1870.] 



Dr. J. Stenhouse on certain Lichens. 



223 



the latter of which were carefully separated by picking — a somewhat tedious 

 operation, as they were much interlaced. 



Vsnea barbata. 



In order to extract the usnic acid from this lichen, it was macerated for 

 about thirty minutes with a dilute solution of so die carbonate, squeezed, 

 again treated once or twice in a similar manner, and the turbid solution 

 precipitated by a slight excess of hydrochloric acid. The crude dark-green 

 acid thus obtained was mixed with milk of lime and a considerable quantity 

 of warm water (40° C), filtered, and the clear lemon-coloured solution of 

 usnate of calcium acidulated with hydrochloric acid. The acid was thus 

 precipitated in pale yellow flocks, which were collected. 



The reason that I adopted this modification of the process I formerly 

 proposed* is, that although usnic acid readily dissolves in milk of lime « 

 when it has been extracted, yet in order to exhaust this and other compact 

 lichens, it requires to be treated a great many times if lime be employed, 

 whilst two or three are sufficient with carbonate of sodium. 



The partially purified usnic acid obtained in the maimer above described 

 was easily rendered quite pure by taking advantage of the peculiar pro- 

 perty which this acid possesses of forming an insoluble calcium salt when 

 boiled with lime. The crude yellow acid was placed in a flask with a 

 quantity of water and an excess of slaked lime, and the mixture boiled for 

 twenty minutes or half an hour. The insoluble calcium usnate was col- 

 lected, well washed with hot water, and the lime then removed by boiling 

 it with a slight excess of hydrochloric acid. The tolerably pure usnic 

 acid was then collected, and well washed with boiling water. It was 

 advisable to continue the digestion with hydrochloric acid for half an hour, 

 as it rendered the acid more compact and easy to collect. By this alternate 

 treatment with lime and acid, a large quantity of some dark-coloured im- 

 purity was removed. This forms the best process for recovering usnic 

 acid in a state of tolerable purity from residues. 



It was found, however, to be better, when considerable quantities of usnic 

 acid were to be prepared, to boil the pale yellow usnic acid paste, as ob- 

 tained by precipitation from the lime solution, with a small quantity of 

 water, to which strong caustic soda solution was gradually added, sufficient 

 to dissolve nearly the whole of the usnic acid. It was then set aside to 

 crystallize, and when cold the very dark coloured supernatant mother 

 liquor decanted, and the crystals of sodic usnate washed once or twice by 

 decantation, with a small quantity of cold water. It was then redissolved 

 and recrystallized once or twice in the same manner. 



The nearly pure sodic usnate was now dissolved in a considerable quantity 

 of hot spirit, filtered, and the boiling solution strongly acidulated with 

 acetic acid. The usnic acid then separated in fine needles, which when 

 cold w r ere collected, well washed with cold spirit (in which they are almost 

 * Ann. der Cheni. und Pharm. vol. lxviii. p. 98. 



