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procurable aramoniacal solution ; but that it is something more is 

 rendered extremely probable, by the fact that large English firms, 

 which, besides preparing orchil and cudbear, also manufacture liquid 

 ammonia, of every degree of strength and purity, still find it advanta- 

 geous to employ urine, instead of pure dilute ammoniacal liquors, in 

 the production of these pigments. Perhaps its true value may depend 

 on its putrefactive state ; the chemical changes in the nitrogenous or 

 other constituents of the decomposing liquid being communicated cata- 

 lylically to the colorific (but colourless) principles of the lichen, thereby 

 inducing an alteration in their physical characters, as well as in their 

 chemical composition. This hypothesis would (at first sight) apparently 

 explain a series of phenomena, of the true nature of which we at present 

 know very little ; and it appears to be supported by the fact, that on 

 the Continent (and particularly in Holland and France), stagnant and 

 putrid waters, which contain a large amount of decaying animal and 

 vegetable matters {e. g., the filthy stream of the Bievre, at Paris), are 

 largely used in the manufacture of orchil and litmus, as macerating 

 agents, because experimentally found most efficient in causing the 

 elimination of these colouring matters. But I cannot yet reconcile 

 this hypothesis, nor the supposition that the changes concerned in the 

 production of these pigments depend essentially on some action of 

 ammonia, or its elements, on the colorific principles of the plant, with 

 the fact that the same colours are capable of being evolved, though in 

 a minor degree, by other alkalies than ammonia, and by liquids cer- 

 tainly not in any state of decomposition [e. g., distilled, or pure spring 

 water). To the putrid urine, lime is sometimes added, materially 

 assisting the colour-metamorphosis, by uniting with, and thereby 

 removing, the carbonic acid of the carbonate of ammonia, generated 

 in the liquid ; thus separating the ammonia, which then acts as a free 

 agent. The necessity for free exposure to atmospheric air, is well 

 illustrated by the simple fact that many of the lichen-colours, which 

 are at first dull in tint, are increased in intensity and brightness by 

 free exposure and prolonged maceration. Some time ago, 1 had 

 occasion to open a small bottle of ' red orchil,' prepared in Glasgow, 

 which had lain for some time on the shelves of a museum. Instead 

 of possessing an intense purple-red colour, and a fine ammoniacal 

 aroma, it was of a very dirty, nondescript, brownish red, had a pulta- 

 ceous or semifluid clotty consistence, and a musty, urinous, disa- 

 greeable smell. But, immediately on spreading it, in thin layers, on 

 paper, thereby exposing a large surface to the action of the air, it 

 acquired its characteristic colour. The following are the chief thick- 



