IV. Examination of the proximate principles of some of the lichens

Abstract

The examination of the proximate principles of the lichens, especially of those which yield red colouring matters with ammonia, attracted the attention of some very eminent chemists at a comparatively early period in the history of organic chemistry, and by the labours of Messrs. Robiquet, Heeren, Dumas and Kane, very considerable progress was made in their investigation. Within the last four or five years, Messrs. Schunck, Rochleder, Heldt and Knop have resumed the subject and greatly extended our acquaintance with this interesting but rather difficult depart­ment of organic research. It is but justice to Mr. Schunck to state that he has been by far the most successful cultivator of this field, and that he has done more to elucidate it than any of his predecessors. Nearly two years ago my attention was directed by Dr. Pereira to a kind of Orcella weed which had been recently imported into London from the Cape of Good Hope, but which had been rejected by the London archil manufacturers as unfit for their use, from the small quantity of colouring matter it yielded when subjected to the usual process. The lichen was of considerable size, from eight to ten inches long, and was pronounced by an eminent botanist, Dr. Scouler of Dublin, to whom submitted it, to be merely a large variety of the Roccella tinctoria . I soon ascertained on a very cursory examination, why the lichen had been rejected by the archil makers, for it only contained a small portion of a crystalline principle which yields a red colour with ammonia; I found in its stead, however, a considerable quantity of another crystalline body on which ammonia had no action, and which appeared to have been hitherto undescribed. As it seemed important to ascertain whether or not the red dyes obtained from the various lichens resulted from the action of am­monia on the same crystalline principle, described by Mr. Schunck under the name of Lecanorin, I procured quantities of the several lichens usually employed by the archil makers, and subjected them to investigation. These lichens consisted, —1st, of a large species of Roccella tinctoria from the west coast of South America; 2nd, of the R. tinctoria from the Cape of Good Hope; 3rd, of the R. Montagnei from Angola; and 4th, of the Lecanora tartarea . I had made considerable progress with the investigation of these lichens, and also with that of the Evernia prunastri , when Mr. Schunck’s elaborate paper on the Angola lichen appeared, from which it was evident that at least two varieties of the red colouring principle existed in these lichens. I now therefore proceed to give a detail of the results of these examinations.