157. LICHEN ACIDS. 151 



ethereal salts previously mentioned. The same is the case with 

 ert/fhric add (sparingly soluble in ether), which is regarded as 

 diorsellinate of erythrite, 1 picroerytlirin (orsellinate of erythrite), 

 and betaerythric acid 2 (orsellinate of betapicroerythrin). 



For picrolichenin compare Alms, Stenhouse, and Groves ; 3 for 



:-lc and liclienostearic acid, Schnedermann and Knop ; 4 for 



rcr'tol'iti'in, Robiquet; 5 stictic acid, Schnedermann and Knop ; 6 



lobaric acid, Knop; 7 atranoric acid (hydrocarbo-usnic acid?), 



, snrdidin, Paterno; 8 calycin, Hesse. 9 



Microscopical examination shows that the majority of these 

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

 hyphae, 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). 10 



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 will be produced, and the 

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

 fine yellowish-green fluorescence by reflected light. Usnic acid is 

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

 erythric and evernic 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. 



1 Compare Heeren, Schweiz. Journ. lix. 313 ; also Schunck, Stenhouse, 

 Strecker, Hesse, already quoted. 



2 See Menschutkin, Bullet, de la Soc. chim. [2], ii. 424, 1864. Lamparter, 

 Annal. d. Chem. und Pharm. cxxxiv. 243, 1865. 



3 Annal. d. Chem. und Pharm. i. 61, 1832 (Amer. Journ. Pharm. xvi. 262) ; 

 ibid, clxxxv. 14, 1877 (Proc. Hoy. Soc. Ix. 68). 



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



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

 Jahresb. f. Pharm. 76, 1845. 



7 Chem. Centralblatt, 173, 1872 (Journ. Chem. Soc. xxv. 639). 



8 Ber. d. d. chem. Ges. x. 1100 and 1382, 1877 (Journ. Chem. Soc. xxxi. 

 89 ; xxxii. 270). 



9 Ber. d. d. chem. Ges. xiii. 1816, 1880 (Pharm. Journ. Trans. [3], xi. 471). 



10 Compare Schwartz in Cohn's ' Beitrage zur Biologic d. Pflanzen,' iii. 

 Part II. and Archiv d. Pharm. [3], xix. 124, 1881. 



