the Roccella tinctoria and Evernia Prunastri. 253 



The first point to be determined is, whether the substance 

 to which I gave the name of erythric acid is the same as the 

 erythric acid of Mr. Stenhouse. Both give when treated with 

 boiling water picro-erythrine. Mr. Stenhouse, however, states 

 that picro-erythrine, when decomposed with alkalies, gives 

 carbonic acid, and a new substance called by him pseudo-orcme, 

 while I was of opinion that it is decomposed into carbonic acid 

 and orcine. Being still in possession of a sufficient quantity of 

 picro-erythrine, I treated some of it with boiling caustic baryta, 

 and after filtering, precipitating the excess of baryta by a 

 stream of carbonic acid gas and evaporating, I obtained a 

 mass of crystals, which, after being treated with aether and 

 crystallized over again, possessed the properties and compo- 

 sition ascribed by Mr. Stenhouse to pseudo-orcine. The 

 aether on evaporation gave a quantity of orcine. This pro- 

 perty of picro-erythrine, the discovery of which is due to Mr. 

 Stenhouse, is so characteristic as to prove at once that the 

 erythric acid examined by him is identical with that described 

 by me. As Mr. Stenhouse seems to have obtained the acid 

 in a state of much greater purity than I did, I shall lay no 

 stress on the differences found by us in its composition, but 

 shall at once accept the numbers given by him as expressing 

 the true composition. 



Mr. Stenhouse found erythric acid to consist in 100 parts 

 of— 



I. 

 Carbon . . 56*85 

 Hydrogen . 5'56 

 Oxygen . . 37 '59 



The formula given by him is C 20 H n O 10 . 



Now I propose to change this formula into C 42 H^ O al . The 

 calculated composition will then be as follows: — 



42 eqs. Carbon . . 3150 56*89 



23 ... Hydrogen . 287 5*18 



21 ... Oxygen . . 2100 37*93 



5537 100-00 



It will be seen that Mr. Stenhouse's numbers agree suffi- 

 ciently well with this formula ; and I shall now show how, by 

 means of it, we may explain the various changes which the 

 acid undergoes. 



If from 1 eq. erythric acid . . "=C 42 H23 21 

 we subtract 1 eq. lecanoric acid =C I8 H 8 O g 



*-'24 "15^13 



and add 1 eq. water ...'.= HO 



we obtain C 94 H lfl O 



24 "W ^14 



