270 On Substances contained in the Roccella tinctoria. 



16006 100-00 100-00 



Erythric acid, therefore, in its conversion into picro-ery- 

 thrine, takes up the elements of 5 equiv. of water. 



Roccellic Acid. 



This acid was discovered by Heeren. If the lichen be ex- 

 tracted with ammonia in the cold, a yellow fluid is obtained 

 which contains erythric acid and roccellic acid dissolved in 

 ammonia. The roccellic acid may be separated by adding 

 chloride of calcium to the fluid, by which a precipitate of roc- 

 cellate of lime is produced, or by precipitating the two acids 

 with muriatic acid and treating the precipitate with boiling 

 water, which dissolves the erythric acid and leaves the roccellic 

 acid behind. But by this method the acid is not obtained so 

 pure as by extraction with alcohol, as the ammonia takes up 

 at the same time a brown substance from the plant, from 

 which it is afterwards difficult to separate the acid. I there- 

 fore prefer the method which I have described above. 



Roccellic acid is a species of fat acid. I have nothing to 

 correct in, and little to add to, the description given of it by 

 Heeren. It is insoluble in water, but easily soluble in alco- 

 hol and aether. From a hot concentrated solution in alcohol 

 it crystallizes on cooling in small needles, forming when dry 

 a white, shining crystalline mass. By the spontaneous evapo- 

 ration of its alcoholic solution, it is obtained in larger and 

 more defined crystals. Its solutions redden litmus paper 

 strongly. Heated on platinum foil it melts to a transparent 

 fluid, which, if allowed to cool, congeals again to a crystalline 

 mass. If further heated it is decomposed, giving off a smell 

 like burning fat, and burns with a bright flame, leaving no 

 residue. Heated in a tube closed at one end, it melts and 

 gives an oily sublimate, leaving little or no residue ; the oily 

 sublimate is soon changed into a crystalline mass, but on 

 being again sublimed it remains fluid. Roccellic acid is 

 insoluble in dilute mineral acids, but soluble in alkalies. 

 When caustic potash is poured on it, it swells up to a gela- 

 tinous mass, which is insoluble in the caustic ley, but soluble in 

 water. The solution on boiling foams like a solution of soap ; 

 strong acids re-precipitate the roccellic acid in flocks. The so- 

 lution on evaporation leaves a crystalline, saponaceous mass. 

 Ammonia behaves in a similar manner. It is also soluble in 



