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V. On Parietin, a yellow colouring matter^ and on the Inorganic 

 Constituents of Lichens. By Robert D. Thomson, M.D.y 

 Lecturer on Practical Chemistry in the University of Glas- 

 goixi*, 



nPHE objects of the present paper are, — 1st, to endeavour 

 to show that, contrary to the generally received opinion, 

 the class of plants termed Lichens require inorganic matter 

 as part of their food, which they must derive from the locali- 

 ties upon which they are fixed ; and, 2nd, to describe the 

 yellow colouring matter obtained from the yellow wall lichen, 

 and to detail its properties, composition, and application as a 

 test for alkalies. 



Although chemists are acquainted with several yellow co- 

 louring matters, few of them have been separated in a pure 

 state and analysed. This may be satisfactorily explained by 

 the circumstance that such substances are obtained with diffi- 

 culty in the same state as that in which they existed in the 

 plant from which they are extracted. The obstacle lies in the 

 facility with which they unite with oxygen, and in their con- 

 sequent conversion into a body of inferior beauty, and of an 

 uncrystallized structure. The yellow colouring matters which 

 have hitherto been analysed are derived from various parts of 

 phenogamous plants, principally the roots and flowers. The 

 subject of the present paper is obtained from a different class 

 of plants — the lichens; but one to which we are indebted for 

 some important colouring substances. The Greeks gave the 

 name >^six^v to a disease of the skin, and likewise to certain 

 plants which were considered to possess the power of healing 

 these cutaneous eruptions. Dioscorides tells us (Mat. Med. 

 b. iv. c. 48) that the lichen, which is familiarly known from 

 its growing on stones and attaching itself to the rough parts 

 of rocks, like a moss, was called by some persons bryon^ and 

 was useful in the cure of sanguineous fluxes and inflamma- 

 tions. Pliny likewise employs the term lichen, but from his 

 describing it as growing on rocks with one leaf from a broad 

 root, and with one small stem, it is obvious he refers to a 

 species of Hepatica (Nat. Hist, xxvii. c. 4). Galen likewise 

 enumerates lichens among the instruments of cure in the treat- 

 ment of impetigenous or cutaneous diseases. Modern bota- 

 nists, up to a comparatively recent period, appear to have 

 overlooked this class of plants, if we may draw this conclu- 

 sion from the catalogue of English plants by John Ray, the 

 second edition of which was published in 1677. In this work 

 the celebrated author describes, under the title of lichen, eight 



* Read before the Philosophical Society of Glasgow, November 29, 1843. 



