190 Dii. R. D. Thomson on Panetin, a Yellow Colouring Matter. 



into test papers, and found that, when properly impregnated with the 

 solution, they were little, if at all inferior to turmeric paper, in their 

 delicate detection of ammonia. Test paper may be prepared extem- 

 poraneously from the alcoholic solution, when it is wished to detect 

 ammonia, by dipping a piece of paper into the alcoholic solution, and 

 then applying it in its wet state to the ammoniacal vapour. The 

 yellow colour is immediately transformed into a reddish purple, but 

 more distinct than the colour that becomes apparent in turmeric paper 

 of old preparation, under similar circumstances, which is a dirty brown. 

 One of the principal recommendations of the liquid test already noticed, 

 is the circumstance of its being capable of preservation without under- 

 going deterioration, while the test papers which have been frequently 

 recommended, although possessing most delicate testing powers when 

 freshly prepared, gradually lose their value by preservation. I believe 

 this to be the explanation of the failure in this country of some conti- 

 nental test papers, which have been recently recommended. It would 

 therefore appear, that the best test paper being that which is of fresh 

 preparation, the most convenient source for its production is that from 

 which it can be most rapidly procured in an efficient state. The ob- 

 servations which have been made upon parietin, in reference to its 

 colouring powers, tend to show that it may be employed with advan- 

 tage for the most delicate purposes to which turmeric is applied. Pari- 

 etin, however, is not acted on by acids ; the natural yellow colour 

 merely becomes brighter, while turmeric, which contains a blue and 

 yellowing colouring principle, has the former reddened by acids, and 

 the latter converted to a brown by alkalies. Moistened yellow pari- 

 etin paper, on the other hand, becomes red or purple when freshly pre- 

 pared, and reddish brown, if long prepared, by coming in contact with 

 ammonia and other alkalies. The other reactions of parietin are 

 simple. The alcoholic solution is precipitated yellow by nitrate of 

 silver and acetate of lead, and other metallic salts. A solution of 

 permuriate of iron renders the colour much darker. The precipitates 

 with silver and lead have not been analysed, from the minute quantity 

 of parietin at my disposal. 



The yellow colour of the Parmelia parietina early attracted the 

 attention of those persons interested in dyes. It was accurately de- 

 scribed by Hoffmann, Amoreux, and Willemet, in 1786.* The latter 

 informs us, that the Swedes in the province of Oeland obtained by 

 means of this lichen and alum a yellow dye for woollen stuffs, and that 

 a flesh tint was also procured from it, fitted for linen and paper ; that 

 goats eat this lichen, and that Haller recommended it as a powerful 

 tonic in diarrhosa. He adds, that he had himself used it in his prac- 

 tice as a tisan, and had found it to prove beneficial in that form of the 



* Memoires couronnds en Tanned, 1786, par rAcademie des Sciences, Belles Lettres, 

 et Arts, de Lyon, sur riitilit6 des lichens, dans la Medicine ct dans Ics Arts. 8vo, 1787. 



