Dr Thomson mi Farietin^ §fc, 187 



ciety) has been making some interesting experiments on the 

 changes in metals, and I have requested him to extend them 

 to rocks, which I hope he will do. 



Richard Solly. 



Sakdon Place, Sheffield, 

 28«A May 1844. 



On Parietin, a Yellow Colouring Matter^ and on the Inorganic 

 Food of Lichens. By Robert D. Thomson, M.D. Com- 

 municated by the Author.* 



The objects of the present paper are, \st^ To endeavour to 

 prove that, contrary to the usually received opinion, the class 

 of plants termed Lichens, require inorganic matter as part of 

 their food, which they must derive from the localities upon 

 which they are fixed ; and, 2d, To describe the yellow colour- 

 ing 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 

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

 state, and analysed. This arises from the difficulty of pro- 

 curing such substances in the same state as that in which 

 they existed in the plant from which they are extracted — 

 depending principally on the facility with which they unite 

 with oxygen, and on their consequent 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, prin- 

 cipally the roots and flowers. The subject of the present 

 paper is procured from a totally different tribe — the lichens — 

 but one to which we are indebted for some important dyes. 

 The Greeks gave the name Xf/%»3v to a disease of the skin, and 

 likewise to certain plants possessing the power of healing these 

 cutaneous eruptions. Dioscoridest tells us 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 



* From the Transactions of the Glasgow Philosophical Society. 

 t Mat. Med. b. iv. cap. 48. 



