Boyal Society. 303 



IV. Evernia Prunastri. 



1 . Evernic acid is obtained by extracting the lichen with milk of 

 lime, precipitating with muriatic acid, drying the precipitate, and 

 digesting in weak spirit till nearly two-thirds are dissolved. The 

 solution yields crystals of evernic acid. The insoluble part is usnic 

 acid. Evernic acid yields only a slight yellow colour with hypo- 

 chlorite of lime. 



Formula of hydrated evernic acid . . C^ His ^n + HO 

 Formula of everniate of potash .... C^ H15 0,3 + KO 

 Formula of everniate of baryta .... C.,4 H^ Oi3 + BaO + Aq. 



2. Evernesic acid is obtained by dissolving evernic acid in a slight 

 excess of caustic potash, passing carbonic acid gas through the solu- 

 tion to saturation, and concentrating the solution : evernesiate of 

 potash crystaUizes out. From this the acid may be separated by 

 means of muriatic acid. It gives a yellow colour with hypochlorite 

 of lime. 



Formula of hydrated acid Cia Hg Oy-t-HO 



Formula of evernesiate of baryta . . C,s H,j 0; + BaO + Aq. 

 Formula of evernesiate of silver . . Cm H,j 0; + AgO. 



Orcine. 



This substance is always obtained when any of the colouring prin- 

 ciples of the lichens which yield red dyes with ammonia are subjected 

 to particular processes. The best way of obtaining it pure is to boil 

 the alpha-, or beta-orsellesic acid, or the erythrelcsic acid in water 

 for about an hour. Carbonic acid is given off, and crystals of co- 

 lourless orcine are deposited. It gives a dark purple red colour with 

 hypochlorite of lime, quickly changing into deep yellow. 



Empirical formula Cis Hu O;. 



Brom-orceide, CioH^^Br 0,3 (empirical), is obtained by pouring bro- 

 mine into a concentrated aqueous solution of orcine ; when pure it 

 forms long white adhering needles ; it has no taste or smell. 



Chlor-orceide, a similar compound, is obtained by passing chlorine 

 gas through a sohition of orcine. 



Usnic Acid. 



'J'liis principle is found in Usnea florida, U. hirta, U. plicata, U. 

 barhala, Ramalinea calicaris, R. Fraxinia, Evernia Prunaslri, and Cla- 

 donia Ranycferina. It is best obtained from Cladonia Ranfjeferina 

 and Usnea fiorida, by the use of lime and muriatic acid. 



Its empirical formula is 0)^ H,- O,,- 



Feb. 17, — " On a Formula for the Elastic Force of Vapour at 

 different Temperatures." By Captain Shortrede. Communicated 

 by Lieut. -Col. Sykcs, F.R.S. 



'I'he autlior adojjts as the basis of his formula the first series of 

 experiments at high temperatures made by the French Academy, 

 and those of Magnus at low temperatures. For the Academy's 



