the Roccella tinctoria and Evernia Prunastri. 255 



the composition of these substances as determined by Mr. 

 Stenhouse ; while the formulae given by that gentleman for 

 picro-erythrine, C^H^O^, and pseudo-orcine, C 10 H 13 O 10 , 

 are not reconcilable with one another, nor with that given by 

 him for erythric acid, viz. C 20 H n O 10 . By adopting the above 

 formulae, the changes which erythric acid undergoes admit of 

 an easy explanation. According to my view, erythric acid f 

 may be considered as a compound oflecanoricacid andpicro- J 

 erythrine'. It is a coupled acid, in which the saturating power 

 of the simple acid is not destroyed. When erythric acid is 

 treated with boiling water, the lecanoric acid is decomposed 

 into orcine and carbonic acid, so that on evaporating the so- 

 lution, a mixture of picro-erythrine and orcine is obtained. 

 When erythric acid is treated with boiling alcohol, the leca- 

 noric acid contained in it is changed into lecanoric aether*, 

 and the liquid contains picro-erythrine. When picro-erythrine 

 is treated with alkalies, it loses carbonic acid, and taking up 

 several equivalents of water, is changed into pseudo-orcinef. 

 When, however, erythric acid is acted on by alkalies, the le- 

 canoric acid contained in it is changed into orcine and car- 

 bonic acid, and the picro-erythrine is changed into pseudo- 

 orcine and carbonic acid, so that a mixture of orcine and 

 pseudo-orcine is obtained. 



As the names picro-erythrine and pseudo-orcine have a i 

 barbarous sound, and the latter is also inappropriate, I pro- 

 pose to change the former into Erypicrine and the latter into 

 Eryglucine. Erythric acid might therefore be called erypicrine- 

 lecanoric acid. 



Mr. Stenhouse has discovered in a South American variety 

 of the Roccella tinctoria an acid, which he calls alpha- orscllic 

 acid; and in a Cape of Good Hope variety of the same plant, 

 a very similar acid, called by him beta-orsellic acid. He has 



* It follows that Mr. Stenhouse's erythric-methylic aether is, in fact, 

 lecanoric-methylic aether. Its formula must therefore be Ci 3 H 8 8 -j-C 2 H 3 

 =€20 H u 9 . Its composition would then be as follows : — 



20 eqs. Carbon . 



11 ... Hydrogen 



9 ... Oxygen . 



2537-2 10000 100-00 100-00 



i I am inclined to think that the orcine, which is found in small quan- 

 tity accompanying the pseudo-orcine derived from the decomposition of 

 picro-erythrine, was produced from erythric acid left undecomposed among 

 the picro-erythrine. 



