contained in the Roccella tinctoria. 265 



ashes are grayish-white. They consist of sulphate of soda, 

 chloride of sodium, oxide of iron, alumina, carbonate of lime 

 and carbonate of magnesia. 



I shall now proceed to describe more fully several of the 

 substances just mentioned. 



Erythric Acid. 



' This body is the most important of those existing in the 

 plant, as it is that one which gives rise to the colouring matters 

 for the production of which the lichen is employed. It is 

 not possible however to obtain much from the plant, since by 

 the action both of boiling water and alcohol it undergoes a 

 rapid change. By the method described above I obtained in 

 one case from 1 lb. of the lichen 60 grains of it. If prepared 

 without the intervention of alkalies, erythric acid is perfectly 

 white and tasteless. It is soluble in water, alcohol and aether. 

 1 part dissolves in 240 parts of boiling water, from which a 

 great part separates on cooling in flocks, or as a crystalline 

 powder. Its solubility in aether distinguishes it from Heeren's 

 erythrine, and its solubility in water from Kane's erythriline. 

 Its solutions redden litmus paper. From a concentrated so- 

 lution in boiling alcohol it is deposited on cooling in needles 

 and star-shaped masses, which consist of minute crystals. It 

 is precipitated from its alcoholic solution by water as a jelly. 

 If the alcoholic solution however be boiled for a length of 

 time, it is converted into erythric aether, in the same way as 

 lecanoric acid is converted by boiling alcohol into leca- 

 noric aether ; and if water be now added to the solution no 

 precipitate is formed, but the erythric aether gradually cry- 

 stallizes in needles from the solution. By the continued 

 action of boiling water erythric acid is converted into picro- 

 erythrine. Heated on platinum foil, it melts and burns away 

 without leaving any residue : heated in a tube closed at one 

 end, it gives an oily sublimate, which after some time cry- 

 stallizes ; this sublimate consists of orcine. Erythric acid is 

 easily soluble in caustic and carbonated alkalies, and in lime 

 and baryta water, and it is reprecipitated from these solutions 

 by acids in form of a jelly, unless they have previously 

 been boiled or left to stand for a considerable time. If a 

 solution of it in baryta water be boiled, carbonate of baryta is 

 deposited, and acids now produce no precipitate of erythric 

 acid. If the excess of baryta be removed by a stream of 

 carbonic acid gas, and the filtered solution be evaporated, 

 there are obtained prismatic crystals, which are easily recog- 

 nised as consisting of orcine by their intensely sweet taste, by 

 their being volatilizable without any residue, by their solution 

 Phil. Mag. S. 3. Vol. 29. No. 194. Oct. 1846. T 



