1. Number and Variety.—A considerable number of lichens have been employed by man on account of the coliouring matter which they yield him. Some of them (e. g., Parmelia parietina and Evernia vulpina) contain colouring principles (e. g., chrysophanic and vulpinic acids). Others (e. g., several species of Roccella, of Lecanora, of Variolaria, &c.) contain principles (e. g. orsellic, erythric, lecanoric, and gyrophoric acids) which are colourless while in the plant, but which, under the influence of alkalies and atmospheric oxygen, yield colouring matters (e. g., orceine). Such principles I shall distinguish as colorific, or colour-making.
2. Colours.—Lichens furnish four principal colours, viz., brown, yellow, purple, and blue.
α. Brown colours are yielded by Gyrophora pustulata and Sticta pulmonaria (see ante, p. 65). The latter lichen, says Professor Guibourt [Hist. Nat. des Drog. simpl. t 2me p. 77, 4me éd. 1849.], produces on silk, by using, as mordants, bitartrate of potash and chloride of tin, a very fine and durable carmelite colour. For use in France, it is principally collected in the Vosges.
β. Yellow colours are yielded by Parmelia parietina (see ante, p. 69) and Evernia vulpina. The former lichen contains, as its yellow colouring principle chrysophanic acid; the latter, according to M. Bébert [Journ. de Pharm. t. xvii. p. 696.], contains a yellow crystallizable acid called vulpinic acid.
γ. and δ. Purple and blue colours are yielded by a considerable number of lichens. In this country, purple colours (orchil and cudbear) only are obtained from them; but in Holland a blue colour (litmus) is also prepared from these. And it appears that the same lichens yield either the one or the other colour according to the method of treatment.
The orchil makers of this country call the cylindrical and flat species of Roccella used in the manufacture of orchil and cudbear, weeds or orchella weeds, and distinguish them according to the countries yielding them (e. g., Angola weed, Canary weed, &c.); while the crustaceous and foliaceous lichens, employed for similar purposes, they term mosses (e. g., tartareous moss, pustulatous moss, rock moss, &c.). A similar distinction is made in French commerce; the term herbe being applied to what the English call a weed, while the name of lichen is given to what our dealers term a moss.
The following is a list of the principal lichens employed by British manufacturers of orchil and cudbear, with their commercial names:—
Angola Orchella weed (R. fuciformis).
Madagascar Orchella weed (R. fuciformis).
Mauritius Orchella weed.
Canary Orchella weed (R. tinctoria).
Cape de Verd Orchella weed (R. tinctoria).
Azores Orchella weed (R. tinctoria).
Madeira Orchella weed (R. tinctoria and R. fuciformis).
South American (Lima), large and round Orchella weed (R. tinctoria?).
South American (Lima), small and flat Orchella weed (R. fuciformis).
Cape of Good Hope Orchella weed (R. hypomecha).
Barbary (Mogadore) Orchella weed (R. tinctoria).
Corsican and Sardinian Orchella weed (R. tinctoria).
Tartareous moss (Lecanora tartarea)
Pustulatous moss (Gyrophora pustulata)
Canary Rock moss (Parmelia perlata?) 
Corsica and Sardinia Rock moss.
Norway Rock moss.
I have not met with the Canary Rock Moss in fructification, and cannot, therefore, positively state its botanical name. I found a similar lichen in commerce under the name of British Rock Moss. The thallus of both corresponds to that of Parmelia perlata, Ach.
Mr. Harman Visger, of Bristol, informs me that "every lichen but the best orchella weed is gone or going rapidly out of use; not from deterioration of their quality, for, being allowed to grow, they are finer than ever; but because the Angola-weed is so superior in quality, and so low-priced and abundant, that the product of a very few other lichens would pay the expense of manufacture."
In France, the Variolaria dealbata, De Cand., and V. orcina or oreina, Ach. (the Parelle d'Auvergne) are employed in the production of orchil. These two lichens constitute the V. corollina, Ach., which must be confounded neither with Lecanora parella, Ach., nor with Isidium corallinum, Ach.
3. Colorific principles.—These in most, if not in all, cases are organic acids: e. g., alpha orsellic, beta orsellic, erythric, lecanoric, gyrophoric, evernic, usnic, &c. acids.
Under the united influence of water, atmospheric oxygen, and ammonia, these colorific principles yield coloured products, which, though probably not identical, pass under the general name of orceine.
