1870.] Dr. J. Stenhouse on certain Lichens. 225 



Calcium Usnate. 



When pure usnic acid was moistened with spirit, and then rubbed up in 

 a mortar with milk of lime, it combined and formed a deep yellow paste, 

 which, on the addition of more water and filtration, yielded a lemon-coloured 

 solution, containing calcium usnate, and hydrate. When this solution 

 was heated it became turbid, and after boiling some time, the whole of the 

 usnic acid was deposited as an insoluble calcium compound, in the form of 

 small deep yellow rhomboidal crystals. Although I made several analyses 

 of this compound, prepared at different times, I was unable to obtain it of 

 a constant composition, probably owing to its being mixed with variable 

 quantities of calcium carbonate and hydrate. 



The formation of this insoluble calcium salt is very characteristic of usnic 

 acid, and is an excellent test of its presence. As with the sodium salt, 

 carbonic anhydride entirely decomposes the calcium compound. Usnic 

 acid appears, therefore, to be a very feeble acid. 



An attempt was made to prepare ethylic usnate by treating usnate of 

 silver with ethylic iodide, but without success. When usnic acid was 

 treated with bromine it was completely decomposed, and converted into 

 an orange-coloured uncrystallizable resin. 



Evernia prunastri — Evernic Acid. 



The evernic and usnic acids that this lichen contains were extracted by 

 the lime process, which consists in macerating the lichen two or three 

 times successively with milk of lime for about half an hour each time. 

 The solution of the mixed acids was then filtered, precipitated by a slight 

 excess of hydrochloric acid, and the precipitate collected and dried. In 

 order to extract the evernic acid from the mixture, it was agitated for 

 about five miuutes with four parts boiling alcohol and filtered. The acids 

 remaining undissolved were treated two or three times with the same 

 quantity of boiling alcohol, and the dissolved evernic acid precipitated by 

 the addition of an equal bulk of water. By this means the evernic acid, 

 being readily soluble in boiling alcohol, was in a great measure separated 

 from the usnic acid, which dissolves with difficulty in that menstruum 

 unless digested with it for a considerable time. The crude evernic acid 

 thus obtained amounted to about one-third of the mixed acids, and was 

 purified by repeated crystallization from strong spirit, taking care not to 

 digest it for any length of time. The process is much facilitated by 

 completely removing the mother liquors by Bunsen's vacuum filter. 



Pure evernic acid, as has been already described by myself* and Hessef, 

 consists of aggregations of minute needles, melting at 164° C. It is a 

 feeble acid, and does not decompose solutions of bicarbonate of sodium in 

 the cold ; as, however, the adhering colouring-matter is somewhat soluble 

 in that menstruum, it may be employed to free the crude acid to a great 



* Ann. der Chem. unci Pharm. vol. lxviii. p. 84, 

 t Ibid. vol. cxvii. p. 298. 



