The author entered into a full analysis of these tests 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 extended use of the lichens as dye agents.  What renders it very probable that efforts in this direction are likely to meet with success is the great similarity of species found all over the world.  It has been repeatedly noticed that the European species, which, of course, are best known, differ little from those of North America.  Dr. Robert Brown remarked the same fact with regard to New Holland species, and Humboldt also recognised