48 Dr. R. D. Thomson 07i Parietiii, Sfc. 



principle so much in vogue at present, "similia similibus cu- 

 rantur." Hoffmann affirms that he never could obtain a yellow 

 colour from this lichen, but that with wine-vinegar he obtained 

 an olive-green or fawn colour, and with true wine-vinegar 

 (aceto vini vero) and copperas a flesh or apricot shade. Of these 

 colours he has appended to his essay specimens, together with 

 forty-nine others obtained from various species of lichens. Dr. 

 John P. Westring, of Nordkoping in Sweden, who prose- 

 cuted an extensive inquiry into the colouring matters of lichens, 

 describes the Lickefi parielinus (Wagglaf) as affording with 

 wool, by infusion for fourteen days and then boiling for half 

 an hour, a fawn colour; by longer boiling a yellow was pro- 

 duced, and this mixture became, by simple infusion and ex- 

 traction, similar to the red wool of Florence. With common 

 salt and nitre, boiled for an hour, a beautiful straw colour was 

 elicited. Upon silk it gave similar colours, differing in their 

 shade from red to yellow, according to the methods employed 

 in dyeing the goods*. 



Subsequently to these observations, which are perhaps in- 

 teresting in an oeconomical point of view, the yellow Parmelia 

 was recommended by Dr. Sande, probably misled by the 

 colour, as a substitute for Peruvian bark, during the last French 

 war. It has also been chemically examined by Herberger, 

 but not apparently with the same results afforded by Scotch 

 specimens, as he found no inorganic constituents, which amount 

 to from 6 to 7 per cent, according to my trials, and obtained 

 a much larger quantity of colouring matter than existed in any 

 plants examined by me. He also found a red colouring matter 

 which did not appear in the process of extraction as followed 

 by me, and which may, therefore, be a product of the oxida- 

 tion of parietin. 



More lately still Dr. Gunprecht extracted yellow oil from 

 the lichen, but in such minute quantity as not to be susceptible 

 of examination. In my own experiments I always found that 

 the colouring matter volatilized when the lichen was heated, 

 just before it caught fire, giving rise to abundant yellow fumes. 

 I have also procured a considerable quantity of sugar in cry- 

 stalline grains, when the alcohol, with which the colouring 

 matter was extracted, was allowed to evaporate spontaneously. 



Glasgow, November 1 843. 



Note. — Since the preceding paper was read, and before I 

 could procure a sufficient quantity of the yellow needles above 

 mentioned for analysis, they have been examined at Giessen 

 by Rochleder and Heldt, and found to consist of C^ Hjg 0,2, 

 so that we have now the following oxides : — 



* Kongl. Vetenskaps Acad. xii. p. 300. ann. 1791. 