The precise chemical changes which these colorific principles undergo when exposed to the joint action of water, air, and ammonia, are not definitely known. Some of these principles are not directly converted into coloured substances, but into intermediate colourless substances. Thus lecanoric acid becomes first orcine and then orceine. Liebig, adopting the formulae which have been given for these three bodies respectively by Schunck, Will, and Dumas, has given the following explanation of the changes: Lecanoric acid, C18H8O8, gives out two atoms of carbonic acid, C2O4, and becomes anhydrous orcine, C16H84, which, with three atoms of water, H3O3, yields one atom of crystallized orcine, C16H11O7; and one atom of crystallized orcine, C16H11O7, with one atom of ammonia NH3, five atoms of oxygen, yield one atom of orceine, C19H9O7, and five atoms of water: but the accuracy of the formulae has been called in question.
4. Test of the Colorific Property of Lichens.—Hellot's test is maceration in a weak solution of ammonia (see Roccella tinctoria).
Another method is by testing an alcoholic tincture of the lichen with a solution of hypochlorite of lime. If the lichen possess any colorific power, a fugitive red colour is produced.
Dr. Stenhouse [Phil. Trans. for 1848.] proposes to estimate the quantity of colorific matter in lichens by means of a solution of hypochlorite of lime. Any convenient quantity of the lichen (say one hundred grains) may be cut into very small pieces, and then macerated with milk of lime till all the coloring principle is extracted. Three or four macerations are quite sufficient for this purpose, if the lichen has been sufficiently comminuted. The clear liquors should be filtered and mixed together. A solution of bleaching powder of known strength should then be poured into the lime solution from a graduated alkalimeter. The moment the bleaching liquor comes in contact with the lime solution of the lichen, a blood red colour is produced, which disappears in a minute or two, and the liquid has only a deep yellow colour. A new quantity of the bleach ing liquid should then be poured into the lime solution, and the mixture carefully stirred. This operation should be repeated so long as the addition of the hypochlorite of lime causes the production of the red colour; for this shows that the lime solution still contains unoxidized colorific principle. Towards the end of the process, the bleaching solution should be added by only a few drops at a time, the mixture being carefully stirred between each addition. We have only to note how many measures of the bleaching liquor have been required to destroy the colouring matter in the solution, to determine the amount of the colorific principle it contained. The following are the results of trials with the same test liquors upon four varieties of lichen:—
|Angola lichen required||200 = 1.00|
|American lichen||120 = 0.69|
|Cape lichen||035 = 0.17|
|Lecanora tartarea, from Germany near Giessen||025 = 0.12|
The amount of colorific principle in a lichen may also be directly determined by extracting the lichen with milk of lime, by precipitating by means of acetic acid, collecting the precipitate on a weighed filter, drying it at the ordinary temperature, and then weighing it.
5. Mode of Extracting the Colorific Principles for Transport.— Dr. Stenhouse suggests the following method: cut the lichens into small pieces, macerate them in wooden vats with milk of lime, and saturate the solution either with muriatic or acetic acid. The gelatinous precipitate is then to be collected on cloths, and dried by a gentle heat.
In this way almost the whole colorific matter can be easily extracted, and the dried extract transported at a small expense from the most distant inland localities, such as the Andes or Himalayas.
Dr. Stenhouse has kindly furnished me with the following table of the lichens, and their colorific principles and coloured products:
|Lichens.||Colorific Principles.||Colouring Principles.||Authority.|
|S. American Orchella-weed||Lima, &c.||Alpha Orsellic acid||C32H15O13+HO||Orceine||C19H10NO||Stenhouse.|
|Cape Orchella-weed||Cape of G. Hope||Beta Orsellic acid||C34H18O14+HO||Ditto||Ditto||Stenhouse.|
|Angola Orchella-weed||Angola, Africa||Erythric acid||C20H10O8+HO||Ditto||Ditto||Stenhouse.|
|Perelle Moss (Lecanora parella)||Switzerland||Lecanoric acid||C18H8O8||Ditto||Ditto||Schunck.|
|tartaerous Moss (L. tartarea)||Norway||Gyrophoric acid||C36H18O18||Ditto||Ditto||Stenhouse.|
|Pustulatous Moss (Gyrophora pustulata)||Norway||Ditto||Ditto||Ditto||Ditto||Stenhouse.|
|Ragged hoary Lichen (Evernia prunastri)||Scotland||Evernic acid||C34H15O15+HO||—||—||Stenhouse.|
|Usnea (florida, plicata, and hirta, &c.||Germany||Usnic acid||C36H17O14||—||—||Rochleder and Heldt.|
|Rein-deer Moss (Cladonia rangiferina)||—||Ditto||Ditto||—||—||Rochleder and Heldt.|
|Ramalina (fastigiata) [calicaris]||—||Ditto||Ditto||—||—||Rochleder and Heldt.|
The Elements of Materia Medica and Therapeutics, Vol. II, 3th American ed., was written by Jonathan Pereira in 1853.