one which he employed to the
greatest extent, because most uniformly applicable, was Helot's
ammonia test. The following combination is that most favorable for
the development of the coloring matter of the lichens--viz., the
presence

1. Of _water_ as a solvent menstruum.
2. Of atmospheric _oxygen_.
3. Of _ammonia_, in the state of vapor or in solution, and
4. Of a moderate degree of _heat_;

And according as the proportion of these combining elements varies,
so do the kind and amount of color educed by them. This combination
is the foundation of all the processes for the manufacture of the
lichen dyes throughout the world, however different these may appear
to be in detail or results.

I believe it may come to be a matter of great commercial importance
to discover, at home or abroad, some cheap and easily-procurable
substitute for the _Roccellas_, which are gradually becoming scarce,
and consequently valuable in European commerce, having sometimes
fetched, in times of scarcity, no less than L1,000 per ton. No
plants can be so easily collected and preserved as
lichens--requiring merely to be cleaned, dried, pulverised, and
packed; and if their bulk be an objection to transport, their whole
colorific matter may be collected in the way I have already
mentioned. Ascending to the verge of eternal snows, and descending
to the ocean level--with a geographical diffusion that is
co-extensive with the surface of our earth, it is difficult to say
where lichens shall not be found. There are myriads of small rocky
islets in the boundless ocean, and there are thousands of miles of
barren rocky coast and sterile mountain range in every part of the
world, which, though at present unfit to bear any of the higher
members of the vegetable kingdom, are yet carpeted and adorned with
a rich covering of lichens, and of those very species too, which I
have already spoken of as prolific in colorific materials. I
sincerely believe, therefore, that a more general attention to the
very simple tests just enumerated, would ultimately result in a
greatly exte