§ 157. LICHEN ACIDS. 151 



ethereal salts previously mentioned. The same is the case with 

 erythric acid (sparingly soluble in ether), which is regarded as 

 diorsellinate-cf erythrite,^ pieroerythrin (orsellinate of erythrite), 

 and betaerythric acid^ (orsellinate of betapicroeiythrin). 



For picrolichenin compare Alms, Stenhouse, and Groves f for 

 cetraric and Ikhenosteario amd, Schnedermann and Knop ,•* for 

 variolmm, Robiquet;^ gtictie acid, Schnedermann and Knop;' 

 lobaric acid, Knop ;^ atranoric acid (hydrocarbo-usnic acid ?), 

 zeorm, sordidin, Patemo;^ calydn, Hesse.' 



Microscopical examination shows that the majority of these 

 acids adhere in the form of minute granules to the exterior of the 

 liyphae, in heteromerous lichens almost exclusively in the cortical 

 portion of the upper surface, or^ in old specimens, on the margin 

 of the thallus (Physicia parietina).!" 



To test for a lichen-acid yielding orcin as a product of decom- 

 position, the substance under examination, or part of. the lichen 

 itself, may be heated with dilute potash, chloroform added, and 

 the warming continued for some time in the water-bath. If such 

 an acid is present, homofluorescin wiU be produced, and the 

 solution will appear reddish-yellow by transmitted, and show a 

 fine yeUovrish-green fluorescence by reflected light. TJsnic acid is 

 said not to give this reaction, which is yielded by lecanoric, 

 erythric and evemic acid (by the last-named after continued 

 boiling with milk of lime). 



Erythric and lecanoric acid are extracted from the lichen by 

 digestion with ammonia, and are precipitated by acetic acid. On 

 warming, erythric acid passes into solution whilst lecanoric acid 

 remains undissolved. 



^ Compare Heeren, Schweiz. Jonrn. lix. 813 ; also Schunck, Stenhonse, 

 Strecker, Hesse, already quoted. 



' See Menschutkin, Bullet, de la Soo. chim. [2], iL 424, 1864. Lamparter, 

 AnnaL d. Chem. und Pharm. oxxxiv. 243, 1865. 



' AnnaL d. Chem. und Pharm. i. 61, 1832 (Amer. Joum. Pharm. xvi. 262) ; 

 ibid, clxxxv. 14, 1877 (Proc. Koy. Soclx. 68). 



« Annal. d. chem. und Pharm. Iv. 144, 159, 1845. 



^ Annal. de chim. et de Phys. xlii 236. 



« Jahresb, f. Pharm. 76, 1845. 



' Chem. Centralblatt, 173, 1872 (Joum. Chem. Soo. xxv. 639). 



8 Ber. d. d. chem. Ges. x. 1100 and 1382, 1877 (Joum. Chem. Soo. xxxi 

 89 ;XKxii, 270). \ 



' Ber.d. d. chem. Ges, xiii. 1816, 1880 (Pharm. Joum. Trans. [3], xi. 471 



'" Compare Schwartz in Cohn's ' Beifcrage zur Blologie d. Pflanzen,' ii. 

 I-art II. and Archiv d. Pharm. [3], xix. 124, 1881. 



